Thesis

Perennial access to water is associated with variation in stand structure in river red gum in San Diego County

The importance of perennial access to water for invasive plants in arid environments was examined at eighteen sites containing Eucalyptus groves in proximity to a water source within San Diego County between June I stand September 30111, 2009. Diameter at breast height (d.b.h.) measurements, a useful indicator of tree size, were measured along transect lines at each site prior to September. D.b.h. distribution curves were examined for significant gaps which should indicate missing cohorts, which were expected where water is intermittent due to variation in germination rates and seedling mortality during drought conditions. In September, the peak of the dry season, each site was evaluated for water presence in the stream bed, evidence of seedling mortality and surface soil moisture. Arc View 9.2 software was used to gather data on elevation, slope (mean, s), proportion of sandy loam type soil present, and to digitize polygons around the canopy area of study sites. Soil type, surface soil moisture, seedling prevalence, slope, and September presence of water were evaluated as predictors of total canopy area, proportion of seedlings present, and proportion of significant gaps in the size class distribution. The total canopy area and proportion of seedlings could not be predicted using the measured variables, however, the proportion of significant gaps was explained by slope (s), seedling proportion, gap cutoff size, soil moisture at Om from the stream bed and September water presence (an indicator of perennial access to water). The low number of gaps seen at sites with access to perennial water sources suggests that continuous access to water is a risk factor for river red gum establishment and recruitment. Alteration of the natural flow regime, from seasonal to perennial, by supplemental water inputs may represent an emerging threat to riparian ecosystems in semi-arid environments. KEYWORDS California, Eucalyptus, impacts, invasive, riparian, water Item only available to the CSUSM community. Authentication with campus user name and password required.

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