Seed dispersal and the Holocene migration of woodland herbs.
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).
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.
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.
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.
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]
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]
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.
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|
|Date:||Aug 1, 1998|
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