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Experimental tests of factors promoting forest invasibility.
Gurevitch, Jessica*,1, Howard, Timothy1, 2, Hyatt, Laura1, 3, 1 SUNY at Stony Brook, Stony Brook, NY2 New York Natural Heritage Program, Albany, NY3 Rider University, Lawrenceville, NJ
ABSTRACT- The purpose of this research was to examine why some forest ecosystems are vulnerable to biological invasions while others appear to be more resistant. Despite the threat that biological invasions pose for native forest plants and animals, little previous experimental work has been carried out in forests to test the importance of hypothesized causes of resistance or susceptibility of forest communities to plant invasions. We report here on the first of several field experiments testing the relative roles of specific factors in determining community vulnerability to invasion by exotic plants. We compared the responses of a suite of common native plants and invasive exotics to experimental gaps and nitrogen addition in contrasting sites: low diversity, low fertility pine barrens and more fertile hardwood forests. Native and invasive species responded differently to the experimental treatments. Natives grew well in both pine barrens and hardwood forests, but invasives grew well only in hardwood forests. Surprisingly, native species grew more rapidly than invasive species overall. Invasive species had a strong positive response to N in the pine barrens; adding N to pine barrens soils was the only way that invasives were able to maintain positive growth rates there. All species responded negatively to the addition of N in the hardwood forests, probably due to fertilizer lowering soil pH. Native species responded very little to the addition of N in the pine barrens. Both native and invasive species had moderate postive responses to the creation of gaps in the pine barrens. Invasive species were suppressed more by the canopy in hardwood forests than in pine barrens, and responded more strongly than did native species to the creation of gaps in the hardwood forest sites.
Key words: experimental, pine barrens, plant invasions, forest community