Nutrient cycling plays a critical role in maintaining biodiversity and ecosystem services in agricultural, urban, and natural lands. However, across landscapes there is substantial unexplained heterogeneity in nutrient cycling. Classic thinking holds that abiotic factors are the source of this spatial heterogeneity with a secondary role of plant biomass. However, recent work suggests that higher trophic levels or variation in traits at the level of plant genotype may also play an important role in structuring nutrient environments. For instance, herbivores may indirectly create heterogeneity in cycling through the induction of chemical and structural changes in plants traits. Phenotypic plasticity due to anti-herbivore defense may then alter nutrient cycling rates by changing the microbial breakdown of plant litter inputs. Alternatively, variation among plant genotypes in the expression of these same traits may overwhelm the influence of phenotypic plasticity on soil processes. Both genetic and environmentally based changes in plant traits have separately been demonstrated to alter soil processes, but their interaction and the relative importance of these sources of variation across local landscapes is unknown.
I address this question by developing a plant trait-mediated, conceptual framework of nutrient cycling. I then evaluate this framework within an old-field ecosystem by focusing on the dominant plant species, Solidago altissima , and its dominant grasshopper herbivore, Melanoplus femurrubrum , using a combination of lab assays, a greenhouse pot experiment, a field mesocosm experiment, and field surveys. First, I demonstrate that goldenrod individuals exhibit both genotypic variation and phenotypic plasticity in plant defensive trait responses across a nutrient and herbivory gradient in the greenhouse. At low nutrient supply, genotypes tolerate herbivory (inducing plant physiological changes that decrease the negative impact on fitness) while at high nutrient supply, the same genotypes induce a resistance response detectable through lower herbivore growth rates. These environmentally mediated changes in plant trait expression then altered the ability of a common microbial community to decompose senesced litter harvested from the same plants. Induced resistance in the population of genotypes grown at high nutrient levels led to decreased litter decomposition of herbivore legacy litter. In contrast, at low nutrient supply, herbivore legacy litter decomposed more efficiently compared to control litter. This suggests that the interaction between herbivory and nutrient supply could cause context-dependent acceleration or deceleration of nutrient cycling. As a result, trait plasticity may mediate effects of multiple environmental conditions on ecosystem processes in this system.
I tested this hypothesis using a three-year, raised bed, field experiment examining the effect of plasticity and locally relevant genetic variation on ecosystem processes in a naturalistic setting. Genotype clone clusters were planted in homogenized soil in enclosed cages with varying nutrient supply and grasshopper herbivory. Again, I documented strong genetically and environmentally-based trait variation in plant allocation, growth, and leaf traits. I next explicitly linked these genetic and plastic functional trait changes to concurrent changes in a variety of soil processes (microbially available carbon, plant available nitrogen, nitrogen mineralization potential, and microbial biomass) and litter decomposition rates. Importantly, partitioning functional trait variation into genetic and environmental components improved explanatory power. I also documented potential differences in herbivore effects on "slow" vs. "fast" cycling in soil microbially available C pools. Within both experiments the magnitude of trait variation measured was similar to the variation expressed by individuals across a focal field.
Taken together, this dissertation demonstrates that plant genotype, herbivores, and nutrients can all modify litter decomposition and other soil processes within ecosystems through differential expression of plant functional traits. Due to the spatially clumped, clonal, and dominant nature of goldenrod, the genetic and herbivory-driven changes documented here could lead to a predictable mosaic of soil process rates across a single old field landscape. This work also highlights the complex interplay between genetically and environmentally-based trait variation in determining population and ecosystem processes within landscapes and improves our understanding of the often-overlooked indirect effects of plant/herbivore interactions on nutrient cycling It suggests that herbivores may shape not only the evolution of plant populations, but also the soil nutrient environment and microbial community in which plants live. This sets up the potential for eco-evolutionary feedbacks between plant defense expression and soil nutrient availability. More broadly, it suggests that biotic factors, in addition to abiotic ones, play a key role in determining local-scale soil nutrient availability patterns and should potentially be accounted for within ecosystem models. These results are particularly salient in a world where anthropogenic nitrogen inputs continue to rise and climate change is predicted to increase herbivory and thus plant defensive trait induction on landscapes.
|Advisor:||Schmitz, Oswald J.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 78/07(E), Dissertation Abstracts International|
|Subjects:||Ecology, Evolution and Development, Soil sciences|
|Keywords:||Ecosystem Processes, Genetic Variation, Induced Defense, Nutrient, Plasticity, Solidago|
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