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Seed dispersal and the Holocene migration of woodland herbs.

INTRODUCTION

Pollen records spanning the 16 000 yr since the last glacial maximum in North America and Eurasia indicate that tree species spread northward from refugia quickly and at different rates (Davis 1976, Delcourt and Delcourt 1987, Webb 1987, Birks 1989). As noted by Davis and others, these data have interesting ecological and evolutionary implications. For example, species-specific dispersal rates imply that the composition of tree communities has changed continually throughout the Holocene and that current communities may not have existed as recently as a few thousand years ago (Davis 1983). In contrast to the information on trees, little is known about how the distribution of herbaceous woodland species has changed over time. The relatively sparse, insect-borne pollen of woodland herbs appears much less reliably in pollen records than does the more abundant, wind-borne pollen of trees. Thus, our understanding of how woodland herbs colonized Northern temperate forests after the last glaciation remains sketchy even though these plants currently cover millions of hectares of understory habitat.

Investigations of the rate at which tree species moved northward following the retreat of the North American ice cap have revealed two discrepancies: the distances seeds move during standard dispersal events correlate poorly with estimated rates of migration, and only by assuming extreme, and presumably rare, seed movements can standard dispersal mechanisms account for the estimated rate of spread (Skellam 1951, Gleason and Cronquist 1964, Webb 1986, Johnson and Webb 1989, Greene and Johnson 1995, Wilkinson 1997; see also the round-table discussion in Bennett 1986). For the three genera of fagaceous trees (Castanea, Fagus, and Quercus), it has been suggested that unusually long-distance dispersal of nuts by Blue Jays (Johnson and Webb 1989) or Passenger Pigeons (Webb 1986) may help to resolve these discrepancies. We are not aware of comparable studies that examine the existence or magnitude of a discrepancy between known dispersal mechanisms and actual rates of long-term colonization in woodland herbs. Although it has been demonstrated that the spread of forest understory plants can be limited by dispersal (e.g., Peterken and Game 1984, Matlack 1994), such studies usually focus on small spatial and short temporal scales. We know of no papers that ask whether standard seed-dispersal mechanisms could have allowed temperate forest herbs to reach their present range over the course of the past 16 000 yr.

We find this question intriguing because the distribution of many woodland herbs extends 1000-2000 km in a north-south direction, yet the majority of these species grow clonally, have little recruitment by seed, and possess no obvious mechanism for long-distance seed dispersal (Bierzychudek 1982). As a result, the seeds of woodland herbs often move [is less than] 1 m and only occasionally move more than a few tens of meters. In some instances, seeds of woodland herbs move such small distances that dispersal may serve primarily as a way of avoiding predators or locating suitable microhabitats rather than as a way of colonizing unoccupied habitat. For example, ant-plant mutualisms represent one of the most common modes of seed dispersal among woodland herbs (Handel et al. 1981). Although the transport of seeds by ants may provide plants with increased opportunities for germination (Handel 1978, Heithaus 1981, Kjellsson 1985b, Casper 1987, Levey and Byrne 1993), ants rarely move seeds more than a few meters. Hence, as dispersal agents ants move seed on a strictly local scale.

An examination of how woodland herbs reached their present range leads naturally to two additional issues. First, what is the importance of the tail of seed-dispersal curves, about which almost nothing is known (Portnoy and Willson 1993, Malanson and Armstrong 1996)? Ecologists usually ignore the tail of dispersal curves because long-distance dispersal events are by definition uncommon, and when they do occur, it is very difficult to follow them to completion. In the present study, we used movement models to examine the tails of dispersal curves. Specifically, we used models to test whether known mechanisms of seed dispersal in woodland herbs (such as ants or rodents) could reasonably account for actual, long-term rates of spread. Where standard mechanisms of seed dispersal could not account for observed rates of spread, we used models to estimate the frequency and distance of the occasional means of transport that must have occurred so that woodland herbs could colonize the vast geographic region that they now occupy. Second, if known dispersal mechanisms cannot explain the current distribution of woodland herbs, this opens the possibility that accidental, long-distance dispersal events, which may differ greatly from standard dispersal mechanisms, play a much more important role than is commonly thought. If true, this would have wide-ranging implications for a suite of ecological and evolutionary issues.

In this paper, we ask whether known dispersal mechanisms can account for the Holocene spread of the temperate woodland herb, Asarum canadense L. (wild ginger). To estimate long-term rates of spread by seeds, we calibrated seed-dispersal diffusion models with life history data and with detailed observations of individual seed movements. We supplement our results for A. canadense with a literature survey on the dispersal capabilities of other plants, and with results from models that examine the tail of dispersal curves for wild ginger and other herbs.

MATERIALS AND METHODS

Biology of Asarum canadense

A. canadense grows in the understory of deciduous forests extending from New Brunswick to southern Manitoba at the northern end of its range, and from North Carolina to eastern Kansas at the southern end of its range (Fernald 1970). A. canadense spreads vegetatively by rhizomes and produces geotropous, self-pollinated flowers. Seeds are produced in mid-June and weigh 13.7 mg ([+ or -] 0.2 SE, n = 50). Like the seeds of many woodland herbs (Handel et al. 1981), A. canadense seeds bear elaisomes and are dispersed by ants. Heithaus (1986) reported that ants dispersed A. canadense seeds [is less than] 1 m, a finding that agrees well with other ant-dispersal systems studied to date. More information about the biology of A. canadense can be found in Heithaus (1981, 1986), Muir (1995), Cain and Damman (1997), and Damman and Cain (1998).

Parameter estimation

We used demographic data and observations of seed carries by ants to calibrate diffusion models for the spatial spread of A. canadense over long periods of time. In particular, we needed to estimate two parameters from these data, [r.sub.m], the intrinsic rate of population increase, and D, the diffusion coefficient. Based on transition matrix analyses reported elsewhere (Damman and Cain 1998), we estimate the intrinsic rate of increase to be [r.sub.m] = 0.12, which is the maximum value observed over 6 yr of study at four spatial locations. In the present paper, we estimated the diffusion coefficient (D) from data on the distance that ants transported A. canadense seeds. For this purpose, we observed 50 seed carries by ants at our study site in a sugar maple-beech-hemlock woodlot in Garreton, Ontario (latitude 44 [degrees] 50' N, longitude 75 [degrees] 40' W). We placed small piles of 10-20 A. canadense seeds on the forest floor in areas where A. canadense grew abundantly. We followed the movement of these seeds either directly by observing carries by ants during the day, or indirectly by noting the distance moved overnight by seeds marked with a fluorescent powder. The two approaches gave comparable results. From these data, we calculated the seed-dispersal diffusion coefficient (D) according to the relation D = E[[1.sup.2]]/4[Tau] (Okubo 1980), where E[[1.sup.2]] is the expected squared distance that ants carried A. canadense seeds and [Tau] is the average time from the dispersal of a seed to its maturation as an adult ramet that itself can produce seeds. For A. canadense, Damman and Cain (1998) estimated [Tau] = 10.1 yr.

Diffusion models

We used models of the long-term spread of woodland herbs to examine two questions: (1) is there a discrepancy between known dispersal capabilities and actual migration rates? And, if so, (2) how often and how far must unusual seed-dispersal events have carried the seeds of woodland herbs in order for these plants to have reached their present range? An answer to the first question could be obtained without resort to models, but a modeling approach is necessary to address the second question.

We modeled the long-term spread of A. canadense and other woodland herbs as a two-dimensional (homogeneous) diffusion process:

(1) [differential]n / [differential]t = D([[differential].sup.2]n / [differential][x.sup.2] + [[differential].sup.2]n / [differential][y.sup.2]).

In this formulation, n(x, y, t) is the density of ramets that are located at coordinate position (x, y) at time t, and D is the diffusion coefficient (a measure of the long-term spread of populations) for dispersal by seed. Eq. 1 represents a reasonable method with which to model long-term displacements in A. canadense because there is no directional component to seed carries by ants over wild ginger's 10-yr generation time. Although the diffusion approach is reasonable, we cannot test its adequacy in detail because we lack time-profile data on the spread of A. canadense and other woodland herbs from known release points. Thus, in the discussion that follows we use diffusion models to illustrate general principles about the spread of woodland herbs rather than to predict in a precise way the movement of any particular species over long periods of time. Among various movement models that we could have used for this purpose, we selected diffusion models because they are biologically realistic (considerable backtracking in seed movement no doubt occurs) and because they provide a well-developed quantitative framework (see Okubo 1980 and Turchin 1998) with which to compare the movement of different species.

We also modified Eq. 1 to include a population growth term, f(n):

(2) [differential]n / [differential]t = D([[differential].sup.2]n / [differential][x.sup.2] + [[differential].sup.2]n / [differential][y.sup.2]) + f(n).

The population growth term can take a variety of forms. For example, as populations expand into new habitat, it may be reasonable (Bennett 1986) to model population growth with a simple exponential growth term, f(n) = [r.sub.m]n, or alternatively, with a logistic growth term, f(n) = [r.sub.m]n(1 - n/K). Our primary interest in this paper concerns the long-term rate of spread of woodland herb populations. With respect to Eq. 2, this means that we can ignore the functional form off(n) because similar results are obtained with exponential, logistic, and other population growth terms (Skellam 1951, Okubo 1980, Andow et al. 1990, Shigesada et al. 1995). In particular, under a variety of reasonable forms off(n), when time is large populations governed by Eq. 2 advance at an asymptotic rate of spread, c (with units of distance/time):

(3) c = [square root of] 4[r.sub.m]D

where [r.sub.m] is the intrinsic rate of increase of populations and D is the diffusion coefficient. Intuitively, it makes sense that the maximal rate of population advance (c) should depend not only on the rate of seed movement (D), but also on how rapidly populations grow ([r.sub.m]) and hence disseminate large numbers of seeds.

Impact of occasional events

We incorporated occasional dispersal events into the diffusion-model approach described above in order to investigate the impact of unusual, long-distance dispersal events on the Holocene spread of woodland herbs. To do this, we specified the diffusion coefficient for local movements, the mean and variance of the distance that seeds were transported on occasional events, and the (per seed) frequency with which such occasional events occurred. We then recalculated the diffusion coefficient by weighting local and long-distance movements by the appropriate frequencies. Finally, we calculated an upper bound ([R.sub.max]) on the distance plants were likely to disperse by seed over long periods of time. Based on the asymptotic rate (c, defined

in Eq. 3) of the spread of populations, we calculated [R.sub.max] from the relation [R.sub.max] = ct = [square root of] 4D[r.sub.m]t, where D is the diffusion coefficient weighted as just described. Advantages to this procedure include that it is simple to compute and that it allows ready comparison to results from diffusion models that do not include occasional dispersal events. A potential difficulty with this approach concerns its violation of the diffusion model assumption that movements consist of a large number of relatively homogeneous, small steps. In practice, however, violation of this assumption appears to make little difference. For example, when local movements have a mean ([+ or -] 1 SD) of 1.0 m ([+ or -] 1.0), and occasional, long-distance dispersal events occur with a frequency of 0.001 and have a mean ([+ or -] 1 SD) of 1000 m ([+ or -] 1000), the diffusion approximation described in this paragraph and a spatially explicit simulation of the movement process yield virtually identical net displacements over time (diffusion approximation: D = 482.3 [m.sup.2]/yr, net displacement after 500 yr = 870.4 m; simulation: D = 481.8 [m.sup.2]/yr, net displacement after 500 yr = 869.9 m).

We corroborated results from diffusion models with a spatially explicit simulation of the "scattered colony" model developed by Shigesada et al. (1995). In these simulations, local populations expanded radially at the rate c (defined by Eq. 3) and offspring colonies were established from the long-distance transport of seeds or other propagules. For each time step in the model, we kept track only of those offspring colonies that had dispersed the farthest from the refugia. We did this for two reasons: (1) our primary interest was in the maximum rate at which offspring colonies reached a given distance from the refugia, and (2) if this was not done the computer simulations ground to a halt due to the exponential production of offspring colonies. Relative to the diffusion approach described in the preceding paragraph, the "scattered colony" simulations have the advantage of directly modeling the spread of woodland herbs as a hierarchical movement process (local movements and occasional long-distance dispersal events). However, because of the extensive time period (16 000 yr) being modeled and the large number of offspring colonies produced, the simulations were very slow. In preliminary analyses, the two approaches gave similar results; thus, in this paper we relied on diffusion models because they were simpler to use.

Literature survey

A survey of the literature on distances dispersed by seeds provided an overview of the dispersal ability of woodland herbs as compared to plants of other growth forms and habitats. We included a study in our literature survey if it provided either an estimate of the mean or maximum distance from the parent plant over which seeds moved, or the data from which we could calculate such values. Dispersal distances reported for wind-dispersed plants usually represent estimates based on measurements of the rate at which seeds fall combined with assumptions about typical wind speeds in the plants' natural habitat (e.g., Feekes 1936, Sheldon and Burrows 1973, Augspurger 1986, Matlack 1987). Because it is often difficult to document the long-distance movements that comprise the tail of a dispersal curve, the maximum distances in our literature survey represent underestimates of the true maximum distance that a seed can move. Many papers reported estimates of mean or maximum dispersal distances obtained under several different treatments. In such instances we selected the treatments that most closely mimicked natural conditions. When more than one treatment mimicked realistic conditions, we chose the conditions giving the longest dispersal distances. Because both the mean and maximum distances moved by the surveyed plant species showed highly skewed distributions, overall comparisons among plant classes and among modes of dispersal were made with the nonparametric Kruskal-Wallis test. Pairwise comparisons within the overall analysis were made with the nonparametric Mann-Whitney U test.

RESULTS

Movement of Asarum canadense seeds by ants

Ants carried A. canadense seeds from 0.05 to 35 m (Fig. 1; mean [+ or -] 1 SD = 1.54 [+ or -] 5.84 m). The two long-distance carries that we observed (23.5 and 35 m) are the farthest we found reported for forest habitats and among the farthest we found reported for any ant-plant mutualism (Table 1; Appendix). Ants carried the seeds of two desert plants for distances greater than the maximum we observed for A. canadense: seeds in Sclerolaena diacantha (Davidson and Morton 1981) and Datura discolor (O'Dowd and Hay 1980) were dispersed by ants up to 77 and 39 m, respectively.

[Figure 1 ILLUSTRATION OMITTED]
TABLE 1. The effect of dispersal mechanism on the mean and maximum
distances moved by the seeds of trees and herbs in woodland and
open habitats. The 75th percentile and maximum values provide an
estimate of the spread of the data around the median.

                     Distance moved by seeds (m)

                            Mean distance

  Dispersal              Median        75th
  mechanism       n    ([dagger])   percentile      Max.

Herbs
  Woodland
    Ant           15       1.38         1.96           9.00
    Ballistic      3       1.50         2.64           3.02
    Bird
    Wind           1       1.18
    Water          1     275

  Open habitat
    Ant            1       0.0023
    Ballistic      4       1.49         2.53           3.28
    Adhesion       4      33.7         71.6          109
    Wind          25       1.37         2.50          25.7
    None          13       0.500        2.04           7

Trees
    Ant            1       2.1
    Bat            2      56.3                        75
    Bird           9      23.73       850         14 300
    Rodent         5      10.3         13.8           20.6
    Wind           5      16.9         33.7           45

                     Distance moved by seeds (m)

                           Maximum distance

                         Median
  Dispersal             ([double       75th
  mechanism       n     dagger])    percentile      Max.

