Effects of abiotic factors and cattle grazing on gypsum outcrop plant communities in the Cimarron Gypsum Hills, northwestern Oklahoma

Patterns of vascular plant species composition in relationship to abiotic factors and grazing history were quantified for gypsum outcrop communities of the Cimarron Gypsum Hills of northwestern Oklahoma. I sampled thirty-nine 100 m2 quadrats at 13 sites during summer 2006, fall 2006, spring 2007, and summer 2007. One hundred and fifty-five species were identified, of which only seven were introduced. The dominant species were native perennial herbs, and the outcrops appear to be "islands" of predominantly native vegetation. Plant assemblages were similar enough to be considered the same community, and richness and diversity differed little among sites. However, differences were detected in species composition among sites, and Detrended Correspondence Analysis showed that these differences along axis 1 were significantly correlated with changes in average annual precipitation and temperature, longitude and elevation, with a weak grazing history gradient shown along axis 2. Indicator Species Analysis identified few significant indicators for a particular grazing history. The native plant community persists in spite of grazing; however non-vegetated substrate demonstrated higher cover on currently-grazed outcrops relative to long-ungrazed ones.