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Effects of disturbance on herbaceous exotic plant species on the floodplain of the Potomac River.

INTRODUCTION

Exotic (non-native) plant species have rapidly invaded the North American flora over the last century (Forcella and Harvey, 1983). In many cases, these species have reached high abundances and have altered the structure of communities and ecosystems, with some becoming dominant components in their communities (Thomas, 1980; Tremmel and Peterson, 1983; Huenneke et al., 1990). In a study of the Potomac River floodplain, Thomas (1980) concluded that the invasive vine Lonicera japonica suppresses the reproduction of native herbs and woody plants and that English ivy (Hedera helix) suppresses the reproduction of native herbs and can kill overstory trees. S. P. Bratton (pers. comm.), in a study of the floodplains of the Susquehanna River, found that the abundances of several wildflower species (e.g., Erythronium americanum, Dicentra canadensis) are negatively correlated with the abundances of woody and herbaceous exotics.

Disturbance may be defined as an event that can change community and ecosystem structure and composition by changing the physical environment and/or resource availability (Drake et al., 1989). In the process, plant biomass is usually removed or destroyed (Rykiel, 1985). Many studies have shown that disturbance can accelerate invasions of exotic plants and may sometimes be a requirement for exotics to establish (Forcella and Harvey, 1983; Huenneke et al., 1990; Hobbs and Mooney, 1991; D'Antonio, 1993). Disturbance may facilitate exotic invasion in two ways: (1) through a simple clearing of space, which increases the availability and suitability of sites for establishment and/or decreases interspecific competition at these sites (Gross and Werner, 1982) and/or (2) through an increase in the availability of, or redistribution of, a limited resource (Connell, 1978; D'Antonio, 1993; Wilson and Tilman, 1993). Disturbances may create refuges for inferior competitors (Bergelson et al., 1993), form cracks in soil which trap seeds (Platt, 1975) or increase soil nutrients (Wilson and Tilman, 1993).

Other studies have shown that disturbance is not a requirement for exotic invasion (Tremmel and Peterson, 1983; McClaran and Anable, 1992; Tyser and Worley, 1992). Some of the conflicting evidence regarding the role of disturbance in exotic invasions may be clarified when examined with reference to Fox and Fox's (1986) distinction between two types of disturbances: endogenous, which occur frequently and to which species in a community are adapted, and exogenous, usually man-made, to which the species in the community are not adapted. From this distinction, it follows that generic "disturbances" do not exist; different types of disturbance may have different effects on a community. Therefore, it is important to understand the context in which the disturbance occurs. A type of disturbance in one community (e.g., flooding) may not be a disturbance in a community in which the member species have adapted to that environmental factor.

McIntyre et al. (1988) suggested that adaptation to an endogenous disturbance may result in resistance to an exogenous disturbance. They showed that wetland communities in Australia which experience more regular endogenous disturbance are more resistant to invasion by exotic plants than other communities. They suggest this resistance results from adaptations to the endogenous disturbance (drought, in this case).

If disturbance can accelerate invasions of exotics, floodplain forests in general may be naturally susceptible to these invasions because of repeated flooding. However, some adaptation by the native species may occur so that smaller, more frequent floods are tolerated and only very large floods act as a disturbance. In addition, floodplain species may exhibit ruderal life-history characteristics that allow them to quickly repopulate bare soil (Menges and Waller, 1983).

Human activities near floodplains may increase the frequency of introductions and the rate of invasion of exotic species by providing seed sources and increasing the amount of disturbance in the landscape over natural levels. Types of exogenous disturbance that may be significant for the Potomac floodplain are the fragmentation of the floodplain forests and recreational use of the land. The influx of visitors to the C&O Canal towpath and the many parks in the area leads to new construction projects, trampled vegetation and a source of exotic seeds.

The purpose of this project is to examine if and how human disturbance, specifically recreational use and forest fragmentation, affects the abundance of exotic species in the Potomac River floodplain. I will also examine whether exotic species are less common near the water's edge; if so, this pattern may result either because the community closer to the water is resistant to invasion by exotic plants because of its member species' adaptations to endogenous disturbance (flooding), or because the exotic species cannot tolerate the frequent flooding. In addition to testing hypotheses about the effects of disturbance on exotic plants, I will provide baseline data on their abundance so that future studies may determine whether their populations are increasing or decreasing.