Herbs
  Woodland
    Ant           12         4.00         5.25         35
    Ballistic      4         3.40         4.03          4.55
    Bird           1        33.0
    Wind           4         4.24         6.17          7.1
    Water          1       400

  Open habitat
    Ant            2        58.0                       77
    Ballistic      5         2.80         3.76          4.25
    Adhesion       8         7.43       213          4423
    Wind          83        12.0        137        10 000
    None          39         7.00        34.0         380

Trees
    Ant            1        10.7
    Bat            2       185                        270
    Bird          11       200         3875        22 000
    Rodent         5        51.5         89.5         151
    Wind          72       165          319          3900

([dagger]) Dispersal mechanism significantly affected the mean
distance that seeds traveled: Kruskal-Wallis [chi square] = 33.3,
df = 9, P < 0.0001. Only dispersal mechanisms having more than
one representative were included in the analysis.

([double dagger]) Dispersal mechanism significantly affected the
maximum distance that seeds traveled: Kruskal-Wallis [chi square] =
89.7, df = 11, P < 0.0001. Only dispersal mechanisms having more
than one representative were included in the analysis.


Long-term dispersal by seeds in Asarum canadense

We used the data on the distance ants moved seeds to calibrate diffusion models of dispersal in A. canadense. Our estimate for the diffusion coefficient (D = E[[1.sup.2]]/4[Tau] see Materials and methods) relied on detailed observations of individual behaviors (the distances ants carried seeds), not on mass-release or mark-recapture experiments. For ant-dispersed seeds in A. canadense, D = 0.89 [m.sup.2]/yr.

We used field-calibrated diffusion models to calculate the distance A. canadense could spread by seed dispersal over long periods of time. In a simple diffusion process (Eq. 1), the probability (P) that at time t the net displacement (r) from a point of origin is greater than some value R is (Okubo 1980):

(4) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where t and D are as defined for Eq. 1. Eq. 4 may be rearranged to yield:

(5) R = [square root of] -4Dt In(P).

For particular values of t and P, R represents an upper bound of the net displacement that is likely to occur by seed dispersal. We set t = 16000 yr and P = 1/ N(t), where N(t) = [N.sub.0]exp([r.sub.m]t) was the size of a population that grew exponentially from time zero to time t ([N.sub.0] is the initial population size and [r.sub.m] is the intrinsic rate of increase). By selecting t and P in this manner, over the last 16 000 yr only one plant from an exponentially growing population is likely to have traveled farther than R (Skellam 1951). Substitution of P = 1/ N(t) into Eq. 5 yields R = [square root of] 4Dt(ln [N.sub.0] + [r.sub.m]t). Note that for [N.sub.0] = 1, this equation reduces to R = ct, where c is defined by Eq. 3. Given the empirically observed values of [r.sub.m] = 0.12/yr (see Materials and methods) and D = 0.89 [m.sup.2]/yr, if we let t = 16000 yr and [N.sub.0] = 1, R equals 10.5 km. As discussed by Okubo (1980), the value of R varies little as No is increased. For example, with [r.sub.m], D, and t held constant at the values just mentioned, R increases [is less than] 0.1 km when No is increased from one to [10.sup.12]. Because the value of No has little practical effect, in results that follow we assume No = 1, and thus, R = [R.sub.max] = ct, where [R.sub.max] is as defined in Materials and methods: Impact of occasional events.

Our field-calibrated diffusion models indicate that A. canadense is likely to have dispersed a maximum of only 10-11 km during the past 16 000 yr. This estimate of the maximum distance moved depends on two empirically calibrated parameters: the intrinsic rate of population increase ([r.sub.m]) and the diffusion coefficient (D). Because our estimate of [r.sub.m] (0.12) may underestimate the true value, we examined the impact of [r.sub.m] for a range of values that are reasonable for herbaceous plants (Silvertown et al. 1993). Even when [r.sub.m] = 1.2, a value that is 10 times greater than our observed value and that exceeds all but one value reported in Silvertown et al. (1993), the maximum distance that A. canadense is likely to have dispersed is [is less than] 35 km (Fig. 2A). Similarly, the diffusion coefficient (D) was estimated from dispersal data, and ants may disperse seeds farther than the longest carry (35 m) that we observed. We therefore calculated the upper bound for the distance A. canadense could disperse over long periods of time under a set of hypothetical, long-distance seed carries by ants. We assumed the hypothetical, long-distance carries occurred with a frequency of 0.001. On this basis we recalculated [R.sub.max] = ct = [square root of] 4[D.sub.h][r.sub.m]t], where [D.sub.h] is a diffusion coefficient based on the (frequency-weighted) combination of empirical (Fig. 1) and hypothetical distances that ants could carry seeds. [R.sub.max] was [is less than] 100 km even for very unrealistic cases, such as hypothetical seed carries by ants on the order of 800-1200 m (Fig. 2B).

[Figure 2 ILLUSTRATION OMITTED]

Literature survey

The relatively small mean and maximum dispersal distances that we report for A. canadense were typical of woodland herbs and herbs in general (Table 1; Appendix). The maximum dispersal distance reported for any woodland herb is 400 m for the water-dispersed seeds of Mimulus guttatus (Waser et al. 1982). After this species, the maximum dispersal distances reported for other woodland herbs are 35 m for the ant-dispersed seeds of A. canadense (this study), 33 m for the bird-dispersed seeds of Phytolacca americana (Hoppes 1988), and 17 m for the ant-dispersed seeds of Sanguinaria canadensis (Pudlo et al. 1980).

The mean dispersal distances of woodland herbs (median = 1.39 m, N = 20) were statistically indistinguishable from those of herbs growing in open habitats (median = 1.07 m, N = 47) (Mann-Whitney U test: Z = 0.93, P = 0.17). However, maximum dispersal distances were lower among woodland herbs (median = 4.0 m, N = 22) than among herbs growing in open, and presumably windier, habitats (median = 10.0 m, N = 137) (Mann-Whitney U test: Z = 1.94, P = 0.026). When pooled across seed-dispersal mechanisms, trees had greater mean (median = 16.1 m, N = 22) and maximum (median = 150 m, N = 91) dispersal distances than did herbs (mean: median = 1.20 m, N = 67; maximum: median = 7.59 m, N = 159) (Mann-Whitney U test: Z = 4.80, P [is less than] 0.0001 for mean distance, and Z = 8.19, P [is less than] 0.0001 for maximum distance).

Mean and maximum dispersal distances usually are collected within fairly homogeneous habitat, and hence, do not include the influence of barriers to dispersal. An alternative approach that does incorporate the impact of barriers to dispersal is to record the rate at which plants colonize recently available habitat. For example, Matlack (1994) measured the distance traveled from nearby source populations by woodland species that had migrated into second growth forests of known age. From these data Matlack calculated maximum yearly rates of spread for a variety of herbs, shrubs, and vines; these maximum migration rates represent a parameter similar to c (see Eq. 3). For A. canadense, Matlack reports a maximum yearly rate of spread of 0.59 m/yr. Extrapolated to 16 000 yr, this rate implies A. canadense would move 9.4 km, a figure in close agreement with the 10.5 km calculated from our diffusion models. If we take the largest rate of spread reported by Matlack (2.5 m/yr for Potentilla canadensis), in 16 000 yr a woodland herb moving at this (maximal) rate would disperse 40 km.

Impact of occasional events

We know of no data that address the frequency and distance of occasional dispersal events in woodland herbs. We used diffusion models to provide a preliminary assessment of the frequencies and distances that would allow woodland herbs to colonize Northern temperate forests over the past 16 000 yr. For models calibrated with data for A. canadense, occasional dispersal events had to have a high frequency ([is greater than or equal to] 0.001 on a per seed basis) and a large magnitude (dispersal distance [is greater than] 1 km) in order for Asarum to have traveled over 200 km in 16000 yr (Fig. 3). Our literature survey (Appendix) indicates that A. canadense disperses as well as or better than most herbaceous plants; thus, the results in Fig. 3 may apply to most herbs. There are, however, exceptions: models consistent with published seed dispersal data for Tussilago farfara show that even in the absence of occasional dispersal events, T. farfara could have dispersed over 900 km in 16 000 yr (Fig. 4). For species with dispersal capabilities similar to Tussilago, occasional dispersal events have little impact unless they occur with high frequency and have a large magnitude; this is because the standard dispersal process already includes long-distance events.

[Figures 3-4 ILLUSTRATION OMITTED]

As shown above for A. canadense (Fig. 2A), long-term displacements calculated from our diffusion models depend on the magnitude of the intrinsic rate of increase, [r.sub.m]. This is evident from the relation [R.sub.max] = [square root of] 4D[r.sub.m]t, where, on a logarithmic scale, [r.sub.m], D, and t have an additive effect on [R.sub.max]. When occasional dispersal events are incorporated into the estimate of the diffusion coefficient, D, it can be shown that

(6) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where t is time, [Tau] is the average time from the dispersal of a seed to its maturation as a seed-producing adult, f is the frequency of occasional dispersal events, [Mu] and [[Sigma].sub.2] are the mean and variance of the distance of standard dispersal events, and [[Mu].sub.r] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the mean and variance of the distance of occasional dispersal events. Eq. 6 indicates that the population growth rate ([r.sub.m]) and the frequency (f) and magnitude ([[Mu].sub.r]) of occasional dispersal events exert complex, nonlinear effects on [R.sub.max]. For example, a graph of [R.sub.max] vs. [r.sub.m] has a characteristic shape: [R.sub.max] increases with the square root of rm. However, the relative impact of [r.sub.m], f, and [[Mu].sub.r] varies greatly, depending on the value of these and other parameters in Eq. 6 (e.g., compare curves 1-3 in Fig. 5). Overall, results in Eq. 6 and Figs. 2-5 demonstrate that demographic parameters (e.g., those that determine [r.sub.m]), the mean and variance of standard dispersal events, and the frequency, mean, and variance of occasional dispersal events have an individual and a joint impact on how rapidly plants colonize new geographic regions.

[Figure 5 ILLUSTRATION OMITTED]

DISCUSSION

Holocene migration of Asarum canadense and other woodland herbs

Of the seed-dispersal mechanisms reported in the literature, wind dispersal, adhesion, bat dispersal, and bird dispersal appear to be the most likely to move seeds far enough to account for the postglacial recolonization of North America and Eurasia by plants (Table 1; Appendix). However, bird and wind dispersal, both of which have the potential to carry seeds long distances, can only rarely account for the estimated rate of spread of trees during the Holocene (e.g., Skellam 1951, Wells 1983, Chambers and Elliott 1989). In addition, long-distance dispersal events may be quite rare in some instances, even for bird- or wind-dispersed trees. For example, Webb (1987) presented evidence that the crossing of Lake Michigan by Fagus grandifolia, a tree with bird- or rodent-dispersed nuts, took almost 2000 yr and required jumps of between 25 and 130 km. Similarly, Greene and Johnson (1995) estimated that, even with their relatively mobile seeds, wind-dispersed trees growing in woodlots separated by [is greater than] 1 km would be effectively isolated for thousands of years.

Because of the lack of fossil pollen data for most woodland herbs, the minimum distance that Asarum canadense moved since the glacial maximum 16 000 yr ago is best estimated from the pollen record for trees that presently occupy the same habitats as Asarum. During the glacial maximum, refugia for trees that currently form the canopy of forests in which A. canadense grows (e.g., Acer saccharum and Fagus grandifolia) lay along the Gulf Coast of the southeastern United States (Watts and Stuiver 1980, Bennett 1985, Delcourt and Delcourt 1987). Delcourt (1979) provides evidence that Fagus may have had isolated refugia as far north as Tennessee. However, pollen records also clearly indicate that areas as far south as northern Georgia and the coastal plain of North Carolina were covered with boreal forest, a vegetation type indicating climates too severe for A. canadense today (Watts 1970, Ritchie 1987).

How far did A. canadense travel in the late-Glacial and Holocene? If its refugia coincided with those of Acer saccharum, A. canadense must have moved 700 km to reach the southern edge of its current range and 1900 km to reach the northern edge of its current range. Alternatively, if its refugia coincided with the isolated F. grandifolia refugia located in Tennessee, then wild ginger must have moved 100 and 1300 km to reach the current southern and northern edges of its range, respectively. It is possible that scattered A. canadense populations existed farther north than the isolated Fagus refugia. However, no refugia for A. canadense existed within 100 km of our study site: our study site and the surrounding area within at least 100 km were first covered by ice for up to 40 000 yr, then submerged under the saltwater Champlain Sea until ~12 000-10 000 yr ago (Terasmae and Mott 1959, Flint 1971). A. canadense exhibits little or no seed dormancy (Baskin and Baskin 1986). The lack of dormancy and the severity of the climate suggest that neither any seed nor any populations of A. canadense could have persisted north of the glacial front. Therefore, we estimate that during the past 16 000 yr A. canadense must have traveled a minimum of 450 km from the southern edge of the glacier to the northern edge of its current range.

Because ants cannot reasonably be posited to move A. canadense seeds as far as 100-450 km in 16 000 yr (Fig. 2), we infer that A. canadense reached our study site and the northern edge of its range by other means. This conclusion is strengthened by the fact that Asarum may have taken much less than 16 000 yr to arrive at our study site. For example, woodland herbs in the genus Viola, many of which rely on ants for dispersal and currently grow with A. canadense, arrived in southern Ontario at the same time as Acer saccharum (~8000 yr before present) (Schwert et al. 1985). If A. canadense reached southern Ontario as rapidly as Viola, this would make it all the more unlikely that wild ginger's northward migration was mediated by ants.

To generalize from A. canadense to the northward migration of other species, the majority of woodland herbs do not have morphological adaptations for long-distance dispersal by wind, adhesion, or ingestion. With very few exceptions, even the woodland herbs with such adaptations have low seed-dispersal distances (Appendix) and low yearly migration rates (Matlack 1994). Seed-eating birds rarely forage in the forest understory, and, in any case, most woodland herbs do not produce seeds until after migratory birds fly north (e.g., migrants pass through our study site in late April to early May, yet seeds are not available until mid-June or later). Although rodents cache seeds, they move seeds relatively short distances ([is less than] 100 m) before burying them (Vander Wall 1993). Rodents eat the seeds of A. canadense and other woodland herbs, but in so doing destroy them. It is extremely unlikely that rodents would remove seeds, travel long distances, and then excrete or cache viable seeds. Finally, as the glaciers retreated most rivers flowed from north to south (Pielou 1991); hence, transport by water is not a plausible explanation of the Holocene migration of forest herbs. Thus, there are no documented dispersal mechanisms that account for long-distance dispersal in most woodland herbs.

