Spatial arrangement of canopy structure and land-use history alter the effect that herbivores have on plant growth

The spatial arrangement of past and present human activities could affect the strength of species interactions through changes to environmental conditions. To better understand how land-use history might mediate the effect of insect herbivory on the growth of four herbaceous plant species at the edges between woodlands and open savannas, we coupled large-scale manipulations of canopy structure by thinning overstory trees to create savanna habitat in non-agricultural and post-agricultural longleaf pine woodlands with local-scale herbivore exclosures at two distances from habitat edges. Light availability, grasshopper abundance, and herbaceous plant cover all increased in patches that received overstory tree thinning. Land-use history altered the effect that edges created by the overstory thinning treatments had on grasshopper herbivory for one plant species (Carphephorus bellidifolius). The edge between land-use histories also altered herbivore effects on three plant species in thinned patches. These edge effects were driven in part by changes in availability of alternative resources for grasshoppers, shady edges, or grasshopper abundance. Canopy manipulation also had important effects on herbivory that were independent of the habitat edge: grasshopper herbivory reduced plant growth of the smallest species (C. bellidifolius) regardless of canopy manipulation, whereas herbivory on two larger species (Solidago nemoralis and So. odora) reduced plant growth mainly in patches with an intact overstory canopy. Collectively, this work suggests that the species-specific effects of grasshopper herbivory on plant growth were driven by both plant traits (e.g., size or growth rates) and landscape-mediated changes to grasshopper abundance, light availability, or availability of alternative resources for grasshoppers created by land-use history and canopy structure. Furthermore, because the heterogeneity in herbivory revealed by our large-scale experiment is likely linked to plant traits, our results provide a means for understanding the profound, but often idiosyncratic, effects of land-use legacies and edges on plant populations and communities.