It is unknown whether woody plant invasions in deciduous forests of Eastern North America (ENA) are enhanced or inhibited by herbivore preferences in the invaded range. Invasive shrubs from Europe and Asia tend to be more productive than native shrub species, and as a result may employ a tolerance strategy toward herbivore damage. We hypothesized that differences in defense traits between natives and invaders lead to differences in leaf palatability to generalist herbivores and that species face tradeoffs between investments in defense, growth, and storage. We measured leaf phenolics, lignin, cellulose, LDMC, and nitrogen content in 20 phylogenetically paired native and invasive species. We conducted feeding trials with two generalist invertebrate herbivores: invasive gypsy moth (Lymantria dispar dispar) and native fall webworm (Hyphantria cunea), and assessed the effects of the five defense-related leaf traits on caterpillar growth. Finally, we evaluated whether negative relationships exist—indicating physiological tradeoffs—between a composite of defense-related leaf traits and previously collected data on annual carbon gain, leaf production and woody carbon storage in this group of species.
Results/Conclusions
Invasive species had higher leaf nitrogen and lignin concentrations than native woody species. Though increased lignin might suggest lower palatability of invaders, it is unclear whether leaf nitrogen concentrations contributed to leaf structure (decreasing palatability) or photosynthetic function (increasing palatability). There were no differences in gypsy moth and fall webworm growth rates when fed native or invasive leaves, and no relationships between caterpillar growth rates and the measured leaf traits. Gypsy moth and fall webworm may cue into leaf characteristics we did not measure, like sugar content or toughness. Surprisingly, these two generalist herbivores had different feeding preferences, though both species grew vigorously on two invaders: Acer platanoides and Viburnum lantana. Thus, labeling herbivores as simply generalists or specialists is insufficient for predicting their effects on invading plant species. Finally, we found no evidence of tradeoffs between defense-related leaf traits, leaf production, and woody carbon storage. Woody invaders continue to spread in ENA forest understories, but we found no evidence of inhibition or facilitation by “generalist” herbivores, despite shrub species’ differences in productivity and leaf traits.