Plant defense against generalist herbivores in the forest understory: a phylogenetic comparison of native and invasive species

Invasive woody plants from Europe and Asia tend to be more productive than co-occurring native species in deciduous forests of Eastern North America, but community-scale drivers of invasion success remain unknown. If increased productivity in invaders relative to natives comes at the expense of reduced relative allocation to defenses, generalist herbivores may preferentially consume invasive species, potentially reducing the effectiveness of invader growth advantage. We compared leaf traits related to herbivory (nitrogen concentration, total phenolics, cellulose and lignin concentrations, and leaf dry matter content) in 20 phylogenetically paired native and non-native, invasive species and evaluated species palatability to two generalist herbivores: one non-native (European gypsy moth, Lymantria dispar) and one native (fall webworm, Hyphantria cunia). We also evaluated potential physiological tradeoffs between defense-related leaf traits, leaf production, and carbon storage in this group of species. Invasive plants had higher leaf N and lignin concentrations than related native species (0.5% and 5%, respectively), but gypsy moth and fall webworm growth were not associated with plant nativity. Plant defense traits did not predict differences in gypsy moth growth, but fall webworm growth was marginally negatively associated with leaf N. We found no evidence of tradeoffs between defense-related leaf traits, leaf production, and carbon storage, indicating a limited role for carbon-based tradeoffs relating to growth and herbivory in these species, although other unmeasured carbon pools including reproduction may impact these relationships. Woody invaders continue to spread in Eastern North American forest understories, but we found no evidence of inhibition or facilitation by generalist insect herbivores, despite differences among species in leaf traits associated with defense.