To summarize, woodland herbs, even those with viable seed banks or refugia populations located just south of the glacial maximum, must have traveled 450 km or more in the past 16 000 yr. Thus, our results and literature review present a paradox: seed-dispersal data and yearly migration rates (Matlack 1994) indicate that most woodland herbs should have moved far less than 100 km over the past 16 000 yr, yet these species actually moved from 450 to 2000 km during this period. Given that there are no documented dispersal mechanisms that can account for this discrepancy, we conclude that occasional events leading to long-distance dispersal were of critical importance in the Holocene colonization of northern temperate forests by woodland herbs (see also Wilkinson 1997, who reached a similar conclusion for the postglacial migration of trees). Such occasional events could include meteorological accidents like tornadoes (Webb 1986) and hurricanes (Campbell 1983), and biotic accidents like the transport of seeds in mud clinging to the feet of vertebrates or the ingestion and subsequent excretion of viable seeds by birds that fly long distances (e.g., Darwin 1859, Webb 1986, Wilkinson 1997).

Implications of occasional, long-distance dispersal events

Our conclusion that occasional, long-distance dispersal events were necessary for the colonization of forest-understory habitat by woodland herbs highlights the importance of the tail of seed dispersal curves (see also Portnoy and Wilson 1993) and has several implications.

1) Population biologists long have argued for the importance of genetics in documenting unusual, long-distance dispersal events. Our results support this argument since they suggest that occasional dispersal events play a critical role. Genetics also may be very useful in discriminating between island and stepping stone models of dispersal; this distinction has considerable relevance for metapopulation models (see point 4 below). Furthermore, in principle genetic data could be used to disentangle the present-day frequency and magnitude of occasional dispersal events. To do this would require two steps. First, ecological data would be used to quantify seed dispersal and plant demography; this would allow estimation of parameters such as the intrinsic rate of population increase ([r.sub.m]) and the diffusion coefficient (D), as we have done for A. canadense. Second, genetic techniques would be used to estimate the frequency with which alleles are transferred between widely separated populations. With such genetic and demographic data, it should then be possible to estimate both the frequency and distance of occasional dispersal events.

2) Several authors have noted that the limited dispersal of seeds in woodland herbs increases the likelihood that these species will adapt to local conditions (Levin and Kerster 1974, Waser et al. 1982). More generally, Ehrlich and Raven (1969) argued strongly that there was so little gene flow between populations that it was not useful to think of species as panmictic. Our results imply that occasional, long-distance dispersal events may be far more important than previously thought. Our diffusion-model results for Asarum canadense suggest that accidental, long-distance dispersal events occurred with a frequency of 0.001 or more (on a per seed basis). There are ~30 000 Asarum ramets located at our study site; on average, 8% of these ramets flower each year and 11 seeds/flower are produced (Damman and Cain 1998). Assuming the frequency of long-distance dispersal events suggested by diffusion models applies to present-day populations, these data indicate that a minimum of 26 Asarum seeds are transported long distances from our study site each year by unusual means. Nothing is known, for Asarum or other woodland herbs, about the percentage of such distantly dispersed seeds that establish at new locations. In addition, there is little information on whether selection for locally adapted genotypes is strong enough to overwhelm the disruptive influence of gene flow. Overall, it remains an open question whether occasional, long-distance transport of seeds prevents the adaptation of woodland herbs to local conditions.

3) Previous studies suggest that dispersal can limit the range of many rare plant species (Peterken and Game 1984, Whitney and Foster 1988, Primack and Miao 1992). Our results support this contention because they indicate that even over long periods of time, rare woodland herbs would be unlikely to reach new habitat patches on their own. In addition, because it would take herbs a hopelessly long period of time to move long distances via the standard processes of seed dispersal, our results suggest that at large spatial and temporal scales, corridors may be of little consequence for the direct (unaided) dispersal of woodland herbs. Consistent with this suggestion, Helliwell (1975) and Fritz and Merriam (1994) found few forest-understory plants growing in hedgerows that could act as dispersal corridors. However, corridors may be of critical importance in allowing the movement of vertebrates, which in turn may act as important, if accidental, long-distance dispersal agents for woodland herbs. In general, because we cannot at present separate the frequency and magnitude of occasional dispersal events, we do not know whether such events would (a) allow rapid recolonization of regions in which local herb populations went extinct (Primack and Miao 1992), or (b) provide woodland herbs with a means of dispersal rapid enough to track projected global climate change (Davis 1989).

4) When occasional, long-distance dispersal events play an important role in colonization of habitat, then distance to the seed source and details of the dispersal mechanism may predict patterns of colonization poorly. For example, the equally rapid northward migration of some trees and herbs in the wake of the retreating glaciers (e.g., Schwert et al. 1985) suggests that typical seed-dispersal distances may tell us little about long-distance dispersal (see also Webb 1986). At a very different spatiotemporal scale, Dale (1989) reported that the density of wind-dispersed herb seeds arriving on a large debris avalanche bore no relationship to the distance from the nearest seed source (where the distance was at least 100 m). In general, once the critical dispersal events involve improbable circumstances, distance from source populations may no longer effectively predict patterns of seed movement (unless the distances are very large). While studies of the colonization of oceanic islands long have accepted the importance of accidental dispersal (e.g., Carlquist 1967, Sorenson 1986), studies at smaller scales and investigations of dispersal within a continuous habitat emphasize more predictable events (e.g., seed-dispersal curves: Ribbens et al. 1994, Lavoral et al. 1995). Studying adaptations for seed dispersal probably allows us to understand how plants fill space within a colonized habitat, but not how plants first colonized the habitat patch. Because the majority of plant species rarely move distances [is greater than] 100 m, at intermediate spatial scales (e.g., 200 m to 20 km) the isolation-by-distance terms incorporated into many metapopulation models (Perry and Gonzalez-Andujar 1993, Hanski 1994, Thrall and Antonovics 1995, Malanson and Armstrong 1996) may distort rather than clarify predictions about colonization patterns for plants. However, at very large spatial scales (e.g., hundreds to thousands of kilometers), it is likely that the degree of movement between populations will decrease with distance. Overall, the spread of plant species and the extent to which isolated populations interact may perhaps best be modeled as a hierarchical movement process (dispersal curves at local spatial scales, "island models" at intermediate spatial scales, "stepping-stone models" at large spatial scales). The stratified diffusion model introduced by Shigesada et al. (1995) provides a recent example of such a model.

5) Most analyses in this paper focused on woodland herbs. However, our results probably apply to many other plant species. For example, even though maximum seed-dispersal distances for herbs that live in open habitats such as grasslands, montane meadows, deserts, and old fields can equal 10 km, our literature survey revealed that 74% of 137 herb species that lived in open habitats had a maximum dispersal distance that was [is less than] 60 m (Appendix; see also Cheplick 1998). Given such limited dispersal distances, we suspect that occasional, accidental transport of seeds may be important in these plant communities as well.

ACKNOWLEDGMENTS

We thank Bill Boecklen, Gregory P. Cheplick, Jon Evans, Brook Milligan, Rich Spellenberg, and Allan Strand for many helpful discussions during the preparation of this paper; Gregory Cheplick for providing us a draft of his book chapter on seed dispersal in grasses; and F. Loops for permission to use the field site. The research was funded in part by a grant from the National Science Foundation (DEB 9407229) to D. H. Howard and M. L. Cain, and by Natural Sciences and Engineering Council Operating, Carleton University GR-5, and Cedar Fund Grants to H. Damman.

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APPENDIX
Maximum and mean seed-dispersal distances as reported in
the literature.

Species                           Habitat   Growth form

Plants of forest understory
    and canopy
  Asarum canadense L.             forest       herb
  A. canadense L.                 forest       herb
  Calathea ovandensis Matuda      forest       herb
  Carex pilulifera L.             forest       herb
  Sanguinaria canadensis L.       forest       herb
  Trillium ovatum Pursh.          forest       herb
  Viola blanda Willd.             forest       herb
  Viola cucullata Ait.            forest       herb
  Viola eriocarpa Schwein         forest       herb
  Viola odorata L.                forest       herb
  Viola papilionacea Pursh.       forest       herb
  Viola pedata L.                 forest       herb
  Viola rostrata Pursh.           forest       herb
  Viola spp.                      forest       herb
  Cnidoscolus stimulosus
    (Michx.) Engelm. & Gray       forest       herb
  Crotalaria rotundifolia
    (Walt.) Poir.                 forest       herb
  Stillingia sylvatica
    (Muell. Arg.) Small           forest       herb
  Carex pauciflora Lightf.        forest       herb
  Geranium maculata L.            forest       herb
  Impatiens capensis Meerb.       forest       herb
  I. capensis Meerb.              forest       herb
  Viola striata Air.              forest       herb
  V. striata Ait.                 forest       herb
  Phytolacca americana L.         forest       herb
  Mimulus guttatus Fisch.
    ex D.C.                       forest       herb
  Aster acuminatus Michx.         forest       herb
  A. acuminatus Michx.            forest       herb
  Aster prenantoides Muhl.        forest       herb
  Eupatorium rugosum Houtt.       forest       herb
  Piper amalgo                    forest       shrub
  Alnus crispa (Ait.) Pursh       forest       shrub
  Halesia monticola Sarg.         forest       shrub
  Purshia tridentata
    (Pursh) DC.                   forest       shrub
  P. tridentata (Pursh) CD.       forest       shrub
  Acacia suaveolens (Sm.)
    Willd.                        forest       tree
  Dipteryx panamensis
    (Pitt.) Rec. & Mell           forest       tree
  Casearia corymbosa H. B. K.     forest       tree
  Cornus controversa Hemsl.       forest       tree
  Fagus grandifolia Ehrh.         forest       tree
  Ficus stupenda Miq.             forest       tree
  Ficus subtecta Corner           forest       tree
  Pinus albicaulis Engelm.        forest       tree
  Pinus edulis Engelm.            forest       tree
  Pinus sp. & Thuja sp.           forest       tree
  Quercus palustris Muenchh.      forest       tree
  Virola surinamensis
    (Rol.) Warb.                  forest       tree
  Fagus silvatica L.              forest       tree
  Juglans nigra L.                forest       tree
  Pinus jeffreyi Murr.            forest       tree
  Quercus macrocarpa Michx.       forest       tree
  Quercus muehlenbergii
    Engelm.                       forest       tree
  Acer palmatum Thunb.            forest       tree
  Acer pseudoplatanus L.          forest       tree
  Acer rubrum L.                  forest       tree
  A. rubrum L.                    forest       tree
  A. rubrum L.                    forest       tree
  Acer saccharum Marsh.           forest       tree
  Acer cappadocicum               forest       tree
  Acer griseum Pax                forest       tree
  Acer platanoides L.             forest       tree
  Ailanthus altissima
    (Mill.) Swingle               forest       tree
  Albizzia julibrissum
    Durazzini                     forest       tree
  Alseis blackiana Hemsl.         forest       tree
  Aspidosperma cruenata
    Woods                         forest       tree
  Astronium graveolans Jacq.      forest       tree
  Betula papyrifera Marsh.        forest       tree
  Bombacopsis quinata
    (Jacq.) Dug.                  forest       tree
  Bombacopsis sessilis
    (Benth.) Pitt.                forest       tree
  Carpinus caroliniana Walt.      forest       tree
  Catalpa bignonioides Walt.      forest       tree
  Cavanillesia platanifolia
    (H. & B.) H. B. K.            forest       tree
  Cedrela odorata L.              forest       tree
  Ceiba pentandra (L.)
    Gaertn.                       forest       tree
  Cespedizia macrophylla
    Seem.                         forest       tree
  Chamaecyperis thyoides
    (L.) BSP.                     forest       tree
  Cochlospermum vitifolium
    (Willd.) Spreng.              forest       tree
  Cordia alliodora (R. &
    P.) Cham.                     forest       tree
  Couratari panamensis
    Standl.                       forest       tree
  Dalbergia retusa Hemsl.         forest       tree
  Eucalyptus regnans F.
    Muell.                        forest       tree
  Fraxinus americana L.           forest       tree
  F. americana L.                 forest       tree
  Fraxinus excelsior L.           forest       tree
  Jacaranda copaia (Aubl.)
    D. Don                        forest       tree
  Juniperus virginiana L.         forest       tree
  Lafoensia punicifolia DC.       forest       tree
  Larix laricina (DuRoi)
    K. Koch                       forest       tree
  Liriodendron tulipifera L.      forest       tree
  L. tulipifera L.                forest       tree
  Lonchocarphus pentaphyllus
    (Poir.) DC.                   forest       tree
  Lonchocarpus velutinus
    Seem.                         forest       tree
  Luehea seemannii Tr. &
    Planch.                       forest       tree
  Luehea speciosa Willd.          forest       tree
  Macrocnemum glabrescens
    (Benth.) Wedd.                forest       tree
  Myroxylon balsamum (L.)
    Harms                         forest       tree
  Ochroma pyramidale (Cav.
    ex Lam.)                      forest       tree
  Picea glauca (Moench) Voss      forest       tree
  Picea engelmannii Parry
    ex Engelm.                    forest       tree
  Pinus contorta Loud.            forest       tree
  Pinus resinosa Ait.             forest       tree
  Pinus strobus L.                forest       tree
  Platanus occidentalis L.        forest       tree
  Platymiscium pinnatum
    (Jacq.) Dug.                  forest       tree
  Platypodium elegans J.
    Vogel                         forest       tree
  P. elegans J. Vogel             forest       tree
  Pseudobombax septenatum
    (Jacq.) Dug.                  forest       tree
  Pseudotsuga menziesii
    (Mirb.) Franco                forest       tree
  Pterocarpus rohrii Vahl         forest       tree
  Tabebuia guayacan (Seem.)
    Hemsl.                        forest       tree
  Tabebuia rosea (Bertol.)
    DC.                           forest       tree
  Tachigalia versicolor
    Standl. & L. O. Wms.          forest       tree
  Terminalia amazonica (J.
    E. Gmel.) Excell in
    Pulle                         forest       tree
  Terminalia oblonga              forest       tree
  Tilia americana L.              forest       tree
  Trichospermum mexicanum
    (DC.) Baill.                  forest       tree
  Triplaris cumingiana
    Fisch. & C. Meyer             forest       tree
  Tsuga canadensis (L.)
    Carr.                         forest       tree
  Vatairea erythrocarpa
    Ducke                         forest       tree
  Parthenocissus quinque-
    folia (L.) Planch.            forest       vine
  Toxicodendron radicans
    Ktze.                         forest       vine
  Vitis vulpina L.                forest       vine
  Clematis virginiana L.          forest       vine

