Grantee Research Project Results
2004 Progress Report: An Experimental Study of Biological Invasions in Forests of the Eastern United States.
EPA Grant Number: R828900Title: An Experimental Study of Biological Invasions in Forests of the Eastern United States.
Investigators: Gurevitch, Jessica , Leger, Elizabeth , Howe, Katherine , Hyatt, Laura , Lerdau, Manuel
Current Investigators: Gurevitch, Jessica , Lerdau, Manuel
Institution: The State University of New York at Stony Brook
EPA Project Officer: Packard, Benjamin H
Project Period: September 20, 2001 through February 19, 2004 (Extended to September 19, 2005)
Project Period Covered by this Report: September 20, 2003 through February 19, 2004
Project Amount: $451,553
RFA: Exploratory Research to Anticipate Future Environmental Issues (2000) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Water , Aquatic Ecosystems
Objective:
The invasion of native ecosystems by nonnative plants and animals is a major problem facing the United States in the 21st century, as well as being an enormous environmental problem globally. Invasive species can cause enormous detrimental effects to natural systems and have enormous economic costs. A major objective of this research project is to test specific hypotheses about what makes different natural communities more or less likely to become invaded by exotic species. We are conducting experiments in a range of protected forests in eastern Long Island, New York. These forests vary in species composition, soils, and the degree to which they are invaded by nonnative species. Although some forests are invaded heavily by alien plants, some hardwood forests and nearly al pitch-pine-dominated pine barrens appear to resist the establishment of nonnative species.
We carried out experiments to test the effects of light, nitrogen, and calcium availability on forest invasibility (2001-2003) and tested whether interactions between nitrogen and herbivory could favor exotic species (2004). We studied the ecosystem impacts of invasion by measuring the dynamics of litter decomposition (mass and nutrient loss) from leaves of native and invasive species (2001-2004). These are the first experimental studies to test specific factors regulating the invasibility of eastern forests, combining ecological community and ecosystem approaches to better understand invasion in these ecological systems.
Progress Summary:
This report summarizes the results from all experiments to date.
Results From Field Experiments Conducted Between 2001 and 2003
Responses to gaps were strongest for invasives in hardwood forest sites; responses of both invasives and natives to gaps in the pine barrens were smaller. Gaps consistently had far greater effects on plant performance than did nutrient manipulations. Invasive species did not grow faster than natives on average, but invasive plants had a greater positive response to gaps. Invasives allocated more biomass to shoots than did natives; natives shifted allocation in response to site differences and nitrogen addition, but invasives did not.
There was an interaction between nitrogen availability and loss of leaves to natural enemies that favored invasive species in hardwood forests. Native species always received more damage by herbivores, and all plants received more damage under nitrogen addition treatments, but in the hardwood forests, nitrogen addition increased herbivory on native species to a much greater degree than on exotic species, to the extent that they were no longer able to tolerate herbivory and had reduced growth rates.
Decomposition rates of both native and exotic taxa were accelerated in invaded hardwood forest communities compared to uninvaded hardwood forests, and all species decomposed more slowly in the pine barrens than in hardwood forest sites. Litter decomposition rates and nutrient release rates of exotic taxa were faster than those of closely related native taxa, despite similar macrodecomposer communities. There were differences in decomposition between community types. Average mass loss was more rapid in hardwood forest than pine barrens sites, mass and nitrogen loss were more rapid in invaded than uninvaded sites, and decomposition was faster for exotic than native species. In addition, percent leaf nitrogen was higher in invaded than uninvaded hardwood forest sites suggesting either that invasive species were altering nutrient dynamics in these sites or that there were underlying differences between sites that were invaded and those that were not. We also found that when litter from different species is placed in mixtures, patterns of mass-loss, nutrient transfer, and decomposer activity and abundance are frequently nonadditive (that is, these patterns cannot be predicted by the dynamics of each species decaying singly).
Our results suggest that gaps created in the pine barrens are unlikely to facilitate invasions, but gaps in these hardwood forests likely are to increase invasion strongly. Nitrogen addition may increase enemy pressure on native species, which could confer a competitive advantage to invasive species. In addition, our litter decomposition experiments suggest that invasive plant taxa may have large effects on ecosystem processes that may facilitate further invasion.
Details of Experiments Conducted in 2004
In the summer of 2004, a final field experiment was conducted to test the interaction between nitrogen and herbivory in hardwood forests and pine barrens. Ten phylogenetically paired native and invasive species (Acer platanoides, Acer rubrum, Lonicera maackii, Diervilla lonicera, Celastrus orbiculata, Celastrus scandens, Elaeagnus umbellata, Ceanothus americanus, Ampelopsis brevipedunculata, and Vitis riparia) were grown in six locations (three hardwood forests, three pine barrens). Plants were grown in both nitrogen-enhanced and ambient conditions, and the number of leaves per plant was tracked throughout the experiment. Above- and belowground biomass of all plants was collected, dried, and weighed.
We tested the effects of nitrogen and herbivory on the following response variables: total size, root-to-shoot ratio (RTS), relative growth rate, carbon-to-nitrogen ratio, and number of leaves lost throughout the course of the experiment. Data were analyzed using the PROC MIXED procedure from SAS, testing the following fixed effects and interactions between them: community type (pine or hardwood), nitrogen treatment, cage treatment, and status (native or nonnative). Random factors also were included in the model (family, site, block, plot), but with the exception of family, are not presented here.
