Contributions of gopher mound and casting disturbances to plant community structure in a Cascade Range meadow complex

Pocket gophers (Geomyidae) are major agents of disturbance in North American grasslands. Gopher mounds bury existing plants and influence community structure through various mechanisms. However, in mountain meadows that experience winter snowpack, gophers also create winter castings, smaller tube-shaped deposits, previously ignored in studies of plant-gopher disturbance relationships. We studied the influences of the Mazama pocket gopher (Thomomys mazama Merriam, 1897) in montane meadows of the Oregon Cascades, quantifying community patterns at larger spatial scales than previously studied in this system and considering, for the first time, effects of both mounds and castings. We measured cover of disturbance and individual plant species along twenty 5 m transects in each of four plots with differing species composition. Total plant cover was negatively correlated with mounds and castings. However, only mounds influenced growth-form dominance, reducing graminoid cover and increasing the forb-graminoid ratio - effects attributable to the greater volume and longevity of mounds. Forb-disturbance relationships were variable among plots, likely due to differences in species' tolerance of burial. Transect-scale richness and heterogeneity (variation in composition within transects) increased with disturbance. We conclude that frequent, small-scale disturbances create a shifting mosaic of vegetation states, reducing graminoid dominance and enhancing species richness and heterogeneity at larger spatial scales.