Plants of open habitats
  Cardamine resedifolia L.        montane      herb
  Achillea moschata Wulfen        montane      herb
  Achillea nana L.                montane      herb
  Agrostis rupestris All.         montane      herb
  Arabis alpina L.                montane      herb
  Cerastium arvense L.            montane      herb
  Cerastium pedunculatum
    Gaudin                        montane      herb
  Poa alpina L.                   montane      herb
  Poa nemoralis L.                montane      herb
  Sagina linnaei Pressl.          montane      herb
  Saxifraga sp.                   montane      herb
  Sempervivum sp.                 montane      herb
  Silene rupestris L.             montane      herb
  Trifolium dubium Sibth          montane      herb
  Trifolium pallescens
    Schreber                      montane      herb
  Adenostyles leucophylla
    (Willd.) Reichenb.            montane      herb
  Carex frigida All.              montane      herb
  Cirsium spinosissimum
    (L.) Scop.                    montane      herb
  Epilobium fieischeri
    Hochst.                       montane      herb
  Erigeron angulosus Gaudin       montane      herb
  Geum reptans L.                 montane      herb
  Hieracium murorum L.            montane      herb
  Hieracium staticifolium
    All.                          montane      herb
  Linaria alpina (L.) Miller      montane      herb
  Oxyria digyna (L.) Hill         montane      herb
  Ranunculis adoneus Gray         montane      herb
  Rumex scutatus L.               montane      herb
  Solidago aplestris Waldst.
    & Kit. ex Willd.              montane      herb
  Taraxacum officinale Weber.     montane      herb
  Tussilago farfara L.            montane      herb
  Rhododendron ferrugineum L.     montane      shrub
  Alnus viridis (Chaix) DC.       montane      shrub
  Myricaria germanica (L.)
    Desv.                         montane      shrub
  Salix spp.                      montane      shrub
  Larix decidua Mill.             montane      tree
  Datura discolor Bernh.          desert       herb
  Sclerolaena diacantha
    (Nees) Benth.                 desert       herb
  Sporobolus airoides Torr.       desert       herb
  Artemesia herba-alba Asso       desert       herb
  Cryptantha flava (A.
    Nels.) Payson                 desert       herb
  Happlopappus squarrosus
    Hook. & Arn.                  desert       shrub
  Bursera graveolens              desert       shrub
  Lithospermum caroliniense
    (Walt.) MacMill.              dune         herb
  Vulpia fasciculata
    (Forskal) Samp.               dune         herb
  Geranium carolinianum L.        field        herb
  Geranium molle L.               field        herb
  Phlox drummondii Hook.          field        herb
  Achyranthes aspera L.           field        herb
  Bidens sp.                      field        herb
  Petiveria alliaceae L.          field        herb
  Abutilon theophrasti
    Medic.                        field        herb
  Agropyron repens (L.)
    Beauv.                        field        herb
  Atriplex patula var.
    hastata L.                    field        herb
  Bromus inermis Leyss.           field        herb
  Capsella bursa-pastoris
    (L.) Medic.                   field        herb
  Carex extensa Good.             field        herb
  Carex sp.                       field        herb
  Dipsacus sylvestris Huds.       field        herb
  Hypericum gentianoides
    (L.) BSP.                     field        herb
  Panicum miliaceum L.            field        herb
  P. miliaceum L.                 field        herb
  Plantago aristata Michx.        field        herb
  Plantago major L.               field        herb
  Poa annua L.                    field        herb
  Poa pratensis L.                field        herb
  Ranunculus scleratus L.         field        herb
  Salicornia herbacea L.          field        herb
  Stipa comatat Trin & Rupr.      field        herb
  Suaeda maritima (L.)
    Dumort.                       field        herb
  Vulpia ciliata (Le Gall)
    Stace & Auquier               field        herb
  Agrostis stolonifera L.         field        herb
  Andropogon glomeratus
    (Walt.) B. S. P.              field        herb
  Andropogon gyrans Ashe          field        herb
  Andropogon longiberbis
    Hackel                        field        herb
  Andropogon virginicus L.        field        herb
  Apocynum cannabinum L.          field        herb
  Apocynum sibiricum Jacq.        field        herb
  Artemesia frigida Willd.        field        herb
  Asclepias syriaca L.            field        herb
  A. syriaca L.                   field        herb
  A. syriaca L.                   field        herb
  Apera spica-venti               field        herb
  Aster tripolium L.              field        herb
  Carduus tenuiflorus Curt.       field        herb
  Carlina vulgaris L.             field        herb
  Centaurea scabiosa L.           field        herb
  Cirsium arvense (L.) Scop.      field        herb
  Cirsium palustre (L.)
    Scop.                         field        herb
  Cirsium undulatum (Nutt.)
    Spreng.                       field        herb
  Cirsium vulgare (Savi)
    Ten. [=Cirsium
    lanceolatum Scop.]            field        herb
  C. vulgare (Savi) Ten.          field        herb
  C. vulgare (Savi) Ten.          field        herb
  Crepis biennis L.               field        herb
  Crepis virens L.                field        herb
  Epilobium angustifolium L.      field        herb
  E. angustifolium L.             field        herb
  Epilobium hirsutum L.           field        herb
  Epilobium palustre L.           field        herb
  Erigeron acer L.                field        herb
  Erigeron canadensis L.          field        herb
  Eupatorium cannabinum L.        field        herb
  E. cannabinum L.                field        herb
  Gentianella germanica
    (Willd.) Borner               field        herb
  Heterotheca latifolia
    Buckl.                        field        herb
  Hieracium umbellatum L.         field        herb
  Holcus lanatus L.               field        herb
  Hypochoeris radicata L.         field        herb
  H. radicata L.                  field        herb
  Juncus bufonius L.              field        herb
  Leontodon autumnalis L.         field        herb
  L. autumnalis L.                field        herb
  Liatris aspera Michx.           field        herb
  L. aspera Michx.                field        herb
  Liatris cylindrica Michx.       field        herb
  Mirabilis hirsuta (Pursh)
    MacM.                         field        herb
  Oenothera biennis L.            field        herb
  Phragmites sp.                  field        herb
  Physalis subglabrata
    Mackenz. & Bush               field        herb
  Rorippa islandica
    (Oeder) Borbas                field        herb
  Rumex obtusifolius L.           field        herb
  Scabiosa columbaria L.          field        herb
  Schizachyrium scoparium
    (Michx.) Nash                 field        herb
  Senecio jacobaea L.             field        herb
  Senecio congestus var.
    palustris (L.) Fern.          field        herb
  Senecio squalidus L.            field        herb
  Senecio viscosus L.             field        herb
  Senecio vulgaris L.             field        herb
  S. vulgaris L.                  field        herb
  Solidago altissima L.           field        herb
  Solidago missouriensis
    Nutt.                         field        herb
  Solidago rigida L.              field        herb
  Sonchus arvensis L.             field        herb
  S. arvensis L.                  field        herb
  Sonchus oleraceus L.            field        herb
  Spergularia media (L.) C.
    Presl. [=S. marginata
    (DC.) Kittel)]                field        herb
  Spergularia marina (L.)
    Griseb. [=S. salina J.
    & C. Presl.]                  field        herb
  Taraxacum officinale
    Weber.                        field        herb
  T. officinale Weber.            field        herb
  T. officinale Weber.            field        herb
  Tragopogon porrifolius L.       field        herb
  T. porrifolius L.               field        herb
  Tragopogon pratensis L.         field        herb
  Trifolium arvense L.            field        herb
  Tussilago farfara L.            field        herb
  Verbascum thapsus L.            field        herb
  Verbena stricta Vent.           field        herb
  Salix alba L.                   field        shrub
  Salsola iberica                 field        shrub
  Ulmus cprocera Salisb.
    [= U. campestris]             field        tree
  Andira inermis (W.
    Wright) H. B. K.              field        tree
  Prunus serotina Ehrh.           field        tree
  P. serotina Ehrh.               field        tree
  Acer negundo L.                 field        tree
  Betula lenta L.                 field        tree
  Betula populifolia Marsh.       field        tree
  Populus sp.                     field        tree
  Typha latifolia L.              marsh        herb
  T. latifolia L.                 marsh        herb
  Phlox pilosa L.                 prairie      herb
  Agrostis hiemalis
    (Walt.) B. S. P.              prairie      herb
  Andropogon gerardi Vitman       prairie      herb
  Andropogon scoparius
    Michx.                        prairie      herb
  Festuca paradoxa Desv.          prairie      herb
  Setaria geniculata
    (Lam.) Beauv.                 prairie      herb
  Silphium laciniatum L.          prairie      herb
  Sorghastrum nutans (L.)
    Nash                          prairie      herb
  Sphenopholis obtusata
    (Michx.) Scribn.              prairie      herb
  Aristida congesta Roem
    & Scholt.                     savanna      herb
  Cenchrus ciliaris L.            savanna      herb
  Schmidtia pappaphoroides
    Stend.                        savanna      herb
  Setaria verticillata
    (L.) Beauv.                   savanna      herb
  Tragus berteronianus
    Schult.                       savanna      herb
  Eragrostis rigidior Pilger      savanna      herb
  Panicum maximum Jacq.           savanna      herb
  Urochloa mosambicensis
    Schult.                       savanna      herb
  Urochloa panicoides
    (Hack) Dandy                  savanna      herb
  Chloris virgata Sm.             savanna      herb
  Enneapogon cenchroides
    (Licht) C. E. Hubbard         savanna      herb
  Zostera marina L.               marine       herb
  Z. marina L.                    marine       herb
  Z. marina L.                    marine       herb

                                  Dispersal         Treatment
Species                           mechanism         ([dagger])

Plants of forest understory
    and canopy
  Asarum canadense L.             ant
  A. canadense L.                 ant
  Calathea ovandensis Matuda      ant               with aril
  Carex pilulifera L.             ant
  Sanguinaria canadensis L.       ant
  Trillium ovatum Pursh.          ant
  Viola blanda Willd.             ant
  Viola cucullata Ait.            ant
  Viola eriocarpa Schwein         ant
  Viola odorata L.                ant
  Viola papilionacea Pursh.       ant
  Viola pedata L.                 ant
  Viola rostrata Pursh.           ant
  Viola spp.                      ant
  Cnidoscolus stimulosus
    (Michx.) Engelm. & Gray       ant + ballistic
  Crotalaria rotundifolia
    (Walt.) Poir.                 ant + ballistic
  Stillingia sylvatica
    (Muell. Arg.) Small           ant + ballistic
  Carex pauciflora Lightf.        ballistic
  Geranium maculata L.            ballistic
  Impatiens capensis Meerb.       ballistic
  I. capensis Meerb.              ballistic
  Viola striata Air.              ballistic
  V. striata Ait.                 ballistic
  Phytolacca americana L.         bird
  Mimulus guttatus Fisch.
    ex D.C.                       water             water
  Aster acuminatus Michx.         wind              field data,
                                                      open
  A. acuminatus Michx.            win               10 km/h
  Aster prenantoides Muhl.        wind              10 km/h
  Eupatorium rugosum Houtt.       wind              10 km/h
  Piper amalgo                    bat
  Alnus crispa (Ait.) Pursh       wind              10 km/h
  Halesia monticola Sarg.         wind              l0 km/h
  Purshia tridentata              rodent            1 [degrees] +
    (Pursh) DC.                                       2 [degrees]
                                                      dispersal
  P. tridentata (Pursh) CD.       rodent
  Acacia suaveolens (Sm.)
    Willd.                        ant
  Dipteryx panamensis
    (Pitt.) Rec. & Mell           bat
  Casearia corymbosa H. B. K.     bird
  Cornus controversa Hemsl.       bird              birds
  Fagus grandifolia Ehrh.         bird
  Ficus stupenda Miq.             bird              from trunk
  Ficus subtecta Corner           bird              from trunk
  Pinus albicaulis Engelm.        bird
  Pinus edulis Engelm.            bird
  Pinus sp. & Thuja sp.           bird
  Quercus palustris Muenchh.      bird
  Virola surinamensis
    (Rol.) Warb.                  bird
  Fagus silvatica L.              rodent
  Juglans nigra L.                rodent
  Pinus jeffreyi Murr.            rodent            chipmunk
  Quercus macrocarpa Michx.       rodent
  Quercus muehlenbergii
    Engelm.                       rodent
  Acer palmatum Thunb.            wind              10 km/h
  Acer pseudoplatanus L.          wind              10 km/h
  Acer rubrum L.                  wind
  A. rubrum L.                    wind              10 km/h
  A. rubrum L.                    wind              field
  Acer saccharum Marsh.           wind
  Acer cappadocicum               wind              10 km/h
  Acer griseum Pax                wind              10 km/h
  Acer platanoides L.             wind              10 km/h
  Ailanthus altissima
    (Mill.) Swingle               wind              10 km/h
  Albizzia julibrissum
    Durazzini                     wind              10 km/h
  Alseis blackiana Hemsl.         wind              12.6 km/h
  Aspidosperma cruenata
    Woods                         wind              12.6 km/h
  Astronium graveolans Jacq.      wind              12.6 km/h
  Betula papyrifera Marsh.        wind              field
  Bombacopsis quinata
    (Jacq.) Dug.                  wind              12.6 km/h
  Bombacopsis sessilis
    (Benth.) Pitt.                wind              12.6 km/h
  Carpinus caroliniana Walt.      wind              10 km/h
  Catalpa bignonioides Walt.      wind              10 km/h
  Cavanillesia platanifolia
    (H. & B.) H. B. K.            wind              12.6 km/h
  Cedrela odorata L.              wind              12.6 km/h
  Ceiba pentandra (L.)
    Gaertn.                       wind              12.6 km/h
  Cespedizia macrophylla
    Seem.                         wind              12.6 km/h
  Chamaecyperis thyoides
    (L.) BSP.                     wind              10 km/h
  Cochlospermum vitifolium
    (Willd.) Spreng.              wind              12.6 km/h
  Cordia alliodora (R. &
    P.) Cham.                     wind              12.6 km/h
  Couratari panamensis
    Standl.                       wind              12.6 km/h
  Dalbergia retusa Hemsl.         wind              12.6 km/h
  Eucalyptus regnans F.
    Muell.                        wind
  Fraxinus americana L.           wind
  F. americana L.                 wind              10 km/h
  Fraxinus excelsior L.           wind              10 km/h
  Jacaranda copaia (Aubl.)
    D. Don                        wind              12.6 km/h
  Juniperus virginiana L.         wind              field
  Lafoensia punicifolia DC.       wind              12.6 km/h
  Larix laricina (DuRoi)
    K. Koch                       wind
  Liriodendron tulipifera L.      wind              10 km/h
  L. tulipifera L.                wind
  Lonchocarphus pentaphyllus                        one-seeded
    (Poir.) DC.                   wind                fruits
  Lonchocarpus velutinus
    Seem.                         wind              12.6 km/h
  Luehea seemannii Tr. &
    Planch.                       wind              12.6 km/h
  Luehea speciosa Willd.          wind              12.6 km/h
  Macrocnemum glabrescens
    (Benth.) Wedd.                wind              12.6 km/h
  Myroxylon balsamum (L.)
    Harms                         wind              12.6 km/h
  Ochroma pyramidale (Cav.
    ex Lam.)                      wind              12.6 km/h
  Picea glauca (Moench) Voss      wind              field
  Picea engelmannii Parry
    ex Engelm.                    wind              field
  Pinus contorta Loud.            wind
  Pinus resinosa Ait.             wind              field
  Pinus strobus L.                wind              field
  Platanus occidentalis L.        wind              10 km/h
  Platymiscium pinnatum
    (Jacq.) Dug.                  wind              12.6 km/h
  Platypodium elegans J.
    Vogel                         wind
  P. elegans J. Vogel             wind
  Pseudobombax septenatum
    (Jacq.) Dug.                  wind              12.6 km/h
  Pseudotsuga menziesii
    (Mirb.) Franco                wind              field
  Pterocarpus rohrii Vahl         wind              12.6 km/h
  Tabebuia guayacan (Seem.)
    Hemsl.                        wind              12.6 km/h
  Tabebuia rosea (Bertol.)
    DC.                           wind              12.6 km/h
  Tachigalia versicolor
    Standl. & L. O. Wms.          wind              12.6 km/h
  Terminalia amazonica (J.
    E. Gmel.) Excell in
    Pulle                         wind              12.6 km/h
  Terminalia oblonga              wind              12.6 km/h
  Tilia americana L.              wind              10 km/h
  Trichospermum mexicanum
    (DC.) Baill.                  wind              12.6 km/h
  Triplaris cumingiana
    Fisch. & C. Meyer             wind              12.6 km/h
  Tsuga canadensis (L.)
    Carr.                         wind              field
  Vatairea erythrocarpa
    Ducke                         wind              12.6 km/h
  Parthenocissus quinque-
    folia (L.) Planch.            bird
  Toxicodendron radicans
    Ktze.                         bird
  Vitis vulpina L.                bird
  Clematis virginiana L.          bird              10 km/h

