Riparian Plant Genotype by Environment Interactions:Eeffects on Leaf Litter Quality, in-Stream Decomposition, and Aquatic Biodiversity

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Litterfall from riparian vegetation provides a critical energy source for many stream food webs. The quality of those litter inputs strongly influences their rate of decomposition in the stream and the diversity and productivity of aquatic organisms they support. The extent to which genetic variation within a single species or hybrid complex contributes to variation in litter quality is not well understood, but could be critical, particularly in high-elevation riparian ecosystems in the southwest, where two species of cottonwoods and their hybrids constitute 80-95% of the riparian vegetation. Furthermore, the effects of environmental stress such as insect herbivory and drought are known to depend on cottonwood genotypes, but how such genotype by environment interactions affect leaf litter quality and in- stream processing is not known for cottonwoods, nor, to our knowledge, for any other riparian species.This study will determine whether aquatic biodiversity and ecosystem processes are similarly linked to cottonwood hybrid zones. Common gardens with pure and hybrid cottonwoods of known genotypes will be used to determine how genetic variation within the cottonwood hybrid complex affects litter quality, in- stream decomposition (including mass loss and nutrient release), and colonization and diversity of microbial decomposers (bacteria and fungi) and invertebrates. This experimental work will be supplemented with natural surveys of litter entering streams to determine the importance and distribution of litter from each of four cottonwood cross-types (Fremont, narrowleaf, F1, and backcross) that are naturally ubiquitous in watersheds from Colorado to Mexico. Finally, controlled experiments crossing cottonwood cross-types and environmental stress (drought and herbivory) will be used to determine how genetic by environment interactions alter litter quality and litter processing and biodiversity in streams. This research will address these critical unknowns in our understanding of aquatic community and ecosystem processes and terrestrial-aquatic interactions.