| Along arid-region rivers, water availability and flood disturbances are two primary factors structuring the riparian community. Data were recorded from 343 study plots along 18 San Pedro River, Arizona, sites that spanned water availability and flood disturbance gradients. Correlation and single and multiple regression analyses were used to determine lateral and longitudinal relations between vegetation composition, structure, and richness (dependent) and flood disturbance and water availability (independent) variables. Quantifying the environmental conditions at which changes in vegetation occur can inform management and conservation strategies.; Woody vegetation community traits were more strongly related to longitudinal gradients than to lateral. Longitudinal species richness increased at wetter sites; richness within functional groups varied along water availability and flood disturbance gradients. Plant communities became more xeric as site water availability decreased, in part due to the replacement of Populus fremontii and Salix gooddingii by Tamarix ramosissima at drier sites. Reflective of this shift in species composition, dry sites had more shrublands and shorter vegetation. Stem densities of young pioneer trees increased downstream, and all age classes of P. fremontii and S. gooddingii were most abundant at wet/high-disturbance sites. Laterally, vegetation biomass was greater on near channel, shallow ground-water surfaces. Species abundances and richness within functional groups varied in response to lateral stress and disturbance gradients.; Two models were developed for riparian management and condition assessment. The threshold model identified that P. fremontii and S. gooddingii maintained tall dense stands with diverse age classes at sites with ground water depths less than 2.9 m, annual ground-water fluctuation less than 0.6 m, and surface flow present more than 73% of the months in a two year period. These thresholds can help define management goals and be used to predict shifts in vegetation community composition. The Riparian Condition Index, designed to diagnose ecosystem condition change from site dewatering, is based on nine vegetation traits that were sensitive to changes in surface flow duration and ground-water levels. Riparian condition classes are based on surface flow duration and ground-water depth and fluctuation; each condition class is associated with ranges for each indicator variable and levels of functional capacity. |