Plants of open habitats
  Cardamine resedifolia L.        ballistic
  Achillea moschata Wulfen        none
  Achillea nana L.                none
  Agrostis rupestris All.         none
  Arabis alpina L.                none
  Cerastium arvense L.            none
  Cerastium pedunculatum
    Gaudin                        none
  Poa alpina L.                   none
  Poa nemoralis L.                none
  Sagina linnaei Pressl.          none
  Saxifraga sp.                   none
  Sempervivum sp.                 none
  Silene rupestris L.             none
  Trifolium dubium Sibth          none
  Trifolium pallescens
    Schreber                      wind
  Adenostyles leucophylla
    (Willd.) Reichenb.            wind
  Carex frigida All.              wind
  Cirsium spinosissimum
    (L.) Scop.                    wind
  Epilobium fieischeri
    Hochst.                       wind
  Erigeron angulosus Gaudin       wind
  Geum reptans L.                 wind
  Hieracium murorum L.            wind
  Hieracium staticifolium
    All.                          wind
  Linaria alpina (L.) Miller      wind
  Oxyria digyna (L.) Hill         wind
  Ranunculis adoneus Gray         wind
  Rumex scutatus L.               wind
  Solidago aplestris Waldst.
    & Kit. ex Willd.              wind
  Taraxacum officinale Weber.     wind
  Tussilago farfara L.            wind
  Rhododendron ferrugineum L.     none
  Alnus viridis (Chaix) DC.       wind
  Myricaria germanica (L.)
    Desv.                         wind
  Salix spp.                      wind
  Larix decidua Mill.             wind
  Datura discolor Bernh.          ant
  Sclerolaena diacantha
    (Nees) Benth.                 ant
  Sporobolus airoides Torr.       none
  Artemesia herba-alba Asso       none              0.25 m height
  Cryptantha flava (A.
    Nels.) Payson                 wind              field data
  Happlopappus squarrosus
    Hook. & Arn.                  ant
  Bursera graveolens              bird
  Lithospermum caroliniense
    (Walt.) MacMill.              none
  Vulpia fasciculata
    (Forskal) Samp.               none
  Geranium carolinianum L.        ballistic
  Geranium molle L.               ballistic
  Phlox drummondii Hook.          ballistic
  Achyranthes aspera L.           adhesion
  Bidens sp.                      adhesion
  Petiveria alliaceae L.          adhesion
  Abutilon theophrasti
    Medic.                        none
  Agropyron repens (L.)
    Beauv.                        none              field
  Atriplex patula var.
    hastata L.                    none              20 km/h
  Bromus inermis Leyss.           none              field
  Capsella bursa-pastoris
    (L.) Medic.                   none              20 km/h
  Carex extensa Good.             none              20 km/h
  Carex sp.                       none              field
  Dipsacus sylvestris Huds.       none
  Hypericum gentianoides
    (L.) BSP.                     none
  Panicum miliaceum L.            none
  P. miliaceum L.                 none
  Plantago aristata Michx.        none
  Plantago major L.               none              20 km/h
  Poa annua L.                    none              20 km/h
  Poa pratensis L.                none              20 km/h
  Ranunculus scleratus L.         none              20 km/h
  Salicornia herbacea L.          none              20 km/h
  Stipa comatat Trin & Rupr.      none              field
  Suaeda maritima (L.)
    Dumort.                       none              20 km/h
  Vulpia ciliata (Le Gall)                          disturbed
    Stace & Auquier               none                by walking
  Agrostis stolonifera L.         wind              20 km/h
  Andropogon glomeratus                             10 km/h, 1 m
    (Walt.) B. S. P.              wind                height
  Andropogon gyrans Ashe          wind              10 km/h, 1 m
                                                      height
  Andropogon longiberbis
    Hackel                        wind
  Andropogon virginicus L.        wind              10 km/h, 1 m
                                                      height
  Apocynum cannabinum L.          wind              10 km/h
  Apocynum sibiricum Jacq.        wind              field data
  Artemesia frigida Willd.        wind              field
  Asclepias syriaca L.            wind              10 km/h
  A. syriaca L.                   wind              field data
  A. syriaca L.                   wind              field data, 1 m
  Apera spica-venti               wind              20 km/h
  Aster tripolium L.              wind              20 km/h
  Carduus tenuiflorus Curt.       wind              16.41 km/h
  Carlina vulgaris L.             wind              16.41 km/h
  Centaurea scabiosa L.           wind              16.41 km/h
  Cirsium arvense (L.) Scop.      wind              16.41 km/h
  Cirsium palustre (L.)
    Scop.                         wind              16.41 km/h
  Cirsium undulatum (Nutt.)
    Spreng.                       wind              field data
  Cirsium vulgare (Savi)
    Ten. [=Cirsium
    lanceolatum Scop.]            wind              20 km/h
  C. vulgare (Savi) Ten.          wind              10 km/h
  C. vulgare (Savi) Ten.          wind              field data
  Crepis biennis L.               wind              20 km/h
  Crepis virens L.                wind              20 km/h
  Epilobium angustifolium L.      wind              20 km/h
  E. angustifolium L.             wind              10 km/h
  Epilobium hirsutum L.           wind              20 km/h
  Epilobium palustre L.           wind              20 km/h
  Erigeron acer L.                wind              16.41 km/h
  Erigeron canadensis L.          wind              20 km/h
  Eupatorium cannabinum L.        wind              10 km/h
  E. cannabinum L.                wind              16.41 km/h
  Gentianella germanica
    (Willd.) Borner               wind              field
  Heterotheca latifolia
    Buckl.                        wind              disk seeds
  Hieracium umbellatum L.         wind              10 km/h
  Holcus lanatus L.               wind              20 km/h
  Hypochoeris radicata L.         wind              20 km/h
  H. radicata L.                  wind              16.41 km/h
  Juncus bufonius L.              wind              20 km/h
  Leontodon autumnalis L.         wind              20 km/h
  L. autumnalis L.                wind              16.41 km/h
  Liatris aspera Michx.           wind
  L. aspera Michx.                wind
  Liatris cylindrica Michx.       wind              10 km/h, 1 m
                                                      height
  Mirabilis hirsuta (Pursh)
    MacM.                         wind              field data
  Oenothera biennis L.            wind              field data
  Phragmites sp.                  wind              20 km/h
  Physalis subglabrata
    Mackenz. & Bush               wind              10 km/h
  Rorippa islandica
    (Oeder) Borbas                wind              20 km/h
  Rumex obtusifolius L.           wind              10 km/h
  Scabiosa columbaria L.          wind              field
  Schizachyrium scoparium                           10 km/h,
    (Michx.) Nash                 wind                1.5 m height
  Senecio jacobaea L.             wind
  Senecio congestus var.
    palustris (L.) Fern.          wind              20 km/h
  Senecio squalidus L.            wind              16.41 km/h
  Senecio viscosus L.             wind              16.41 km/h
  Senecio vulgaris L.             wind              20 km/h
  S. vulgaris L.                  wind              16.41 km/h
  Solidago altissima L.           wind              10 km/h
  Solidago missouriensis
    Nutt.                         wind              field
  Solidago rigida L.              wind              field data
  Sonchus arvensis L.             wind              20 km/h
  S. arvensis L.                  wind              16.41 km/h
  Sonchus oleraceus L.            wind              16.41 km/h
  Spergularia media (L.) C.
    Presl. [=S. marginata
    (DC.) Kittel)]                wind              20 km/h
  Spergularia marina (L.)
    Griseb. [=S. salina J.
    & C. Presl.]                  wind              20 km/h
  Taraxacum officinale
    Weber.                        wind              20 km/h
  T. officinale Weber.            wind              16.41 km/h
  T. officinale Weber.            wind              10 km/h
  Tragopogon porrifolius L.       wind              10 km/h
  T. porrifolius L.               wind              16.41 km/h
  Tragopogon pratensis L.         wind              20 km/h
  Trifolium arvense L.            wind              10 km/h
  Tussilago farfara L.            wind
  Verbascum thapsus L.            wind
  Verbena stricta Vent.           wind              field data
  Salix alba L.                   wind              20 km/h
  Salsola iberica                 wind              tumbleweed,
                                                      field data
  Ulmus cprocera Salisb.
    [= U. campestris]             wind              20 km/h
  Andira inermis (W.
    Wright) H. B. K.              bat
  Prunus serotina Ehrh.           bird
  P. serotina Ehrh.               bird
  Acer negundo L.                 wind              10 km/h
  Betula lenta L.                 wind
  Betula populifolia Marsh.       wind              10 km/h
  Populus sp.                     wind              20 km/h
  Typha latifolia L.              wind              20 km/h
  T. latifolia L.                 wind              10 km/h
  Phlox pilosa L.                 ballistic
  Agrostis hiemalis
    (Walt.) B. S. P.              wind              tumbling
  Andropogon gerardi Vitman       wind
  Andropogon scoparius
    Michx.                        wind
  Festuca paradoxa Desv.          wind
  Setaria geniculata
    (Lam.) Beauv.                 wind
  Silphium laciniatum L.          wind              field data
  Sorghastrum nutans (L.)
    Nash                          wind
  Sphenopholis obtusata
    (Michx.) Scribn.              wind              with glumes
  Aristida congesta Roem
    & Scholt.                     adhesion          10 m/s
  Cenchrus ciliaris L.            adhesion          10 m/s
  Schmidtia pappaphoroides
    Stend.                        adhesion          10 m/s
  Setaria verticillata
    (L.) Beauv.                   adhesion          10 m/s
  Tragus berteronianus
    Schult.                       adhesion          10 m/s
  Eragrostis rigidior Pilger      none              10 m/s
  Panicum maximum Jacq.           none              10 m/s
  Urochloa mosambicensis
    Schult.                       none              10 m/s
  Urochloa panicoides
    (Hack) Dandy                  none              10 m/s
  Chloris virgata Sm.             wind              10 m/s
  Enneapogon cenchroides
    (Licht) C. E. Hubbard         wind              10 m/s
  Zostera marina L.               water
  Z. marina L.                    water
  Z. marina L.                    water

                                      Dispersal distance (m)

Species                                     Maximum

Plants of forest understory
    and canopy
  Asarum canadense L.                 35
  A. canadense L.                      0.94([double dagger])
  Calathea ovandensis Matuda           3.25
  Carex pilulifera L.                  1.4
  Sanguinaria canadensis L.           17
  Trillium ovatum Pursh.               1.8
  Viola blanda Willd.                  3.8
  Viola cucullata Ait.                 2.1
  Viola eriocarpa Schwein              5.4
  Viola odorata L.                     0.02
  Viola papilionacea Pursh.            4.8
  Viola pedata L.                      5.1
  Viola rostrata Pursh.                4.2
  Viola spp.                           1.5
  Cnidoscolus stimulosus
    (Michx.) Engelm. & Gray
  Crotalaria rotundifolia
    (Walt.) Poir.
  Stillingia sylvatica
    (Muell. Arg.) Small
  Carex pauciflora Lightf.
  Geranium maculata L.                 4.55
  Impatiens capensis Meerb.            1.7([double dagger])
  I. capensis Meerb.                   3.5
  Viola striata Air.                   3.3
  V. striata Ait.                      3([double dagger])
  Phytolacca americana L.             33
  Mimulus guttatus Fisch.
    ex D.C.                          400
  Aster acuminatus Michx.              5.25
  A. acuminatus Michx.                 5.08([double dagger])
  Aster prenantoides Muhl.             3.11
  Eupatorium rugosum Houtt.            7.1
  Piper amalgo                       700
  Alnus crispa (Ait.) Pursh            6.65
  Halesia monticola Sarg.             18.5
  Purshia tridentata
    (Pursh) DC.                       20.96
  P. tridentata (Pursh) CD.           16([double dagger])
  Acacia suaveolens (Sm.)
    Willd.                            10.75
  Dipteryx panamensis
    (Pitt.) Rec. & Mell              100
  Casearia corymbosa H. B. K.        200
  Cornus controversa Hemsl.           42.5
  Fagus grandifolia Ehrh.           4000
  Ficus stupenda Miq.                 75
  Ficus subtecta Corner               75
  Pinus albicaulis Engelm.          3500
  Pinus edulis Engelm.            22 000
  Pinus sp. & Thuja sp.           15 000
  Quercus palustris Muenchh.        1900
  Virola surinamensis
    (Rol.) Warb.                      49
  Fagus silvatica L.                  13
  Juglans nigra L.                   151
  Pinus jeffreyi Murr.                68.94
  Quercus macrocarpa Michx.           51.5
  Quercus muehlenbergii
    Engelm.                           45.3
  Acer palmatum Thunb.                11.6
  Acer pseudoplatanus L.              60.1
  Acer rubrum L.                     160([double dagger])
  A. rubrum L.                        98.7([double dagger])
  A. rubrum L.                       475
  Acer saccharum Marsh.              100
  Acer cappadocicum                   48.2
  Acer griseum Pax                    29.2
  Acer platanoides L.                 50.3
  Ailanthus altissima
    (Mill.) Swingle                  111.6
  Albizzia julibrissum
    Durazzini                         16.2
  Alseis blackiana Hemsl.            377
  Aspidosperma cruenata
    Woods                            258
  Astronium graveolans Jacq.         184
  Betula papyrifera Marsh.           475
  Bombacopsis quinata
    (Jacq.) Dug.                     324
  Bombacopsis sessilis
    (Benth.) Pitt.                    88
  Carpinus caroliniana Walt.          19.4
  Catalpa bignonioides Walt.          16.8
  Cavanillesia platanifolia
    (H. & B.) H. B. K.               224
  Cedrela odorata L.                 410
  Ceiba pentandra (L.)
    Gaertn.                          314
  Cespedizia macrophylla
    Seem.                            775
  Chamaecyperis thyoides
    (L.) BSP.                         68.8
  Cochlospermum vitifolium
    (Willd.) Spreng.                  96
  Cordia alliodora (R. &
    P.) Cham.                        140
  Couratari panamensis
    Standl.                          229
  Dalbergia retusa Hemsl.            117
  Eucalyptus regnans F.
    Muell.                           150
  Fraxinus americana L.               70.71
  F. americana L.                     70.1([double dagger])
  Fraxinus excelsior L.               40.3
  Jacaranda copaia (Aubl.)
    D. Don                           538
  Juniperus virginiana L.           1200
  Lafoensia punicifolia DC.          192
  Larix laricina (DuRoi)
    K. Koch                           80
  Liriodendron tulipifera L.         111.9
  L. tulipifera L.                    86.6([double dagger])
  Lonchocarphus pentaphyllus
    (Poir.) DC.                       93
  Lonchocarpus velutinus
    Seem.                            239
  Luehea seemannii Tr. &
    Planch.                          350
  Luehea speciosa Willd.             181
  Macrocnemum glabrescens
    (Benth.) Wedd.                   340
  Myroxylon balsamum (L.)
    Harms                            159
  Ochroma pyramidale (Cav.
    ex Lam.)                         228
  Picea glauca (Moench) Voss         475
  Picea engelmannii Parry
    ex Engelm.                       200
  Pinus contorta Loud.                40
  Pinus resinosa Ait.                475
  Pinus strobus L.                   475
  Platanus occidentalis L.            62.8
  Platymiscium pinnatum
    (Jacq.) Dug.                     191
  Platypodium elegans J.
    Vogel                            110([double dagger])
  P. elegans J. Vogel                137
  Pseudobombax septenatum
    (Jacq.) Dug.                     139
  Pseudotsuga menziesii
    (Mirb.) Franco                   800
  Pterocarpus rohrii Vahl            268
  Tabebuia guayacan (Seem.)
    Hemsl.                           223
  Tabebuia rosea (Bertol.)
    DC.                              246
  Tachigalia versicolor
    Standl. & L. O. Wms.             141
  Terminalia amazonica (J.
    E. Gmel.) Excell in
    Pulle                            335
  Terminalia oblonga                 276
  Tilia americana L.                  15
  Trichospermum mexicanum
    (DC.) Baill.                     147
  Triplaris cumingiana
    Fisch. & C. Meyer                211
  Tsuga canadensis (L.)
    Carr.                           1600
  Vatairea erythrocarpa
    Ducke                            176
  Parthenocissus quinque-
    folia (L.) Planch.                24
  Toxicodendron radicans
    Ktze.                             33
  Vitis vulpina L.                    24
  Clematis virginiana L.               3.23

