Multiscale controls on woody riparian vegetation : distribution, diversity, and tree regeneration in four western Oregon watersheds

Graduation date: 2005I studied riparian forests of four western Oregon watersheds (dry south to wet north) to determine the multiscale controls on woody riparian vegetation. I conducted separate analyses of controls on plant distribution, diversity, and tree\ud regeneration using vegetation and environmental data collected in two related field studies: (1) a multiscale riparian forest inventory; (2) a comparative study of\ud natural forest gaps and interiors. Climatic moisture, indexed by vapor pressure deficit in summer, was the primary correlate of compositional change between riparian sites at all scales analyzed, demonstrating that the majority of riparian species responded directly or indirectly to the landscape scale climate gradient. Additional variation in composition was explained by measures of local topography and disturbance. Climate, as indexed by modeled gross primary productivity (GPP), explained the majority of the variation in multiple regression models of plant diversity that included local and landscape scale variables. As GPP increased from dry to wet climates, understory light and moisture heterogeneity decreased, coincident with declines in alpha, beta, and hectare scale diversity, suggesting that climate controls diversity indirectly through its effects on local conditions. Tree regeneration varied sharply across the climate gradient; seedling\ud frequency and diversity declined and nurse log use increased from the driest to wettest climates. Life history attributes of riparian tree species provided important\ud clues to their regeneration success in different environments. These relationships\ud were explored in a model that linked species shade and drought tolerance with expected variation in the environment caused by climate and disturbance. The model accurately predicted regeneration patterns for four of five functional groups of tree species. The studies in this dissertation provided compelling evidence of regional\ud variation in riparian vegetation composition, diversity, and dynamics, illustrating that these communities are strongly shaped by landscape scale as well as local scale factors. Moreover, climate-related differences among riparian sites were at least as important as the local variation within them in explaining spatial vegetation patterns. These findings argue for a multiscale perspective of riparian forest ecology that closely integrates larger scale controls, such as climate, with local hydrologic processes