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

Abstract

AbstractQuestionsHow 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?LocationShortgrass steppe, northeast Colorado, USA, in the North American Great Plains.MethodWe 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.ResultsAlthough 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 C4 shortgrass (Bouteloua gracilis) to co‐dominance by a C3 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, C3 perennial graminoids and sub‐shrubs have increased in relative abundance in both grazed and ungrazed communities.ConclusionsOur long‐term experiment shows that community responses to grazing in this semi‐arid grassland occur very slowly, but are predictable, with C4 shortgrasses eventually giving way to taller C3 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 C3 grass abundance, and the slow rate at which taller C3 grasses can coalesce into mono‐dominant patches that outcompete C4 shortgrasses. Increased abundance of C3 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 C3 plants under increasing atmospheric [CO2].