PARENT SESSION
Poster Session #46: Riparian Ecology.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Groundwater and surface water thresholds for maintaining cottonwood-willow stands along the San Pedro River, Arizona.

Lite, Sharon^*,1, Stromberg, Juliet², ¹ Arizona State University, Tempe, AZ² Arizona State University, Tempe, AZ

ABSTRACT- Groundwater and surface flow declines are threatening the integrity of riparian ecosystems throughout the southwestern United States. Due in large part to these altered hydrologic conditions, pioneer tree communities are shifting from native Fremont cottonwood-Goodding willow (Populus fremontii-Salix gooddingii) forests to woodlands of the exotic tree/shrub saltcedar (Tamarix ramosissima). Saltcedar woodlands often have different ecological functions and sometimes offer lower quality wildlife habitat than cottonwood-willow forests. While previous work has focused on the physiological responses of these species to decreases in water availability, little attention has been given to shifts in community and population structure in response to decreases in surface flow frequency and groundwater levels. This study identified how cottonwood, willow, and saltcedar stand canopy cover, vegetation volume, basal area, and stem density varied across a spatial gradient of water availability along the San Pedro River in Arizona. Pearson correlation analyses were used to test for significant relationships between structural traits and hydrologic variables. Results showed that saltcedar woodlands are the dominant pioneer tree community at drier sites, whereas cottonwood-willow forests dominate the more mesic sites. These shifts in species composition correspond to changes in community structure. Sites at the dry end of the hydrologic gradient had lower total vegetation volumes, higher stem densities, and higher percentages of small stems than sites with perennial flow and shallow groundwater. Cottonwoods and willows had more size classes at the wetter sites, indicating higher age class diversity in these areas. These variations in species composition and community structure can have implications for the functioning of the ecosystem. For example, increased densities of small stems may trap more sediment and cause changes in channel and flood plain morphology. The decrease in total vegetation volume may produce less desirable wildlife habitat for some species while the higher stem densities may favor others.

KEY WORDS: San Pedro River Arizona, vegetation structure and composition, riparian vegetation, hydrologic gradient