Nitrogen additions and natural enemies in eastern forests: native and exotic species responses to resource manipulation in two community types.
Leger, Elizabeth*,1, Howe, Katherine1, 2, Gurevitch, Jessica1, Woo, Eliza1, Hickman, Jonathan1, Lerdau, Manuel1, 1 State University of New York, Stony Brook, NY, United States2 The Nature Conservancy of Indiana, Indianapolis, IN
ABSTRACT- Nitrogen availability impacts both plant growth and the preferences of herbivores. Here we consider how these two factors may interact to affect the performance of native and exotic species, using phylogenetic pairs of species grown in pine barrens and hardwood forests on Long Island, New York. Plants were grown in ambient and nitrogen-enhanced conditions in both of these community types, and we measured plant size, growth rates, and the number of leaves lost to herbivores throughout the course of the experiment. Nitrogen additions produced an initial increase in plant growth relative to control plants in both community types, but by the end of the experiment, the effect of nitrogen on plant size and growth rates varied significantly by the community where plants were grown and whether a plant was native or exotic. Nitrogen additions in pine barrens, which are naturally lower in nutrients (including nitrogen) than hardwood forests, caused native plants to increase in size and relative growth rate relative to control plants, while exotic plants did not perform significantly better under enhanced nitrogen treatments. In contrast, an interaction between herbivory and nitrogen favored exotic species in hardwood forests. Plants that received additional nitrogen lost significantly more leaves than plants in control plots in hardwood forests. These effects were strongest for native species, and the increase in the number of leaves lost by native species in nitrogen treatments led to a decrease in plant size and relative growth rate. Exotic species also experienced increased leaf-loss in nitrogen treatment plots in hardwood forests, but significantly less so than native species, and exotic plants were able to compensate for this loss, showing no declines in size or growth rate despite the increased damage. This type of interaction between increased nutrients and increased damage may be common in situations where native plants are growing with high nitrogen levels. If native species experience higher overall rates of damage by natural enemies, interactions between nitrogen and natural enemies could help to shift the competitive advantage to exotic species in nitrogen-enhanced environments.
Key words: invasive species, nitrogen, herbivory
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