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

Abstract

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 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. Conclusions Our 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].

Plant community responses to bison reintroduction on the Northern Great Plains, United States: a test of the keystone species concept

“…The plant communities of the NGP have undoubtedly evolved in the presence of large grazers (Knapp et al ), and large grazer exclusion or livestock removal in the tallgrass and shortgrass prairie is expected to result in lower annual forb cover compared to grazed sites (Fuhlendorf & Engle ; Valone & Sauter ; Manier & Hobbs ). Our findings are comparable with studies in the shortgrass prairie where livestock removal resulted in heightened forb abundance (Augustine et al ). However, our data suggest that removal may additionally lead to landscape homogenization (evidenced by lower compositional heterogeneity compared to bison treatment).…”

“…However, counter to what would be expected in the mesic tallgrass ecosystem, we did not observe significant differences with livestock retention areas in the dominance of C 4 grasses over C 3 grasses (Knapp et al ; Towne et al ; Fuhlendorf et al ). Similar studies looking at cattle grazing in the shortgrass steppe showed very little functional group difference between grazing treatments in less than 20 years, with functional group shifts not occurring for almost 70 years (Augustine et al ). However, more productive ecotones like the mixed‐grass prairie will likely show these shifts sooner than the arid shortgrass steppe (Huston ), and this is likely reflected in the partial transition observed in our study.…”

A Three-Dimensional Assessment of Soil δ13C in a Subtropical Savanna: Implications for Vegetation Change and Soil Carbon Dynamics

“…This estimate is coincident with historical accounts indicating this site has been grazed continuously by domestic livestock since the mid to late 1800s [51,52]. Since cattle in this region feed preferentially on C 4 grasses, this gives co-occurring C 3 forbs (many of which are unpalatable) a competitive advantage and allows them to increase their biomass and productivity within the grassland matrix [29,77,78].…”

Cattle, conservation, and carbon in the western Great Plains

“…This change could lead to increased forage productivity, but could also lead to lower soil water availability (Tietjen et al 2017). Derner et al (2019) found that C stocks did not change even after significant vegetation change (Augustine et al 2017), yet uncertainty remains about soil C changes following vegetation changes due to variable and inadequate understanding of C allocation patterns among dominant grasses (Derner and Schuman 2007).…”