PARENT SESSION
Poster Session #8: Riparian Ecology.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Controls of plant species diversity in a desert riparian ecosystem.

Bagstad, Kenneth¹, Stromberg, Juliet¹, ¹

ABSTRACT- Riparian ecosystems in the southwestern United States face increasing threats of dewatering for urban and agricultural use. Depletion of surface and groundwater can alter the riparian plant community, leading to a loss of biodiversity and ecological function. In this study, we examined the plant community of eighteen sites along the San Pedro River in southeastern Arizona, to determine whether species diversity measures change across gradients of groundwater depth and surface flow frequency. Herbaceous species richness (species/m²) in the May/June dry season increased significantly with a qualitative ranking of site wetness for floodplain and streamside zones. After the late summer rainy season, richness was more dependent on site elevation than local hydrology. Multiple regression analysis showed that floodplain width, site relative wetness, and winter rainfall were most important in predicting species richness for the dry season, while only floodplain width and elevation predicted richness for the rainy season. Beta diversity (Whittaker's B_w), representing species turnover across a lateral gradient extending from the channel to the edge of the floodplain, increased in response to floodplain width and site relative wetness after the rainy season; no predictors were statistically significant for the dry season. Overall, local hydrologic and geomorphologic characteristics and site elevation all influenced species diversity patterns. Understanding patterns of spatial and temporal variability of plant diversity will aid in the development of vegetation monitoring techniques, including indices of ecosystem integrity, for use in management, conservation, and restoration activities.

KEY WORDS: riparian, groundwater decline, species diversity, ecosystem integrity