I tested my hypotheses regarding the effects of disturbance on exotic plants by comparing the frequency and species richness of exotics in three sites: an undisturbed reference site, a completely forested site disturbed by recreational use and a site disturbed by recreational use with fragmented forest cover. I also examined how the frequency and species richness of exotics changed with distance from the water and, therefore, with different flooding frequency. I tested three hypotheses: (1) the frequency and species richness of exotic species are lower in a forested, undisturbed reference site (Ruppert Island) than in a forested site on the mainland that is subject to heavier recreational use (Great Falls National Park); (2) the frequency and species richness of exotic species are lower in a completely forested mainland site (Great Falls National Park) than in a mainland site with fragmented forest cover (Violets Lock); (3) the frequency and species richness of exotic species within a site increase with distance from the river's edge.

METHODS

Study sites. - All three study sites were located on the floodplain of the Maryland side of the Potomac River. For further information, refer to the following USGS 7.5 min quadrangles: Falls Church, VA-MD (Ruppert Island); Rockville, MD-VA (Great Falls); and Seneca, MD (Violets Lock).

Ruppert Island, owned since 1886 by the Sycamore Island Canoe Club, was chosen as a reference site. The only known human disturbance on the island has been relatively minor (archaeological digs were conducted there in the 1960s). The island is approximately 2.25 ha and 100 m from the mainland; the only access to the island is by boat.

I selected two study sites on the mainland (which receives heavy recreational use due to the presence of the C&O Canal towpath and numerous parks) so that they were comparable to each other and the reference site in terms of physical characteristics such as size, elevation and slope. Ideally, the three sites would differ only in the degree of disturbance and forest fragmentation.

The forested mainland site was located in Great Falls National Park at the intersection of Cool Spring and the Potomac River. There was a band of at least 300 m of unbroken forest between the river and the towpath; forest stretched beyond the towpath for several hundred more meters. The fragmented forest mainland site was located in the C&O Canal National Historical Park near the southern terminus of Violets Lock Road. Here the canal and towpath were much closer (within 20 m) to the river so that they actually intersected my sampling area. The total width of forest bordering the river ranged (approximately) from 80-120 m; just outside the forest was a golf course.

I estimated the degree of forest fragmentation at each site by examining cover patterns on topographic maps and by visiting each site. I did not quantify the degree of fragmentation at the sites since I was comparing sites with no visible fragmentation (Ruppert Island and Great Falls) to a site with marked fragmentation (Violets Lock). However, I wanted to establish some relationship between fragmentation and the light levels in the understory. Therefore, I took measurements of light intensity, using a Li-Cor quantum photometer (model LI-185B), at 40 randomly selected locations within each site. All light measurements were made between 1100 h and 1400 h and were taken at 1 m above the ground. A measurement in a cleared space, unobscured by trees, buildings, etc., was also taken the same day just before the readings at the site. The readings are expressed as a proportion of the control reading in order to standardize the measurements, which were taken on different days with different weather conditions.

Light intensity data were arc-sine transformed and tested for normality using the Kolmogorov-Smirnov test for goodness of fit; the null hypothesis of normality was rejected ([Alpha] = 0.05). The data were ranked (using PROC RANK in SAS); differences among the three sites in the amount of light penetrating the canopy were evaluated with a one-way ANOVA (PROC GLM). This procedure is equivalent to a Kruskal-Wallace k-sample test (SAS Institute, 1985). Comparisons among means were made using the Ryan-Einot-Gabriel-Welsch multiple range test. The ANOVA resulted in a significant F-value (F = 20.64, P [less than] 0.0001); the proportion of full sunlight intensity reaching 1 m above the ground was significantly lower at the Great Falls site than those at the Violets Lock and Ruppert Island sites which were not significantly different. The variance of the readings was greatest at Violets Lock.

