Grazing moderates increases in C₃ grass abundance over seven decades across a soil texture gradient in shortgrass steppe

Questions How does long‐term exclusion of cattle grazing influence plant community composition in a semi‐arid grassland? Can spatial variation in the effects of cattle grazing exclusion be explained by variation in soil texture? Location Shortgrass steppe, northeast Colorado, USA , in the North American Great Plains. Method We used 16 long‐term (72 yr) cattle exclosures to examine the effects of grazers on plant communities and evaluate whether grazer effects interact with soil texture. Results Although shortgrass steppe communities are relatively unaffected by grazing in the short‐term (one to two decades), exclusion of cattle grazing for seven decades caused a compositional shift from dominance by a C 4 shortgrass ( Bouteloua gracilis ) to co‐dominance by a C 3 midgrass ( Pascopyrum smithii ) and B. gracilis . The strength of this shift was highly variable across sites. Soil texture was correlated with the abundance of certain plant species, but did not explain variation in the magnitude of grazer effects. Grazed communities contained perennial and annual growth forms with a diversity of strategies to co‐exist with grazers and B. gracilis . Ungrazed communities included increased abundance of annual, ruderal forbs and three woody plant species. Grazing effects occurred against a backdrop of changing plant communities: during the past seven decades, C 3 perennial graminoids and sub‐shrubs have increased in relative abundance in both grazed and ungrazed communities. Conclusions Our long‐term experiment shows that community responses to grazing in this semi‐arid grassland occur very slowly, but are predictable, with C 4 shortgrasses eventually giving way to taller C 3 grasses and ruderal forbs. Spatial variation in grazing effects across sites (and lack of a relationship with soil texture) may reflect the importance of fine‐scale heterogeneity in C 3 grass abundance, and the slow rate at which taller C 3 grasses can coalesce into mono‐dominant patches that outcompete C 4 shortgrasses. Increased abundance of C 3 species over the past seven decades, both in the presence and absence of grazing, may be related to recovery from the severe drought and dust storms of the 1930s as well as enhanced growth of C 3 plants under increasing atmospheric [ CO 2 ].