Recent interest in using trait-based approaches to understand and predict ecosystem processes and evolutionary responses to environmental change (both biotic and abiotic), highlights the need to understand the relative importance of genetic and environmental sources of intraspecific trait variation within local populations of dominant species. Here, I combine plant defense theory with functional approaches to quantify genetic trait variation and phenotypic trait plasticity of nine goldenrod (Solidago altissima) genotypes derived from a local field population in Connecticut, USA to herbivory along a nutrient supply gradient. I found that increasing nutrient supply changed the dominant plant defense strategy from tolerance to induced resistance. Induced resistance was detected through decreased herbivore growth rates and a behavioral feeding shift of grasshoppers to older leaf tissue. This could not be fully accounted for through stoichiometric changes in leaf tissue quality. A multi-dimensional phenotype approach revealed that abiotic and biotic environments (nutrients and herbivory) accounted for almost as much whole-plant trait variation (31%) as did plant genotype (36%). Increasing nutrient supply and herbivory resulted in independent and differential effects on whole-plant trait expression. Increasing both treatments concurrently produced a unique plant phenotype with increased leaf carbon content and allocation to asexual reproduction (ExE). Notably, individual genotypes exhibited different magnitudes of multivariate trait plasticity to nutrient and herbivory gradients. However, the population of genotypes as a whole within a given environment expressed an approximately equal magnitude of trait variation across both permissive (high nutrient, no herbivory) and stressful (low nutrient, high herbivory) environments. Quantifying plasticity in defensive strategy in concert with correlated whole-plant trait expression changes across multiple abiotic and biotic factors may be key to providing a mechanistic understanding of how heterogeneous landscapes impact community interactions and ecosystem processes.
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