Plants of open habitats
  Cardamine resedifolia L.            <1
  Achillea moschata Wulfen            <1
  Achillea nana L.                     4
  Agrostis rupestris All.             <1
  Arabis alpina L.                    <1
  Cerastium arvense L.                <1
  Cerastium pedunculatum
    Gaudin                            <1
  Poa alpina L.                       <1
  Poa nemoralis L.                    50
  Sagina linnaei Pressl.              10
  Saxifraga sp.                       40
  Sempervivum sp.                     <1
  Silene rupestris L.                 10
  Trifolium dubium Sibth              10
  Trifolium pallescens
    Schreber                           6
  Adenostyles leucophylla
    (Willd.) Reichenb.                85
  Carex frigida All.                  <1
  Cirsium spinosissimum
    (L.) Scop.                        30
  Epilobium fieischeri
    Hochst.                           50
  Erigeron angulosus Gaudin           85
  Geum reptans L.                      4
  Hieracium murorum L.                10
  Hieracium staticifolium
    All.                              75
  Linaria alpina (L.) Miller          12
  Oxyria digyna (L.) Hill              1
  Ranunculis adoneus Gray
  Rumex scutatus L.                   12
  Solidago aplestris Waldst.
    & Kit. ex Willd.                   4
  Taraxacum officinale Weber.         50([double dagger])
  Tussilago farfara L.                20([double dagger])
  Rhododendron ferrugineum L.         25
  Alnus viridis (Chaix) DC.           70
  Myricaria germanica (L.)
    Desv.                            100
  Salix spp.                         100
  Larix decidua Mill.                 15
  Datura discolor Bernh.              39
  Sclerolaena diacantha
    (Nees) Benth.                     77
  Sporobolus airoides Torr.           15.2
  Artemesia herba-alba Asso            0.5
  Cryptantha flava (A.
    Nels.) Payson                     31.3
  Happlopappus squarrosus
    Hook. & Arn.                       3
  Bursera graveolens                  35
  Lithospermum caroliniense
    (Walt.) MacMill.                 199
  Vulpia fasciculata
    (Forskal) Samp.                    0.21
  Geranium carolinianum L.             4.25
  Geranium molle L.                    2.8
  Phlox drummondii Hook.               1.5
  Achyranthes aspera L.             4423.4
  Bidens sp.
  Petiveria alliaceae L.             156.6
  Abutilon theophrasti
    Medic.                             1.04
  Agropyron repens (L.)
    Beauv.
  Atriplex patula var.
    hastata L.                        55
  Bromus inermis Leyss.                7
  Capsella bursa-pastoris
    (L.) Medic.                       35
  Carex extensa Good.                 31
  Carex sp.                            3
  Dipsacus sylvestris Huds.            1.5
  Hypericum gentianoides
    (L.) BSP.                          0.5
  Panicum miliaceum L.                 1.5([double dagger])
  P. miliaceum L.                      3
  Plantago aristata Michx.            18
  Plantago major L.                   38
  Poa annua L.                       180
  Poa pratensis L.                    25
  Ranunculus scleratus L.             35
  Salicornia herbacea L.              36
  Stipa comatat Trin & Rupr.         100
  Suaeda maritima (L.)
    Dumort.                          380
  Vulpia ciliata (Le Gall)
    Stace & Auquier                    1.1
  Agrostis stolonifera L.             61
  Andropogon glomeratus
    (Walt.) B. S. P.                  13.8
  Andropogon gyrans Ashe              10.3
  Andropogon longiberbis
    Hackel                             7.35
  Andropogon virginicus L.             8.6
  Apocynum cannabinum L.              83.3
  Apocynum sibiricum Jacq.
  Artemesia frigida Willd.            25
  Asclepias syriaca L.                18.1
  A. syriaca L.
  A. syriaca L.
  Apera spica-venti                  140
  Aster tripolium L.                1500
  Carduus tenuiflorus Curt.            2.12
  Carlina vulgaris L.                  1.47
  Centaurea scabiosa L.                1.56
  Cirsium arvense (L.) Scop.          11.35
  Cirsium palustre (L.)
    Scop.                              6.81
  Cirsium undulatum (Nutt.)
    Spreng.
  Cirsium vulgare (Savi)
    Ten. [=Cirsium
    lanceolatum Scop.]               900
  C. vulgare (Savi) Ten.              11.6([double dagger])
  C. vulgare (Savi) Ten.              32
  Crepis biennis L.                  350
  Crepis virens L.                   650
  Epilobium angustifolium L.      10 000
  E. angustifolium L.                 35.7([double dagger])
  Epilobium hirsutum L.             3800
  Epilobium palustre L.             7300
  Erigeron acer L.                     3.61
  Erigeron canadensis L.            2500
  Eupatorium cannabinum L.             7.81
  E. cannabinum L.                     5.89([double dagger])
  Gentianella germanica
    (Willd.) Borner                    1.1
  Heterotheca latifolia
    Buckl.                             5
  Hieracium umbellatum L.              3.03
  Holcus lanatus L.                  340
  Hypochoeris radicata L.           1900
  H. radicata L.                       2([double dagger])
  Juncus bufonius L.                 100
  Leontodon autumnalis L.            610
  L. autumnalis L.                     1.64([double dagger])
  Liatris aspera Michx.                9.5
  L. aspera Michx.                     9([double dagger])
  Liatris cylindrica Michx.            9
  Mirabilis hirsuta (Pursh)
    MacM.                              2
  Oenothera biennis L.
  Phragmites sp.                    2500
  Physalis subglabrata
    Mackenz. & Bush                    1.47
  Rorippa islandica
    (Oeder) Borbas                    80
  Rumex obtusifolius L.                2.48
  Scabiosa columbaria L.               1.3
  Schizachyrium scoparium
    (Michx.) Nash                      4.11
  Senecio jacobaea L.                 36
  Senecio congestus var.
    palustris (L.) Fern.            8800
  Senecio squalidus L.                 2.53
  Senecio viscosus L.                  2.57
  Senecio vulgaris L.               1200
  S. vulgaris L.                       2.9([double dagger])
  Solidago altissima L.               14.9
  Solidago missouriensis
    Nutt.                             45
  Solidago rigida L.
  Sonchus arvensis L.               1100
  S. arvensis L.                      10([double dagger])
  Sonchus oleraceus L.                 6.56
  Spergularia media (L.) C.
    Presl. [=S. marginata
    (DC.) Kittel)]                   130
  Spergularia marina (L.)
    Griseb. [=S. salina J.
    & C. Presl.]                     340
  Taraxacum officinale
    Weber.                           620
  T. officinale Weber.                 2.27([double dagger])
  T. officinale Weber.                 1.49([double dagger])
  Tragopogon porrifolius L.            5.77
  T. porrifolius L.                    1.8([double dagger])
  Tragopogon pratensis L.           1900
  Trifolium arvense L.                 0.75
  Tussilago farfara L.              4000
  Verbascum thapsus L.                13
  Verbena stricta Vent.
  Salix alba L.                     6500
  Salsola iberica                   4069
  Ulmus cprocera Salisb.
    [= U. campestris]                330
  Andira inermis (W.
    Wright) H. B. K.                 104.4
  Prunus serotina Ehrh.               35
  P. serotina Ehrh.                    9([double dagger])
  Acer negundo L.                     41.1
  Betula lenta L.                     85
  Betula populifolia Marsh.           64.1
  Populus sp.                       3900
  Typha latifolia L.                5400
  T. latifolia L.                     46.9([double dagger])
  Phlox pilosa L.                      3.6
  Agrostis hiemalis
    (Walt.) B. S. P.                  11
  Andropogon gerardi Vitman            0.19
  Andropogon scoparius
    Michx.                             0.11
  Festuca paradoxa Desv.               0.29
  Setaria geniculata
    (Lam.) Beauv.                      0.39
  Silphium laciniatum L.               2.75
  Sorghastrum nutans (L.)
    Nash                               0.24
  Sphenopholis obtusata
    (Michx.) Scribn.                   0.46
  Aristida congesta Roem
    & Scholt.                          2.31
  Cenchrus ciliaris L.                 7.26
  Schmidtia pappaphoroides
    Stend.                             7.59
  Setaria verticillata
    (L.) Beauv.                        3.28
  Tragus berteronianus
    Schult.                            2.24
  Eragrostis rigidior Pilger          10.19
  Panicum maximum Jacq.                7.95
  Urochloa mosambicensis
    Schult.                            4.46
  Urochloa panicoides
    (Hack) Dandy                       3.16
  Chloris virgata Sm.                  5.56
  Enneapogon cenchroides
    (Licht) C. E. Hubbard             13.11
  Zostera marina L.                  200
  Z. marina L.                         4.5([double dagger])
  Z. marina L.                        50([double dagger])

                                      Dispersal distance (m)

Species                                       Mean

Plants of forest understory
    and canopy
  Asarum canadense L.                  1.54
  A. canadense L.                      0.85([double dagger])
  Calathea ovandensis Matuda           1.14
  Carex pilulifera L.                  0.4
  Sanguinaria canadensis L.            1.38
  Trillium ovatum Pursh.               0.4
  Viola blanda Willd.                  1
  Viola cucullata Ait.                 1.5
  Viola eriocarpa Schwein              1.2
  Viola odorata L.                     0.01
  Viola papilionacea Pursh.            2.1
  Viola pedata L.                      1.4
  Viola rostrata Pursh.                1.2
  Viola spp.                           0.75
  Cnidoscolus stimulosus
    (Michx.) Engelm. & Gray            9
  Crotalaria rotundifolia
    (Walt.) Poir.                      9
  Stillingia sylvatica
    (Muell. Arg.) Small                9
  Carex pauciflora Lightf.             1
  Geranium maculata L.                 3.02
  Impatiens capensis Meerb.            0.24
  I. capensis Meerb.
  Viola striata Air.                   0.79([double dagger])
  V. striata Ait.                      1.5
  Phytolacca americana L.
  Mimulus guttatus Fisch.
    ex D.C.                          275
  Aster acuminatus Michx.              1.18
  A. acuminatus Michx.
  Aster prenantoides Muhl.
  Eupatorium rugosum Houtt.
  Piper amalgo                        37.8
  Alnus crispa (Ait.) Pursh
  Halesia monticola Sarg.
  Purshia tridentata
    (Pursh) DC.                        9.55
  P. tridentata (Pursh) CD.            6.21([double dagger])
  Acacia suaveolens (Sm.)
    Willd.                             2.1
  Dipteryx panamensis
    (Pitt.) Rec. & Mell               75
  Casearia corymbosa H. B. K.
  Cornus controversa Hemsl.           15.32
  Fagus grandifolia Ehrh.
  Ficus stupenda Miq.                  6.67
  Ficus subtecta Corner                5.36
  Pinus albicaulis Engelm.           100
  Pinus edulis Engelm.            14 300
  Pinus sp. & Thuja sp.
  Quercus palustris Muenchh.        1100
  Virola surinamensis
    (Rol.) Warb.
  Fagus silvatica L.                   4.13
  Juglans nigra L.                    11.60
  Pinus jeffreyi Murr.                20.58
  Quercus macrocarpa Michx.           10.3
  Quercus muehlenbergii
    Engelm.                            6.4
  Acer palmatum Thunb.
  Acer pseudoplatanus L.
  Acer rubrum L.
  A. rubrum L.
  A. rubrum L.
  Acer saccharum Marsh.
  Acer cappadocicum
  Acer griseum Pax
  Acer platanoides L.
  Ailanthus altissima
    (Mill.) Swingle
  Albizzia julibrissum
    Durazzini
  Alseis blackiana Hemsl.
  Aspidosperma cruenata
    Woods
  Astronium graveolans Jacq.
  Betula papyrifera Marsh.
  Bombacopsis quinata
    (Jacq.) Dug.
  Bombacopsis sessilis
    (Benth.) Pitt.
  Carpinus caroliniana Walt.
  Catalpa bignonioides Walt.
  Cavanillesia platanifolia
    (H. & B.) H. B. K.
  Cedrela odorata L.
  Ceiba pentandra (L.)
    Gaertn.
  Cespedizia macrophylla
    Seem.
  Chamaecyperis thyoides
    (L.) BSP.
  Cochlospermum vitifolium
    (Willd.) Spreng.
  Cordia alliodora (R. &
    P.) Cham.
  Couratari panamensis
    Standl.
  Dalbergia retusa Hemsl.
  Eucalyptus regnans F.
    Muell.                            30
  Fraxinus americana L.
  F. americana L.
  Fraxinus excelsior L.
  Jacaranda copaia (Aubl.)
    D. Don
  Juniperus virginiana L.
  Lafoensia punicifolia DC.
  Larix laricina (DuRoi)
    K. Koch                            4.54
  Liriodendron tulipifera L.
  L. tulipifera L.
  Lonchocarphus pentaphyllus
    (Poir.) DC.                       16.87
  Lonchocarpus velutinus
    Seem.
  Luehea seemannii Tr. &
    Planch.
  Luehea speciosa Willd.
  Macrocnemum glabrescens
    (Benth.) Wedd.
  Myroxylon balsamum (L.)
    Harms
  Ochroma pyramidale (Cav.
    ex Lam.)
  Picea glauca (Moench) Voss
  Picea engelmannii Parry
    ex Engelm.
  Pinus contorta Loud.                 4.84
  Pinus resinosa Ait.
  Pinus strobus L.
  Platanus occidentalis L.
  Platymiscium pinnatum
    (Jacq.) Dug.
  Platypodium elegans J.
    Vogel                             45
  P. elegans J. Vogel
  Pseudobombax septenatum
    (Jacq.) Dug.
  Pseudotsuga menziesii
    (Mirb.) Franco
  Pterocarpus rohrii Vahl
  Tabebuia guayacan (Seem.)
    Hemsl.
  Tabebuia rosea (Bertol.)
    DC.
  Tachigalia versicolor
    Standl. & L. O. Wms.
  Terminalia amazonica (J.
    E. Gmel.) Excell in
    Pulle
  Terminalia oblonga
  Tilia americana L.
  Trichospermum mexicanum
    (DC.) Baill.
  Triplaris cumingiana
    Fisch. & C. Meyer
  Tsuga canadensis (L.)
    Carr.
  Vatairea erythrocarpa
    Ducke
  Parthenocissus quinque-
    folia (L.) Planch.                 9
  Toxicodendron radicans
    Ktze.
  Vitis vulpina L.
  Clematis virginiana L.

