Land-use history and fire management drive context-dependent plant-herbivore interactions

Land-use legacies often interact with other factors such as fire suppression to generate persistent changes in soil conditions and plant communities in many ecosystems. However, little is known about how these habitat modifications affect higher trophic levels or interactions between plants and herbivores. Working in longleaf pine woodlands, I found that agricultural land-use history altered grasshopper community composition and decoupled positive relationships between plant cover and grasshopper abundance. Herbivory on a palatable plant species was also affected by the interaction between land-use history and fire frequency, and exhibited a hump-shaped response to neighboring plant cover, but herbivore density did not affect herbivory rates. In an experiment that manipulated neighborhood (10m2) plant density and frequency within post-agricultural and non-agricultural habitats, I found that the frequency of neighboring plants determined the amount of herbivore damage received by focal plants. These patterns were driven by herbivore foraging decisions made at the neighborhood scale, highlighting that behavioral decisions are important for generating associational effects. Collectively, these results suggest that the effect of historic agricultural land use extends beyond plants and soils to leave a strong legacy on present-day plant-herbivore interactions.;Using a landscape-scale experiment, I showed that the effect of excluding herbivores on plant growth can be contingent upon canopy structure, historical land-use, and the spatial arrangement of these two factors. Land-use history and canopy structure also affected the degree to which herbivores limit seedling establishment across a suite of plant species. Furthermore, we found strong negative effects of herbivores on the performance of some demographic stages of all four herbaceous plant species we tested, particularly juvenile plant survival, but only in habitats with intact overstory canopies. The exception was the slowest growing plant species, which also suffered reduced juvenile survival due to herbivory in some thinned treatments. Overstory tree thinning substantially increased adult flowering and size for all four species and the effect of thinning on adult performance was often greater in non-agricultural habitats. Collectively, this work experimentally separates the habitat-induced effects from the herbivore-induced effects on plant performance and highlights that context-dependent plant-herbivore interactions depend on plant demographic stages and species traits.