Plant defense strategies as functional traits: Nutrient availiability alters the ecosystem impact of herbivores in Solidago altissima

Background/Question/Methods

Recently, scientists have become very interested in connecting genetic material to ecosystem processes through plant traits (e.g. genes to ecosystems). However, genetic effects are modified by interactions with the environment that can change phenotypic expression of key traits. If phenotypes are quite plastic than the environment may be a better predictor of the impact of plants on ecosystem processes than genotype. A preliminary greenhouse experiment demonstrated that S. altissima alters plant defense strategies across nutrient environments with characteristic suites of plant trait responses to Melanoplus femurrubrum herbivory (leaf toughness, nutrient content etc.) in each context. Because these traits are also a major determinant of decomposition in ecosystems, plasticity in plant defense strategies across contexts may play a strong role in mediating ecosystem processes. However, it is a challenge to understand how these individual plant responses scale within real ecosystems that are characterized by spatially clumped genotypes and nutrient environments. Here, we present results for a multi-year field mesocosm experiment designed to explicitly test the role of induced plant defense strategies and nutrient environment on ecosystem processes. We planted four genotypes of S. altissima from the greenhouse experiment within raised beds of homogenized soil and then imposed herbivory and nutrient treatments. 

Results/Conclusions

Over the two-year interval of the experiment we monitored the raised beds for changes in plant traits, community dynamics, soil N and C, litter decomposition and microbial biomass. We found that plant trait expression within the field mesocosm experiment mirrored the results in greenhouse experiment with higher resistance at high nutrient levels and tolerance at low nutrient levels and a different magnitude response between genotypes. Herbivory also increased leaf toughness and LMA and occurred in conjunction with below-ground alterations in dynamics such as a decrease in litter decomposition and an increase in soil C mineralization. In addition, laboratory assays are examining how microbial communities with different herbivore and nutrient treatment histories perceive leaf litter from plants grown in different herbivore and nutrient contexts. Taken together, this experiment and decomposition assay presents a comprehensive look at whether phenotypic plasticity in the expression of plant defensive strategies has characteristic impacts on plant functional traits and thus ecosystem processes across contexts. Using this framework, functional traits may be useful tool for linking the evolution of plant defensive syndromes to ecosystem context and possibly bring to light eco-evolutionary feedbacks within old-field ecosystems.