Effects of warming and nitrogen on above- and below-ground herbivory of an exotic invasive plant and its native congener

Warming and atmospheric nitrogen deposition could impact plant community composition by altering competitive interactions, however, the effects of these environmental changes on plant invasions via above- and below-ground herbivory are unknown. Here we report the effects of warming and nitrogen addition on aboveground insect defoliation and belowground root-knot nematode infection of a native plant and an introduced invasive congener. Warming increased belowground nematode infection only, while nitrogen addition increased both nematodes and defoliation. Defoliation rates were similar for the exotic invasive and native species and the increases with nitrogen addition were large (almost doubled) but comparable. However, roots of native plant were more intensively infected (i.e., knot density) than roots of the exotic invasive plant (similar to 4-fold in ambient conditions) and this difference increased under elevated temperature (similar to 30-fold higher) in which total nematode infection were nearly tenfold higher. Compared to the exotic invasive plant, the native plant had a higher proportion of fine roots and specific leaf area, but lower photosynthesis ability irrespective of warming and nitrogen deposition treatments. The nematode preferred fine roots to coarse roots for both plant species. Our study indicates that above- and below-ground herbivory of plants differ in their sensitivity to varying drivers of environmental change, which may alter plant interactions and makes it difficult to predict future community structure. Together with the dramatic response of belowground nematodes to warming, this suggests that future modeling or experimental studies on species' responses to environmental change should simultaneously consider above- and below-ground communities.