These results were surprising because although I chose the Ruppert island site as a completely forested, undisturbed reference site, the proportion of light reaching 1 m above the ground was significantly greater than at the Great Falls site, also chosen as a completely forested site (Table 1). Differences in the flooding patterns at the two sites may help to explain why more light penetrates the Ruppert Island canopy. Because Ruppert Island is in the middle of the river, where water velocity is higher than near the banks, and is located near Chain Bridge, an area that historically has had very high flood velocities (Yanosky, 1982), damage to the forest on the island is probably more severe than at the other two sites. My subjective impression of the forest at Ruppert Island is that there were small, natural gaps in the canopy that may be the result of flooding. The fact that the variance of the light readings is greater at the Violets Lock site than at Ruppert Island (Table 1) lends support to this interpretation. At Violets Lock, the fragmentation followed a regular pattern, and large areas of the site were completely cleared of vegetation. Therefore, there would be a great difference between adjacent readings depending on whether the reading was taken in a patch of forest or in a cleared area. The pattern at Ruppert Island is one of small, irregular openings in the canopy in which all vegetation was not removed. Presumably there would be a smaller difference between readings taken in forest and in "clearings" than at the Violets Lock site. I do not believe my assumptions about the levels of exogenous disturbance (fragmentation) present at each site were significantly violated.
TABLE 1. - The mean proportion of full sunlight intensity reaching
1 m above the ground, at each site


Site              Mean       SE     Variance


Violets Lock      0.254    0.045     0.127
Great Falls(*)    0.029    0.004     0.004
Ruppert Island    0.160    0.031     0.062


* Significant


I also made assumptions about the community composition of the three sites. Communities are known to vary in their susceptibility to invasion (Fox and Fox, 1986; McIntyre et al., 1988); therefore, to compare the extent of invasion under different conditions, the communities sampled should be initially similar. To test these assumptions, I placed three 15 x 30 m plots within both the Ruppert Island and Great Falls sites. I recorded all species present in the plots, in both the canopy ([greater than]1 m tall) and understory ([less than]1 m tall), along with an index of percent cover (5 = 75-100%, 4 = 50-75%, 3 = 25-50%, 2 = 5-25%, 1 = 1-5%, + = [less than]1%; Barbour et al., 1987). In each plot, I also estimated total percent cover of all species in the canopy and understory.

Similar tree species were present in the canopy at both the Great Falls and Ruppert Island sites (Table 2). The understory, however, showed differences between the sites. Many species that were not present at Great Falls were found on Ruppert Island, including Thalictrum pubescens and Polygonatum biflorum. Other species were seen only at the Great Falls site, such as Ellisea nyctelea and Osmorhiza sp. Some species that were present at both sites had much greater cover values at one site. For example, Boehemeria cylindrica reached 1-5% cover on the island but always had [less than]1% cover at Great Falls; Laportea canadensis reached 5-25% cover on Ruppert Island but was infrequent at Great Falls. In general, exotic herbs had higher cover values at Great Falls, and there was a higher diversity and cover of native herbs on Ruppert Island. Total cover in both layers (canopy and understory) was higher at Great Falls.

I believe my assumptions about the community composition at the Ruppert Island and Great Falls sites are justified by this data; the vegetation at these sites is similar enough, especially in the canopy, so that they may be regarded as parts of the same community. Variation in flooding regimes between the two sites help to explain the differences in the understories as revealed by my surveys. Differences in species composition and cover in the understory could also be a result of less intense competition with exotic species for the natives on the island.

No such descriptions and comparisons of communities were done for the fragmented site (Violets Lock), as it was intentionally selected to be different from the reference and the forested mainland sites. I assumed that, because the Violets Lock site is located at the same distance from the river and the same elevation above the river as the other two sites, historically, all sites had similar community composition.
TABLE 2. - Community composition of two sites based on surveys of
three 15 x 30 m plots at each site. All species present in each plot
are listed along with an index of percent cover (5 = 75-100%,
4 = 50-75%, 3 = 25-50%, 2 = 5-25%, 1 = 1-5%, + = [less than]1%,
0 = not present) for each plot. Species marked with an asterisk are
exotic


                               Great Falls     Ruppert Island


Canopy species


Acer negundo                     4, 5, 2          4, 5, 5
A. saccharinum                   0, 0, 0          0, 0, 2
Asimina triloba                  3, 4, 4          4, 2, 0
Celtis occidentalis              3, 3, 2          0, 4, 0
Lindera benzoin                  2, 2, 1          +, 0, 0
Morus rubra                      1, 1, 0          0, 0, 0
Ostrya virginica                 0, 1, 0          0, 0, 0
Parthenocissus quinquefolia      0, 1, 0          +, +, +
Platanus occidentalis            2, 0, 4          5, 0, 0
Staphylea trifolia               0, 0, 0          1, 0, 0
Toxicodendron radicans           0, 1, 0          0, +, 0
Vitis sp.                        0, 1, 1          0, 0, 0


