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Plant diversity response to grazing on spring-fed riparian areas of California's oak woodlands.
JACKSON, RANDALL1, ALLEN-DIAZ, BARBARA1, 1
ABSTRACT- We monitored species composition at 3 spring-fed sites in each of 3 watersheds from 1992 thru 2000. Each site maintained 2 geomorphologically-defined vegetation types - a spring-fed wetland zone and a resultant 1st-order creek. All sites had similar Fall-Winter-Spring, moderately grazed histories of 100 g⋅m-2 residual dry matter (RDM) in surrounding uplands until 1993 when sites within watersheds were randomly assigned to grazing exclusion (UG, 200 to 300 g upland RDM⋅m-2), light grazing (LG, 150 to 180 g upland RDM⋅m-2), or moderate grazing (MG, 90 to 110 g upland RDM⋅m-2) treatments. We used Sorensen's index (%D) on species abundance (cover) data to track interannual plot dissimilarity. UG wetlands were on average 63% different interannually while LG and MG wetlands were 46% and 45% different, respectively. No significant differences in %D were found among grazing treatments at creeks, although creek plots exhibited mean interannual differences of 54%. We regressed %D values calculated for 1992 v. each subsequent year on time-since-treatment finding a significant positive trend (2% per year) indicating subtle directional change at wetlands regardless of grazing treatment. No directional trends were evident at creeks. No significant differences in species richness (relative to pretreatment 1992 values) were observed at wetland areas, though LG wetlands maintained greater species evenness and diversity (Shannon-Weaver and Simpson indices) relative to 1992 pretreatment values than either UG or MG plots. At creeks, MG plots maintained greater relative species richness (1.8× pretreatment values cf. 1.1× pretreatment values for UG and LG), evenness, and diversity than LG and UG plots which were not significantly different from each other. Our results indicate that light grazing on spring-fed wetlands and moderate grazing on their downslope creeks may increase plant diversity and reduce interannual fluctuations in composition.
KEY WORDS: grazing, diversity, riparian