Location: Rangeland Resources & Systems ResearchTitle: Grazing moderates increases in C3 grass abundance over seven decades across a soil texture gradient in shortgrass steppe
|Milchunas, Daniel - Colorado State University|
Submitted to: Journal of Vegetation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2016
Publication Date: 2/2/2017
Citation: Augustine, D.J., Derner, J.D., Milchunas, D., Blumenthal, D.M., Porensky, L.M. 2017. Grazing moderates increases in C3 grass abundance over seven decades across a soil texture gradient in shortgrass steppe. Journal of Vegetation Science. 28(3):562-572. doi:10.1111/jvs.12508.
Interpretive Summary: We studied the effects of livestock grazing on plant communities in the shortgrass steppe of eastern Colorado. A network of 16 exclosures was built in 1939 at our study site, creating the opportunity to compare these ungrazed sites to adjacent sites that have been grazed by livestock at moderate, growing-season stocking rates. Shortgrass steppe plant communities have previously been found to change very little when livestock are removed for 1 or 2 decades, and to exhibit only moderate changes following 5 decades of livestock removal. We found that after 7 decades of livestock removal, shortgrass plant communities changed substantially, due to a shift from dominance by warm-season shortgrasses to dominance by cool-season, mid-height grasses and annual forbs. Grazed communities have been relatively stable over the past 7 decades, and include species with a wide diversity of strategies for coping with grazing. The long-term record also shows that cool-season plant species have been increasing over the past 7 decades both in the presence and absence of grazing (but more so in the absence of grazing), which may be related to ongoing increases in the concentration of carbon dioxide in the atmosphere.
Technical Abstract: Questions: How does long-term grazing exclusion influence plant community composition in a semiarid grassland? Can spatial variation in the effects of grazing exclusion be explained by variation in soil texture? Location: The shortgrass steppe of northeastern Colorado, USA, located in the North American Great Plains. Method: We used 16 long-term (72 yr) 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 (1-2 decades), exclusion of cattle grazing for 7 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 coexist 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 7 decades, C3 perennial graminoids and subshrubs have increased in relative abundance in both grazed and ungrazed communities. Conclusions: Our long-term experiment shows that community responses to grazing in this semiarid grassland occur very slowly, but are predictable, with C4 shortgrasses gradually 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 monodominant patches that outcompete C4 shortgrasses. Increased abundance of C3 species over the past 7 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]. Nomenclature USDA Plants Database (http://plants.usda.gov; accessed on 18 Feb 2016)