Understory species


Acer negundo                     0, 0, 0          0, 1, 0
A. saccharinum                   +, +, +          +, 0, 4
Albizzia julibrissin(*)          0, 0, 0          0, +, 0
Alliaria officinalis(*)          5, 5, 3          1, 2, 0
Allium vineale(*)                0, +, 0          0, 0, 0
Arisaema triphyllum              0, 0, 0          0, +, 0
Asimina triloba                  1, 2, +          3, 2, +
Boehemeria cylindrica            0, +, 0          +, 1, 1
Calystegia sepium                0, 0, 0          0, 0, +
Celtis occidentalis              +, +, 0          +, +, 0
Dioscorea villosa                0, 0, 0          0, +, 0
Duchesnea indica(*)              0, 0, +          0, 0, 0
Ellisea nyctelea                 1, +, 0          0, 0, 0
Fraxinus pennsylvanica           0, 0, 0          0, 0, +
Galium aparine                   1, 1, +          0, 0, 0
Geum canadense                   0, 0, 0          0, +, 0
Glecoma hederacea(*)             5, 4, 5          0, +, 0
Hedera helix(*)                  0, 0, 0          1, 1, 0
Hydrophyllum canadense           1, 3, 3          +, 0, 0
H. virginianum                   0, 0, 0          0, 0, 1
Impatiens sp.                    +, +, 0          +, +, +
Laportea canadensis              +, 0, 0          2, 2, 0
Lindera benzoin                  0, 0, +          +, +, +
Lonicera japonica(*)             0, 0, 0          0, +, 0
Osmorhiza sp.                    +, +, 0          0, 0, 0
Ostrya virginica                 0, 0, 0          0, 0, +
Oxalis stricta                   0, 0, 0          +, +, 0
Parthenocissus quinquefolia      +, 1, +          2, 2, +
Podophyllum peltatum             +, 0, 0          3, 0, 0
Polygonatum biflorum             0, 0, 0          1, +, 0
Polygonum sp.                    0, +, +          0, +, +
Rosa multiflora(*)               0, 0, 0          0, +, 0
Saururus cernuus                 0, 0, 0          0, 0, +
Senecio aureus                   0, 0, 0          0, 1, 0
Smilacina racemosa               +, 0, 0          0, 0, 0
Smilax rotundifolia              0, 0, 0          0, +, +
Stellaria media(*)               1, +, 0          0, 0, 0
Thalictrum pubescens             0, 0, 0          1, 1, 0
Toxicodendron radicans           1, +, 0          0, +, +
Urtica dioica(*)                 0, 0, 0          0, 0, +
Verbesina alternifolia           0, 0, +          +, +, 1
Verbesina occidentalis           0, 0, 0          0, 0, +
Vinca major                      1, 1, +          0, 0, 0
Viola sororia                    0, +, 0          +, 1, +
Viola striata                    0, 0, +          0, 0, 0
Vitis sp.                        0, 0, +          0, 0, +


Total % cover - canopy          95, 90, 90      85, 90, 85
- understory                    90, 90, 90      75, 75, 50


Exotic frequency and species richness. - In each of the three sites, 100, 1-[m.sup.2] plots were randomly located along a transect parallel to the river's edge 1 m from the water. Another 100 such plots were located along a transect 10 m from the water and a final 100 plots were placed 20 m inland. These nine transects were located independently of the 15 x 30 plots used for analysis of community composition. The transects will be referred to as follows: V1, V10, V20 (Violets Lock site at 1 m, 10 m, 20 m, respectively); G1, G10, G20 (Great Falls National Park transects); R1, R10, R20 (Ruppert Island transects).

I surveyed 10 randomly selected 100-[cm.sup.2] subplots in each 1-[m.sup.2] plot for the presence or absence of ten common exotic species (Alliaria officinalis, Glecoma hederacea, Stellaria media, Lonicera japonica, Hedera helix, Duchesnea indica, Urtica dioica, Veronica hederaefolia, Lamium purpureum and Rosa multiflora). I recorded 11 data values for each 1-[m.sup.2] plot: 10 frequency values (the proportion of subplots in which the exotic was found), one for each species, and a species richness value (the number of surveyed exotics present in the 10 subplots).