Exotic species were neither consistently nor significantly larger than natives in either community type (no significant main effect of status on plant weight, F = 0.02, P = 0.8993, or Community x Status interaction, F = 0.63, P = 0.4488). RTS ratios did vary by community and status (significant Community x Status interaction on RTS, F = 6.95, P = 0.0310). Invasive species significantly increased their RTS ratio when grown in pine barrens (P = 0.0321), whereas natives did not ( P = 0.6997, Figure 1).
Figure 1. RTS Ratios of Native and Exotic Plants Growing in Hardwood Forests and Pine Barrens. Invasive plants significantly increase their RTS ratio when grown in pine barren communities, whereas native plants show a similar but nonsignificant trend. Values are untransformed means and standard errors.
Adding nitrogen affected plant size differently in pine barrens and hardwood forests (significant Community x Nitrogen interaction for plant size, P = 0.0004). Plants increased their overall size and relative growth rate when nitrogen was added in pine barrens ( F = 5.73, P = 0.0203), but not when nitrogen was added in hardwood forests (F = 1.72, P = 0.1949). Native and invasive species responded differently to the nitrogen treatment in the two community types (significant three-way Community x Nitrogen x Status interaction, F = 4.61, P = 0.0359). Both native and invasive species tended to increase in size in response to nitrogen treatments in the pine barren communities (Figure 2b), but the difference between control and nitrogen treatments was strongest and only statistically significant for native plants (Natives: F = 5.31, P = 0.0248; Exotics: F = 1.10, P = 0.2982). In contrast, plants did not significantly increase their size when nitrogen was added in hardwood forests (Figure 2a), and native plants tended to decrease in size under nitrogen-enhanced conditions (Natives: F = 2.66, P = 0.1082; Exotics: F = 0.03, P = 0.8671; RGR: F = 0.07, P = 0.7915).
Figure 2. Effect of Nitrogen Addition on Plant Size of Native and Exotic Plants Grown in Hardwood Forests (a) and Pine Barrens (b). Values are least square means of log-transformed mass measurements.
Native plants lost more leaves than exotic plants overall (F = 9.54, P = 0.0068), with exotic plants loosing an average of 3.4 leaves (+/- 0.14 standard error [SE]), whereas native plants lost an average of 4.0 leaves (+/- 0.12 SE). Adding nitrogen significantly increased the chances that all plants would loose leaves through the course of the experiment (F = 6.45, P = 0.0297), with plants in the nitrogen-addition plots losing an average of 4.0 leaves (+/- 0.14 SE) and plants in the control treatments losing an average of 3.42 (+/- 0.12 SE) leaves.
Nitrogen addition affected leaf-loss differently in pine barrens and hardwood forests (F = 6.33, P = 0.0146). Plants that received additional nitrogen in hardwood forests lost significantly more leaves than plants in the control treatment, and though the trend was similar in pine barrens, the difference between control and nitrogen enhanced treatments was not statistically significant (Figure 3a, hardwood forests: F = 11.92, P = 0.0010; Figure 3b, pine barrens: F = 0.93, P = 0.3380). Native plants consistently lost more leaves than exotic plants, and they also lost more leaves than exotics when grown in increased nitrogen treatments. Contrasts demonstrate that the loss of leaves by natives in the nitrogen plots in hardwood forests was strongly significant (F = 14.47, P = 0.0003), whereas the pattern, though in the same direction, was not as strong for exotic species (F = 1.15, P = 0.2874). Neither native (F = 0.661, P = 0.4191) nor exotic (F = 0.31, P = 0.5824) plants lost significantly more leaves in the nitrogen-treated plots in the hardwood forests.
Figure 3. Effect of Nitrogen Addition on Leaf-Loss of Native and Exotic Plants Grown in Hardwood Forests (a) and Pine Barrens (b). Values are untransformed means and standard errors, though analyses were conducted on transformed data.
We conducted a third decomposition experiment in 2004 examining the differences in decomposition of native and exotic invasive litter when decomposed singly and in two-species combinations in a single site. Three congeneric pairs of native and exotic invasive species were used. Each species was decomposed singly as well as in each possible two-species combination. Samples were collected from the field in the fall of 2004 and currently are being analyzed for carbon and nitrogen.
Future Activities:
We are completing our analysis of carbon-to-nitrogen ratio data for our field experiments and continuing our analysis and writing of our experiments testing factors regulating forest invasibility in the eastern United States and litter decomposition between native and invasive species and in invaded and uninvaded sites.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 35 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Ashton IW, Hyatt LA, Howe KM, Gurevitch J, Lerdau MT. Invasive species accelerate decomposition and litter nitrogen loss in a mixed deciduous forest. Ecological Applications 2005;15(4):1263-1272. |
R828900 (2004) R828900 (Final) |
Exit Exit |
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Gurevitch J, Padilla DK. Are invasive species a major cause of extinctions? Trends in Ecology & Evolution 2004;19(9):470-474. |
R828900 (2004) R828900 (Final) |
Exit Exit |
Supplemental Keywords:
forest invasion, invasibility, herbivory, exotic plants, decomposition, pine barrens, Long Island, northeastern US, experimental manipulation, field experiments, invasion ecology, water, land, soil, ecological effects, population, susceptibility, nitrogen oxides, ecosystem, terrestrial, habitat, public policy, conservation, biology, ecology, northeast, midatlantic, New York, NY, forestry,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Ecosystem/Assessment/Indicators, Ecosystem Protection, Forestry, Monitoring/Modeling, Ecological Effects - Environmental Exposure & Risk, Ecological Risk Assessment, Biology, Exp. Research/future, Futures, emerging environmental problems, extinction risk, ecological exposure, biodiversity, endangered species, forest, biopollution, runoff, shrubland, Eastern U.S., exploratory research, invasive species, forests, ecological dynamics, rainfallProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.