Plants of open habitats
  Cardamine resedifolia L.
  Achillea moschata Wulfen
  Achillea nana L.
  Agrostis rupestris All.
  Arabis alpina L.
  Cerastium arvense L.
  Cerastium pedunculatum
    Gaudin
  Poa alpina L.
  Poa nemoralis L.
  Sagina linnaei Pressl.
  Saxifraga sp.
  Sempervivum sp.
  Silene rupestris L.
  Trifolium dubium Sibth
  Trifolium pallescens
    Schreber
  Adenostyles leucophylla
    (Willd.) Reichenb.
  Carex frigida All.
  Cirsium spinosissimum
    (L.) Scop.
  Epilobium fieischeri
    Hochst.
  Erigeron angulosus Gaudin
  Geum reptans L.
  Hieracium murorum L.
  Hieracium staticifolium
    All.
  Linaria alpina (L.) Miller
  Oxyria digyna (L.) Hill
  Ranunculis adoneus Gray              0.25
  Rumex scutatus L.
  Solidago aplestris Waldst.
    & Kit. ex Willd.
  Taraxacum officinale Weber.
  Tussilago farfara L.
  Rhododendron ferrugineum L.
  Alnus viridis (Chaix) DC.
  Myricaria germanica (L.)
    Desv.
  Salix spp.
  Larix decidua Mill.
  Datura discolor Bernh.
  Sclerolaena diacantha
    (Nees) Benth.
  Sporobolus airoides Torr.            3.33
  Artemesia herba-alba Asso            0.16
  Cryptantha flava (A.
    Nels.) Payson                      2.36
  Happlopappus squarrosus
    Hook. & Arn.
  Bursera graveolens                   7
  Lithospermum caroliniense
    (Walt.) MacMill.                  58.80
  Vulpia fasciculata
    (Forskal) Samp.                    0.07
  Geranium carolinianum L.             3.28
  Geranium molle L.                    1.78
  Phlox drummondii Hook.               0.78
  Achyranthes aspera L.               34.4
  Bidens sp.                         108.8
  Petiveria alliaceae L.              32.9
  Abutilon theophrasti
    Medic.
  Agropyron repens (L.)
    Beauv.                             7
  Atriplex patula var.
    hastata L.
  Bromus inermis Leyss.                1.72
  Capsella bursa-pastoris
    (L.) Medic.
  Carex extensa Good.
  Carex sp.                            1.07
  Dipsacus sylvestris Huds.            0.2
  Hypericum gentianoides
    (L.) BSP.
  Panicum miliaceum L.                 0.5
  P. miliaceum L.
  Plantago aristata Michx.
  Plantago major L.
  Poa annua L.
  Poa pratensis L.
  Ranunculus scleratus L.
  Salicornia herbacea L.               0.13
  Stipa comatat Trin & Rupr.           3
  Suaeda maritima (L.)
    Dumort.                            0.10
  Vulpia ciliata (Le Gall)
    Stace & Auquier                    0.19
  Agrostis stolonifera L.
  Andropogon glomeratus
    (Walt.) B. S. P.
  Andropogon gyrans Ashe
  Andropogon longiberbis
    Hackel
  Andropogon virginicus L.
  Apocynum cannabinum L.
  Apocynum sibiricum Jacq.            25.7
  Artemesia frigida Willd.             3
  Asclepias syriaca L.
  A. syriaca L.                       13.80
  A. syriaca L.                       11([double dagger])
  Apera spica-venti
  Aster tripolium L.
  Carduus tenuiflorus Curt.
  Carlina vulgaris L.
  Centaurea scabiosa L.
  Cirsium arvense (L.) Scop.
  Cirsium palustre (L.)
    Scop.
  Cirsium undulatum (Nutt.)
    Spreng.                           18.4
  Cirsium vulgare (Savi)
    Ten. [=Cirsium
    lanceolatum Scop.]
  C. vulgare (Savi) Ten.
  C. vulgare (Savi) Ten.               1
  Crepis biennis L.
  Crepis virens L.
  Epilobium angustifolium L.
  E. angustifolium L.
  Epilobium hirsutum L.
  Epilobium palustre L.
  Erigeron acer L.
  Erigeron canadensis L.
  Eupatorium cannabinum L.
  E. cannabinum L.
  Gentianella germanica
    (Willd.) Borner                    0.3
  Heterotheca latifolia
    Buckl.                             1.65
  Hieracium umbellatum L.
  Holcus lanatus L.
  Hypochoeris radicata L.
  H. radicata L.
  Juncus bufonius L.
  Leontodon autumnalis L.
  L. autumnalis L.
  Liatris aspera Michx.                2.49([double dagger])
  L. aspera Michx.                     2.5
  Liatris cylindrica Michx.            2.5
  Mirabilis hirsuta (Pursh)
    MacM.                              0.41
  Oenothera biennis L.                 1.83
  Phragmites sp.
  Physalis subglabrata
    Mackenz. & Bush
  Rorippa islandica
    (Oeder) Borbas
  Rumex obtusifolius L.
  Scabiosa columbaria L.               0.3
  Schizachyrium scoparium
    (Michx.) Nash
  Senecio jacobaea L.                  2.5
  Senecio congestus var.
    palustris (L.) Fern.
  Senecio squalidus L.
  Senecio viscosus L.
  Senecio vulgaris L.
  S. vulgaris L.
  Solidago altissima L.
  Solidago missouriensis
    Nutt.
  Solidago rigida L.                   4.9
  Sonchus arvensis L.
  S. arvensis L.
  Sonchus oleraceus L.
  Spergularia media (L.) C.
    Presl. [=S. marginata
    (DC.) Kittel)]
  Spergularia marina (L.)
    Griseb. [=S. salina J.
    & C. Presl.]
  Taraxacum officinale
    Weber.
  T. officinale Weber.
  T. officinale Weber.
  Tragopogon porrifolius L.
  T. porrifolius L.
  Tragopogon pratensis L.
  Trifolium arvense L.
  Tussilago farfara L.
  Verbascum thapsus L.                 4
  Verbena stricta Vent.                1.03
  Salix alba L.
  Salsola iberica                   3050
  Ulmus cprocera Salisb.
    [= U. campestris]
  Andira inermis (W.
    Wright) H. B. K.                  37.63
  Prunus serotina Ehrh.                7.06
  P. serotina Ehrh.                    1([double dagger])
  Acer negundo L.
  Betula lenta L.
  Betula populifolia Marsh.
  Populus sp.
  Typha latifolia L.
  T. latifolia L.
  Phlox pilosa L.                      1.2
  Agrostis hiemalis
    (Walt.) B. S. P.                   2.22
  Andropogon gerardi Vitman            0.06
  Andropogon scoparius
    Michx.                             0.03
  Festuca paradoxa Desv.               0.08
  Setaria geniculata
    (Lam.) Beauv.                      0.11
  Silphium laciniatum L.               1.09
  Sorghastrum nutans (L.)
    Nash                               0.06
  Sphenopholis obtusata
    (Michx.) Scribn.                   0.08
  Aristida congesta Roem
    & Scholt.
  Cenchrus ciliaris L.
  Schmidtia pappaphoroides
    Stend.
  Setaria verticillata
    (L.) Beauv.
  Tragus berteronianus
    Schult.
  Eragrostis rigidior Pilger
  Panicum maximum Jacq.
  Urochloa mosambicensis
    Schult.
  Urochloa panicoides
    (Hack) Dandy
  Chloris virgata Sm.
  Enneapogon cenchroides
    (Licht) C. E. Hubbard
  Zostera marina L.
  Z. marina L.
  Z. marina L.                         1.27

Species                               References

Plants of forest understory
    and canopy
  Asarum canadense L.             present study
  A. canadense L.                 Heithaus (1986)
  Calathea ovandensis Matuda      Horvitz and Schemske (1994)
  Carex pilulifera L.             Kjellsson (1985a)
  Sanguinaria canadensis L.       Pudlo et al. (1980)
  Trillium ovatum Pursh.          Mesler and Lu (1985)
  Viola blanda Willd.             Beattie and Lyons (1975)
  Viola cucullata Ait.            Beattie and Lyons (1975)
  Viola eriocarpa Schwein         Beattie and Lyons (1975)
  Viola odorata L.                Beattie and Lyons (1975)
  Viola papilionacea Pursh.       Beattie and Lyons (1975)
  Viola pedata L.                 Beattie and Lyons (1975)
  Viola rostrata Pursh.           Beattie and Lyons (1975)
  Viola spp.                      Culver and Beattie (1978)
  Cnidoscolus stimulosus
    (Michx.) Engelm. & Gray       Stamp and Lucas (1990)
  Crotalaria rotundifolia
    (Walt.) Poir.                 Stamp and Lucas (1990)
  Stillingia sylvatica
    (Muell. Arg.) Small           Stamp and Lucas (1990)
  Carex pauciflora Lightf.        Hutton (1976)
  Geranium maculata L.            Stamp and Lucas (1983)
  Impatiens capensis Meerb.       Stamp and Lucas (1983)
  I. capensis Meerb.              Primack and Miao (1992)
  Viola striata Air.              Stamp and Lucas (1983)
  V. striata Ait.                 Beattie and Lyons (1975)
  Phytolacca americana L.         Hoppes (1988)
  Mimulus guttatus Fisch.
    ex D.C.                       Waser et al. (1982)
  Aster acuminatus Michx.         Hughes et al. (1988)
  A. acuminatus Michx.            Matlack (1987)
  Aster prenantoides Muhl.        Matlack (1987)
  Eupatorium rugosum Houtt.       Matlack (1987)
  Piper amalgo                    Fleming (1981)
  Alnus crispa (Ait.) Pursh       Matlack (1987)
  Halesia monticola Sarg.         Matlack (1987)
  Purshia tridentata
    (Pursh) DC.                   Vander Wall (1994)
  P. tridentata (Pursh) CD.       Vander Wall (1995)
  Acacia suaveolens (Sm.)
    Willd.                        Andersen (1988)
  Dipteryx panamensis             Morrison
    (Pitt.) Rec. & Mell             (in DeSteven and Putz 1985)
  Casearia corymbosa H. B. K.     Howe (1977)
  Cornus controversa Hemsl.       Masaki et al. (1994)
  Fagus grandifolia Ehrh.         Johnson and Adkisson (1985)
  Ficus stupenda Miq.             Laman (1996)
  Ficus subtecta Corner           Laman (1996)
  Pinus albicaulis Engelm.        Hutchins anti Lanner (1982)
  Pinus edulis Engelm.            Vander Wall and Balda (1977)
  Pinus sp. & Thuja sp.           Reimers (in Vander Wall and
                                    Balda 1977)
  Quercus palustris Muenchh.      Darley-Hill and Johnson (1981)
  Virola surinamensis
    (Rol.) Warb.                  Howe et al. (1985)
  Fagus silvatica L.              Jensen (1985)
  Juglans nigra L.                Stepanian and Smith (1986)
  Pinus jeffreyi Murr.            Vander Wall (1993)
  Quercus macrocarpa Michx.       Stepanian and Smith (1986)
  Quercus muehlenbergii
    Engelm.                       Stepanian and Smith (1986)
  Acer palmatum Thunb.            Matlack (1987)
  Acer pseudoplatanus L.          Matlack (1987)
  Acer rubrum L.                  Green (1980)
  A. rubrum L.                    Matlack (1987)
  A. rubrum L.                    Greene and Johnson (1995)
  Acer saccharum Marsh.           Green (1980)
  Acer cappadocicum               Matlack (1987)
  Acer griseum Pax                Matlack (1987)
  Acer platanoides L.             Matlack (1987)
  Ailanthus altissima
    (Mill.) Swingle               Matlack (1987)
  Albizzia julibrissum
    Durazzini                     Matlack (1987)
  Alseis blackiana Hemsl.         Augspurger (1986)
  Aspidosperma cruenata
    Woods                         Augspurger (1986)
  Astronium graveolans Jacq.      Augspurger (1986)
  Betula papyrifera Marsh.        Greene and Johnson (1995)
  Bombacopsis quinata
    (Jacq.) Dug.                  Augspurger (1986)
  Bombacopsis sessilis
    (Benth.) Pitt.                Augspurger (1986)
  Carpinus caroliniana Walt.      Matlack (1987)
  Catalpa bignonioides Walt.      Matlack (1987)
  Cavanillesia platanifolia
    (H. & B.) H. B. K.            Augspurger (1986)
  Cedrela odorata L.              Augspurger (1986)
  Ceiba pentandra (L.)
    Gaertn.                       Augspurger (1986)
  Cespedizia macrophylla
    Seem.                         Augspurger (1986)
  Chamaecyperis thyoides
    (L.) BSP.                     Matlack (1987)
  Cochlospermum vitifolium
    (Willd.) Spreng.              Augspurger (1986)
  Cordia alliodora (R. &
    P.) Cham.                     Augspurger (1986)
  Couratari panamensis
    Standl.                       Augspurger (1986)
  Dalbergia retusa Hemsl.         Augspurger (1986)
  Eucalyptus regnans F.
    Muell.                        Cremer (1965 in Harper 1977)
  Fraxinus americana L.           Green (1980)
  F. americana L.                 Matlack (1987)
  Fraxinus excelsior L.           Matlack (1987)
  Jacaranda copaia (Aubl.)
    D. Don                        Augspurger (1986)
  Juniperus virginiana L.         Greene and Johnson (1995)
  Lafoensia punicifolia DC.       Augspurger (1986)
  Larix laricina (DuRoi)
    K. Koch                       Brown et al. (1988)
  Liriodendron tulipifera L.      Matlack (1987)
  L. tulipifera L.                Green (1980)
  Lonchocarphus pentaphyllus      Augspurger and Hogan (1983),
    (Poir.) DC.                     Augspurger, (1986)
  Lonchocarpus velutinus
    Seem.                         Augspurger (1986)
  Luehea seemannii Tr. &
    Planch.                       Augspurger (1986)
  Luehea speciosa Willd.          Augspurger (1986)
  Macrocnemum glabrescens
    (Benth.) Wedd.                Augspurger (1986)
  Myroxylon balsamum (L.)
    Harms                         Augspurger (1986)
  Ochroma pyramidale (Cav.
    ex Lam.)                      Augspurger (1986)
  Picea glauca (Moench) Voss      Greene and Johnson (1995)
  Picea engelmannii Parry
    ex Engelm.                    Green and Johnson (1996)
  Pinus contorta Loud.            Greene and Johnson (1989)
  Pinus resinosa Ait.             Greene and Johnson (1995)
  Pinus strobus L.                Greene and Johnson (1995)
  Platanus occidentalis L.        Matlack (1987)
  Platymiscium pinnatum
    (Jacq.) Dug.                  Augspurger (1986)
  Platypodium elegans J.
    Vogel                         Augspurger (1983a, b)
  P. elegans J. Vogel             Augspurger (1986)
  Pseudobombax septenatum
    (Jacq.) Dug.                  Augspurger (1986)
  Pseudotsuga menziesii
    (Mirb.) Franco                Greene and Johnson (1995)
  Pterocarpus rohrii Vahl         Augspurger (1986)
  Tabebuia guayacan (Seem.)
    Hemsl.                        Augspurger (1986)
  Tabebuia rosea (Bertol.)
    DC.                           Augspurger (1986)
  Tachigalia versicolor
    Standl. & L. O. Wms.          Augspurger (1986)
  Terminalia amazonica (J.
    E. Gmel.) Excell in
    Pulle                         Augspurger (1986)
  Terminalia oblonga              Augspurger (1986)
  Tilia americana L.              Matlack (1987)
  Trichospermum mexicanum
    (DC.) Baill.                  Augspurger (1986)
  Triplaris cumingiana
    Fisch. & C. Meyer             Augspurger (1986)
  Tsuga canadensis (L.)
    Carr.                         Greene and Johnson (1995)
  Vatairea erythrocarpa
    Ducke                         Augspurger (1986)
  Parthenocissus quinque-
    folia (L.) Planch.            Hoppes (1988)
  Toxicodendron radicans
    Ktze.                         Hoppes (1988)
  Vitis vulpina L.                Hoppes (1988)
  Clematis virginiana L.          Matlack (1987)