Data analysis. - All statistical analysis was performed using the Statistical Analysis System (SAS); significance testing is at the [Alpha] = 0.05 level.

Frequency and species richness of exotics were arcsine transformed and tested for normality using the Kolmogorov-Smirnov test for goodness of fit. I rejected the null hypothesis of normality at the [Alpha] = 0.05 level for all variables. I therefore used PROC RANK to rank the variables before subjecting the data to one-way ANOVAs (PROC GLM). This procedure is equivalent to a Kruskal-Wallace k-sample test (SAS Institute, 1985). Comparisons among means were made using the Ryan-Einot-Gabriel Welsch multiple range test.

In order to examine the differences in the frequency and species richness of exotics among the transects, I carried out a total of nine ANOVAs: eight in which the frequency of eight of the exotic species were dependent variables and one in which the species richness of exotics was the dependent variable. I did not carry out ANOVAs using the frequencies of Lamium purpureum and Rosa multiflora as dependent variables because these species were very rare or absent in all plots.
TABLE 3. - Mean species richness of exotic species per square meter
plot, by transect (V = Violets Lock, G = Great Falls, R = Ruppert
Island; 1, 10, and 20 refer to distance from the river, in meters).
Degrees of freedom = 8, 891. Means with the same "group" letter are
not significantly different


Transect    Mean        SE          F        Group


V1          0.040     0.020     351.71(*)    F, G
V10         2.580     0.074                  B
V20         3.200     0.112                  A
G1          0.000     0.000                  G
G10         1.120     0.091                  D
G20         1.790     0.074                  C
R1          0.030     0.017                  F, G
R10         0.170     0.038                  F
R20         0.530     0.074                  E


* Significant


Because the subplots I used to obtain frequency values were small (100 [cm.sup.2]), it is possible that they were not independent observations, especially since many of the surveyed exotic species are viney or rhizomatous. Therefore, I also tested the hypotheses regarding the frequency of exotic species a second time by using a chi-square analysis (PROC CATMOD in SAS) to examine the patterns of exotic species presence and absence in the 1-[m.sup.2] plots.

RESULTS

Exotic species richness. - A one-way ANOVA with the species richness of exotic species per 1-[m.sup.2] plot as the dependent variable resulted in a significant F-value (Table 3). The highest means were at the Violets Lock site (V10 and V20 transects); in general, the means were lower at the Ruppert Island site although the G1 transect had the absolute lowest mean. At all three sites, mean species richness of exotics increased significantly with distance from the water.

Frequency of exotic species. - Alliaria officinalis and Glecoma hederacea had the highest mean frequencies of the 10 exotic species (Table 4). All eight ANOVAs with the frequencies of exotic species as dependent variables were significant (Table 4). The chi-square tests confirmed these results in every case except for Hedera helix, which was most likely the result of the low frequency of H. helix. Hedera helix showed no significant differences in frequency among the transects.

Four species (Stellaria media, Lonicera japonica, Veronica hederaefolia and Duchesnea indica) exhibited similar patterns of frequency among the nine transects, and were significantly more frequent at the Violets Lock site (Table 4). Within this site, these species were more frequent farther from the water. Lonicera japonica and Veronica hederaefolia had higher mean frequencies at the 10-m transect than at the 20 m transect. All four species were absent or had very low mean frequencies at the 1-m transect.

Urtica dioica had highest mean frequencies at the V20, R10 and R20 transects (Table 4). Alliaria officinalis and Glecoma hederacea were unusual in that some of their higher mean frequencies were found at the Great Falls site. Alliaria officinalis had the highest mean at the V10 and G20 transects (which were not significantly different from each other); Glecoma hederacea was most frequent at the G20 transect. Glecoma hederacea increased in frequency with distance from the water at all three sites. Alliaria officinalis followed this pattern only [TABULAR DATA FOR TABLE 4 OMITTED] at the Great Falls site. There was no significant difference in the frequency of Alliaria officinalis among transects at the Ruppert Island site; at the Violets Lock site, the 10-m transect had the highest mean.

