PARENT SESSION
Oral Session #70: Riparian Ecology. Presiding: R. Cosgriff.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas I.

Woody plant diversity in western Oregon riparian forests: Multiple scales of control.

Sarr, Daniel¹, Hibbs, David¹, ¹

ABSTRACT- Understanding the processes regulating plant diversity and the scales of their operation is a central pursuit of plant ecology. In this study we analyzed patterns of woody plant diversity in riparian forests of 4 western Oregon watersheds sampling the strong climatic gradient in the region, from the wet northern Coast Range to the dry eastern Siskiyou mountains. We sampled woody plant diversity in sixteen 1-ha sample reaches in each watershed, using eighteen (40 m2) microplots on topographic transects within each 1-ha plot. Geomorphology and forest stand characteristics were quantified in each microplot. The nested approach allowed comparisons of patch-scale, hectare-scale, and watershed level patterns of species richness. To evaluate fine-scale disturbance, we revisited 24 streams to sample vegetation in naturally occurring forest gaps. Woody plant richness increased markedly with aridity, from 20.3 species/ha in the Coast Range to 33 species/ha in the Siskiyou mountains, and patterns were similar at all the sample scales. Changes in diversity were correlated with light competition at the patch-scale, and habitat differentiation along soil moisture gradients, both of which varied strongly along the climate gradient. Northern forests had higher foliar cover, and more muted topographic moisture gradients than southern forests. Disturbance gaps harbored more species than non-gaps in all four watersheds. Our analyses suggest that species diversity in these riparian forests is regulated through a hierarchy of controls comprising climate, environmental heterogeneity, and disturbance.

KEY WORDS: riparian ecology, plant diversity, forests, disturbance