Plants of open habitats
  Cardamine resedifolia L.        Stocklin and Baumler (1996)
  Achillea moschata Wulfen        Stocklin and Baumler (1996)
  Achillea nana L.                Stocklin and Baumler (1996)
  Agrostis rupestris All.         Stocklin and Baumler (1996)
  Arabis alpina L.                Stocklin and Baumler (1996)
  Cerastium arvense L.            Stocklin and Baumler (1996)
  Cerastium pedunculatum
    Gaudin                        Stocklin and Baumler (1996)
  Poa alpina L.                   Stocklin and Baumler (1996)
  Poa nemoralis L.                Stocklin and Baumler (1996)
  Sagina linnaei Pressl.          Stocklin and Baumler (1996)
  Saxifraga sp.                   Stocklin and Baumler (1996)
  Sempervivum sp.                 Stocklin and Baumler (1996)
  Silene rupestris L.             Stocklin and Baumler (1996)
  Trifolium dubium Sibth          Stocklin and Baumler (1996)
  Trifolium pallescens
    Schreber                      Stocklin and Baumler (1996)
  Adenostyles leucophylla
    (Willd.) Reichenb.            Stocklin and Baumler (1996)
  Carex frigida All.              Stocklin and Baumler (1996)
  Cirsium spinosissimum
    (L.) Scop.                    Stocklin and Baumler (1996)
  Epilobium fieischeri
    Hochst.                       Stocklin and Baumler (1996)
  Erigeron angulosus Gaudin       Stocklin and Baumler (1996)
  Geum reptans L.                 Stocklin and Baumler (1996)
  Hieracium murorum L.            Stocklin and Baumler (1996)
  Hieracium staticifolium
    All.                          Stocklin and Baumler (1996)
  Linaria alpina (L.) Miller      Stocklin and Baumler (1996)
  Oxyria digyna (L.) Hill         Stocklin and Baumler (1996)
  Ranunculis adoneus Gray         Scherff et al. (1994)
  Rumex scutatus L.               Stocklin and Baumler (1996)
  Solidago aplestris Waldst.
    & Kit. ex Willd.              Stocklin and Baumler (1996)
  Taraxacum officinale Weber.     Stocklin and Baumler (1996)
  Tussilago farfara L.            Stocklin and Baumler (1996)
  Rhododendron ferrugineum L.     Stocklin and Baumler (1996)
  Alnus viridis (Chaix) DC.       Stocklin and Baumler (1996)
  Myricaria germanica (L.)
    Desv.                         Stocklin and Baumler (1996)
  Salix spp.                      Stocklin and Baumler (1996)
  Larix decidua Mill.             Stocklin and Baumler (1996)
  Datura discolor Bernh.          O'Dowd and Hay (1980)
  Sclerolaena diacantha
    (Nees) Benth.                 Davidson and Morton (1981)
  Sporobolus airoides Torr.       Knipe and Springfield (1972)
  Artemesia herba-alba Asso       Friedman and Orshan (1975)
  Cryptantha flava (A.
    Nels.) Payson                 Casper (1987)
  Happlopappus squarrosus
    Hook. & Arn.                  Louda (1982)
  Bursera graveolens              Clark and Clark (1981)
  Lithospermum caroliniense
    (Walt.) MacMill.              Westelaken and Maun (1985)
  Vulpia fasciculata
    (Forskal) Samp.               Watkinson (1978)
  Geranium carolinianum L.        Stamp and Lucas (1983)
  Geranium molle L.               Stamp and Lucas (1983)
  Phlox drummondii Hook.          Stamp and Lucas (1983)
  Achyranthes aspera L.           Bullock and Primack (1977)
  Bidens sp.                      Bullock and Primack (1977)
  Petiveria alliaceae L.          Bullock and Primack (1977)
  Abutilon theophrasti
    Medic.                        Primack and Miao (1992)
  Agropyron repens (L.)
    Beauv.                        Hume and Archbold (1986)
  Atriplex patula var.
    hastata L.                    Feekes (1936)
  Bromus inermis Leyss.           Hume and Archbold (1986)
  Capsella bursa-pastoris
    (L.) Medic.                   Feekes (19:36)
  Carex extensa Good.             Feekes (19136)
  Carex sp.                       Hume and Archbold (1986)
  Dipsacus sylvestris Huds.       Werner (1975)
  Hypericum gentianoides
    (L.) BSP.                     Primack and Miao (1992)
  Panicum miliaceum L.            McCanny and Cavers (1989)
  P. miliaceum L.                 McCanny and Cavers (1987)
  Plantago aristata Michx.        Primack and Miao (1992)
  Plantago major L.               Feekes (1936)
  Poa annua L.                    Feekes (19136)
  Poa pratensis L.                Feekes (1936)
  Ranunculus scleratus L.         Feekes (1936)
  Salicornia herbacea L.          Feekes (1936)
  Stipa comatat Trin & Rupr.      Hume and Archbold (1986)
  Suaeda maritima (L.)
    Dumort.                       Feekes (1936)
  Vulpia ciliata (Le Gall)
    Stace & Auquier               Carey and Watkinson (1993)
  Agrostis stolonifera L.         Feekes (1936)
  Andropogon glomeratus
    (Walt.) B. S. P.              Campbell (1983)
  Andropogon gyrans Ashe          Campbell (1983)
  Andropogon longiberbis
    Hackel                        Campbell (1983)
  Andropogon virginicus L.        Campbell (1983)
  Apocynum cannabinum L.          Matlack (1987)
  Apocynum sibiricum Jacq.        Platt and Weis (1977)
  Artemesia frigida Willd.        Hume and Archbold (1986)
  Asclepias syriaca L.            Matlack (1987)
  A. syriaca L.                   Platt and Weis (1977)
  A. syriaca L.                   Morse and Schmidt (1985)
  Apera spica-venti               Feekes (1936)
  Aster tripolium L.              Feekes (1936)
  Carduus tenuiflorus Curt.       Sheldon and Burrows (1973)
  Carlina vulgaris L.             Sheldon and Burrows (1973)
  Centaurea scabiosa L.           Sheldon and Burrows (1973)
  Cirsium arvense (L.) Scop.      Sheldon and Burrows (1973)
  Cirsium palustre (L.)
    Scop.                         Sheldon and Burrows (1973)
  Cirsium undulatum (Nutt.)
    Spreng.                       Platt and Weis (1977)
  Cirsium vulgare (Savi)
    Ten. [=Cirsium
    lanceolatum Scop.]            Feekes (1936)
  C. vulgare (Savi) Ten.          Matlack (1987)
  C. vulgare (Savi) Ten.          Klinkhamer et al. (1988)
  Crepis biennis L.               Feekes (1936)
  Crepis virens L.                Feekes (1936)
  Epilobium angustifolium L.      Feekes (1936)
  E. angustifolium L.             Matlack (1987)
  Epilobium hirsutum L.           Feekes (1936)
  Epilobium palustre L.           Feekes (1936)
  Erigeron acer L.                Sheldon and Burrows (1973)
  Erigeron canadensis L.          Feekes (1936)
  Eupatorium cannabinum L.        Matlack (1987)
  E. cannabinum L.                Sheldon and Burrows (1973)
  Gentianella germanica
    (Willd.) Borner               Verkaar et al. (1983)
  Heterotheca latifolia
    Buckl.                        Venable and Levin (1985)
  Hieracium umbellatum L.         Matlack (1987)
  Holcus lanatus L.               Feekes (1936)
  Hypochoeris radicata L.         Feekes (I 936)
  H. radicata L.                  Sheldon and Burrows (1973)
  Juncus bufonius L.              Feekes (1936)
  Leontodon autumnalis L.         Feekes (1936)
  L. autumnalis L.                Sheldon and Burrows (1973)
  Liatris aspera Michx.           Levin and Kerster (1969)
  L. aspera Michx.                Levin and Kerster (1974)
  Liatris cylindrica Michx.       Levin and Kerster (1974)
  Mirabilis hirsuta (Pursh)       Platt and Weis
    MacM.                           (1977), Platt (1976)
  Oenothera biennis L.            Platt and Weis (1977)
  Phragmites sp.                  Feekes (1936)
  Physalis subglabrata
    Mackenz. & Bush               Matlack (1987)
  Rorippa islandica
    (Oeder) Borbas                Feekes (1936)
  Rumex obtusifolius L.           Matlack (1987)
  Scabiosa columbaria L.          Verkaar et al. (1983)
  Schizachyrium scoparium
    (Michx.) Nash                 Campbell (1983)
  Senecio jacobaea L.             Poole and Cairns (in Harper
                                    1977), McEvoy and Cox (1987)
  Senecio congestus var.
    palustris (L.) Fern.          Feekes (1936)
  Senecio squalidus L.            Matlack (1987)
  Senecio viscosus L.             Sheldon and Burrows (1973)
  Senecio vulgaris L.             Feekes (1936)
  S. vulgaris L.                  Sheldon and Burrows (1973)
  Solidago altissima L.           Matlack (1987)
  Solidago missouriensis
    Nutt.                         Hume and Archbold (1986)
  Solidago rigida L.              Platt and Weis (1977)
  Sonchus arvensis L.             Feekes (1936)
  S. arvensis L.                  Sheldon and Burrows (1973)
  Sonchus oleraceus L.            Sheldon and Burrows (1973)
  Spergularia media (L.) C.
    Presl. [=S. marginata
    (DC.) Kittel)]                Feekes (1936)
  Spergularia marina (L.)
    Griseb. [=S. salina J.
    & C. Presl.]                  Feekes (1936)
  Taraxacum officinale
    Weber.                        Feekes (1936)
  T. officinale Weber.            Sheldon and Burrows (1973)
  T. officinale Weber.            Matlack (1987)
  Tragopogon porrifolius L.       Matlack (1987)
  T. porrifolius L.               Sheldon and Burrows (1973)
  Tragopogon pratensis L.         Feekes (1936)
  Trifolium arvense L.            Matlack (1987)
  Tussilago farfara L.            Bakker (in Harper 1977)
  Verbascum thapsus L.            Salisbury (in Harper 1977)
  Verbena stricta Vent.           Platt and Weis (1977)
  Salix alba L.                   Feekes (1936)
  Salsola iberica                 Stallings et al. (1995)
  Ulmus cprocera Salisb.
    [= U. campestris]             Feekes (1936)
  Andira inermis (W.
    Wright) H. B. K.              Janzen et al. (1976)
  Prunus serotina Ehrh.           Smith (1975)
  P. serotina Ehrh.               Hoppes (1988)
  Acer negundo L.                 Matlack (1987)
  Betula lenta L.                 Matlack (1989)
  Betula populifolia Marsh.       Matlack (1987)
  Populus sp.                     Feekes (1936)
  Typha latifolia L.              Feekes (1936)
  T. latifolia L.                 Matlack (1987)
  Phlox pilosa L.                 Levin and Kerster (1974)
  Agrostis hiemalis
    (Walt.) B. S. P.              Rabinowitz and Rapp (1979)
  Andropogon gerardi Vitman       Rabinowitz and Rapp (1981)
  Andropogon scoparius
    Michx.                        Rabinowitz and Rapp (1981)
  Festuca paradoxa Desv.          Rabinowitz and Rapp (1981)
  Setaria geniculata
    (Lam.) Beauv.                 Rabinowitz and Rapp (1981)
  Silphium laciniatum L.          Pleasants and Jurik (1992)
  Sorghastrum nutans (L.)
    Nash                          Rabinowitz and Rapp (1981)
  Sphenopholis obtusata
    (Michx.) Scribn.              Rabinowitz and Rapp (1981)
  Aristida congesta Roem
    & Scholt.                     Ernst et al. (1992)
  Cenchrus ciliaris L.            Ernst et al. (1992)
  Schmidtia pappaphoroides
    Stend.                        Ernst et al. (1992)
  Setaria verticillata
    (L.) Beauv.                   Ernst et al. (1992)
  Tragus berteronianus
    Schult.                       Ernst et al. (1992)
  Eragrostis rigidior Pilger      Ernst et al. (1992)
  Panicum maximum Jacq.           Ernst et al. (1992)
  Urochloa mosambicensis
    Schult.                       Ernst et al. (1992)
  Urochloa panicoides
    (Hack) Dandy                  Ernst et al. (1992)
  Chloris virgata Sm.             Ernst et al. (1992)
  Enneapogon cenchroides
    (Licht) C. E. Hubbard         Ernst et al. (1992)
  Zostera marina L.               Churchill et al. (1985)
  Z. marina L.                    Orth et al. (1994)
  Z. marina L.                    Ruckelshaus (1996)

([dagger]) When measured under more than one condition, we selected
the seed-dispersal treatment that led to the greatest dispersal
distance. Estimates of dispersal distances for plants bearing
wind-dispersed seeds often depended on combining a measured
rate of fall with typical infructescence heights and assumed
wind speeds.

([dagger]) Because we used only the largest mean or maximum
dispersal distances, these values were excluded from the analyses.


MICHAEL L. CAIN,(1) HANS DAMMAN,(2) AND ANGELA MUIR(2)

(1) Department of Biology, New Mexico State University, Las Cruces, New Mexico 88003 USA

(2) Ottawa-Carleton Institute of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6

Manuscript received 17 October 1996; revised 29 April 1997; accepted 29 May 1997; final version received 7 August 1997.
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Author:Cain, Michael L.; Damman, Hans; Muir, Angela
Publication:Ecological Monographs
Date:Aug 1, 1998
Words:18588
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