DISCUSSION

The exotic species frequency and species richness data do not contradict hypothesis #1 which predicted that exotic species frequency and species richness would be greater in a forested site which was disturbed by recreational use (Great Falls) than in a forested undisturbed reference site (Ruppert Island; Table 4). These data also do not contradict hypothesis #2 which predicted that the species richness and frequency of exotics in a fragmented forest mainland site (Violets Lock) would be higher than at completely forested Great Falls (Table 4). Both forest fragmentation and recreational and residential use of land encourage the invasion of most species of exotic plants.

However, there is some variation among species in response to disturbance. Alliaria officinalis and Glecoma hederacea, the two most common species, were significantly more frequent at Great Falls than at Violets Lock or Ruppert Island (Table 4). This pattern may result from an inability to tolerate high light levels. Both species had significantly higher mean frequencies at the Violets Lock site than at Ruppert Island, which suggests that recreational disturbance encouraged their spread. However, the increased forest fragmentation at Violets Lock did not result in higher mean frequencies than at Great Falls; in fact, higher light levels resulted in significantly lower mean frequencies.

The data also are in agreement with hypothesis #3 which predicted that species richness and frequency of the exotic species would increase with distance from the water (Table 4). All exotic species were absent or infrequent 1 m from the water. This pattern may result from the fact that exotics could not tolerate the increased frequency and severity of flooding, or native species may have specific adaptations for quick recolonization of the bare soil after a flood, which prevents the exotic species from establishing.

There is some variation among exotic species in patterns of frequency at the 10- and 20-m positions. Lonicera japonica and Veronica hederaefolia were more frequent at the V10 transect than the V20 transect (Table 4). Mowing of the immediate vicinity around the towpath, which coincided with the V20 transect, may have reduced the number and size of some exotic plants.

When the results are examined together, it becomes apparent that all types of disturbance are not equal in promoting or inhibiting invasions of exotics. Frequency and species richness of exotics decreased with increased flooding, but increased with increasing man-made disturbance. The results of this study lend support to the ideas of Fox and Fox (1986): a distinction can be made between exogenous and endogenous disturbances. Although an exogenous disturbance was effective in promoting exotic establishment in the Potomac floodplain, it may be that floodplain communities are actually less vulnerable to exotic invasions because of the member species' adaptations to an endogenous disturbance. Comparisons between floodplain and upland communities of the rate and extent of invasions of exotic species should be made.

My findings agree with those of previous authors (Forcella and Harvey, 1983; Huenneke et al., 1990; Hobbs and Mooney, 1991; D'Antonio, 1993) who said that disturbance can encourage exotic invasions. Fragmentation of the forest canopy on the Potomac floodplain allows more light to reach the herbaceous layer of the forest. This increase in a limiting resource alters the relationships of the species within the community, and results in a new set of environmental conditions in which some exotic species can grow and reproduce. Use of the land near the Potomac River for recreation and residential purposes also helps to create sites for exotic establishment through damage and removal of native vegetation, and spreads exotic seeds.

The results of this study may be useful for park managers in predicting the responses of exotic species to certain actions. In general, fragmentation of forests increases the numbers of exotic species and their importance in the floodplain community. However, simply preserving tracts of forested land does not guarantee a low abundance of exotics; Alliaria officinalis and Glecoma hederacea had high mean frequency values in the forested but disturbed Great Falls National Park site. Identifying sensitive areas that are the most important to preserve in their natural state and reducing human traffic in those areas may be one way to reduce exotic populations.

Acknowledgments. - I would like to thank James Lawrey and Ted Bradley of George Mason University; Robert Unnasch of The Nature Conservancy; Paul Rosa, The Potomac Conservancy; Rodney Bartgis, The Maryland Nature Conservancy; Elaine Lahn, Institute for Urban Ecology, and Chris Lea, National Park Service, for their help. Susan P. Bratton, Jeffrey R. Hapeman and Austin R. Mast allowed me to cite data from their studies of herbaceous plants along the Susquehanna River. Peter Jones, caretaker for Sycamore Island, and the Sycamore Island Canoe Club provided a canoe and allowed me to work on Ruppert Island.

LITERATURE CITED

BARBOUR, M. G., J. H. BURK AND W. D. PITTS. 1987. Terrestrial plant ecology. Benjamin Cummings Publishing Co., Inc. Menlo Park, Calif. 604 p.

BERGELSON, J., I. A. NEWMAN AND E. M. FLOURESROUX. 1993. Rates of weed spread in spatially heterogenous environments. Ecology, 74:999-1011.

BRATTON, S. P., J. R. HAPEMAN AND A. R. MAST. 1995. The lower Susquehanna river gorge and floodplain (U.S.A.) as a riparian corridor for vernal, forest floor herbs. Conserv. Biol., in press.

CONNELL, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science, 199:1302-1309.

D'ANTONIO, C. M. 1993. Mechanisms controlling invasion of coastal plant communities by the alien succulent Carpobrotus edulis. Ecology, 74:83-95.

DRAKE, J. A., H. A. MOONEY, F. DICASTRI, R. H. GROVES, F. J. KRUGER, M. REJMANEK AND M. WILLIAMSON (EDS.). 1989. Biological invasions: a global perspective. John Wiley and Sons, Chichester. 525 p.

FORCELLA, F. AND ST. J. HARVEY. 1983. Relative abundance in an alien weed flora. Oecologia, 59:292-295.

FOX, M. D. AND B. J. FOX. 1986. The susceptibility of natural communities to invasion., p. 57-66. In: R. H. Groves and J. J. Burton, (eds.). Ecology of biological invasions: an Australian perspective. Australian Academy of Science, Canberra, Australia. 166 p.

GROSS, K. L. AND P. A. WERNER. 1982. Colonizing abilities of "biennial" plant species in relation to ground cover: implications for their distribution in a successional sere. Ecology, 63:921-931.

HOBBS, R. J. AND H. A. MOONEY. 1991. Effects of rainfall variability and gopher disturbance on serpentine annual grassland dynamics. Ecology, 72:59-68.

HUENNEKE, L. F., S. P. HAMBURG, R. KOIDE, H. A. MOONEY AND P.M. VITOUSEK. 1990. Effects of soil resources on plant invasion and community structure in Californian serpentine grassland. Ecology, 71:478-491.

McCLARAN, M. P. AND M. E. ANABLE. 1992. Spread of introduced Lehmann lovegrass along a grazing intensity gradient. J. Appl. Ecol., 29:92-98.

McINTYRE, S., P. Y. LADIGES AND G. ADAMS. 1988. Plant species-richness and invasion by exotics in relation to disturbance of wetland communities on the Riverine Plain, NSW. Aust. J. Ecol., 13: 361-373.

MENGES, E. S. AND D. M. WALLER. 1983. Plant strategies in relation to elevation and light in floodplain herbs. Am. Nat., 122:454-473.

MOONEY, H. A. AND J. A. DRAKE (EDS.). 1986. Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York. 321 p.

PLATT, W. J. 1975. The colonization and formation of equilibrium plant species associations on badger disturbances in a tall-grass prarie. Ecol. Monogr., 45:285-305.

RYKIEL, E. J. 1985. Towards a definition of ecological disturbance. Aust. J. Ecol., 10:361-365.

SAS INSTITUTE. 1985. SAS user's guide: statistics. SAS Institute, Cary, North Carolina. 956 p.

THOMAS, L. K., JR. 1980. The impact of three exotic plant species on a Potomac island. U.S. Natl. Park Serv. Monogr. no. 13. U.S. Department of the Interior. 179 p.

TREMMEL, D.C. AND K. M. PETERSON. 1983. Competitive subordination of a Piedmont old field successional dominant by an introduced species. Am. J. Bot., 70:1125-1132.

TYSER, R. W. AND C. A. WORLEY. 1992. Alien flora in grasslands adjacent to road and trail corridors in Glacier National Park, Montana (U.S.A). Conserv. Biol., 6:253-262.

WILSON, S. D. AND D. TILMAN. 1993. Plant competition and resource availability in response to disturbance and fertilization. Ecology, 74:599-611.

YANOSKY, T. M. 1982. Effects of flooding upon woody vegetation along parts of the Potomac River flood plain. U.S. Geol. Surv. Prof Pap. 1206. 21 p.
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Author:Pyle, Laura L.
Publication:The American Midland Naturalist
Date:Oct 1, 1995
Words:5093
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