Grazing history effects on rangeland biomass, cover and diversity responses to fire and grazing utilization

Authors: Vermeire, Lance; Strong, Dustin; Waterman, Richard

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2018
Publication Date: 6/12/2018
Citation: Vermeire, L.T., Strong, D.J., Waterman, R.C. 2018. Grazing history effects on rangeland biomass, cover and diversity responses to fire and grazing utilization. Rangeland Ecology and Management. 71(6):770-775. https://doi.org/10.1016/j.rama.2018.05.001.
DOI: https://doi.org/10.1016/j.rama.2018.05.001

Interpretive Summary: Removal of large grazers from rangelands that evolved with significant grazing pressure can alter natural processes and may change the magnitude or direction of community responses to other disturbances that follow. Three moderately grazed pastures were paired with 12-ha areas that had livestock excluded for 15 years. Six levels of disturbance were assigned to each with combinations of fire (fall fire or no fire) and grazing utilization (0, 50, or 75% biomass removal) to determine grazing history effects on rangeland response to disturbance. Livestock exclusion increased cool-season perennial grass (1232 vs. 980 ± 50 kg ha-1) and forb biomass (173 vs. 62 ± 19 kg ha-1) and reduced warm-season perennial grass (36 vs. 180 ± 25 kg ha-1) with no effect on total current-year biomass. Diversity was greater in grazed pasture than exclosures. Every biomass, cover and diversity measure was affected by fire, grazing utilization, or both except sub-shrub biomass. Contrary to expectations and despite numerous treatment effects, grazing history only impacted fire effects for old standing dead material and effects on grazing utilization were limited to old dead, bare ground, richness and dominance. The only fire by grazing utilization interaction was for bare ground. Fire reduced annual grass (64 vs. 137 ± 29 kg ha-1), forbs (84 vs. 133 ± 29 kg ha-1) and diversity with no differences in total current-year biomass (1557 vs. 1594 ± 66 kg ha-1). Grazing to 75% utilization reduced total current-year biomass (1467 vs. 1656 ± 66 kg ha-1) and dominance. Limited interaction among disturbances indicates effects of livestock exclusion were not sufficient to alter change in biomass or diversity in response to fire or grazing utilization.

Technical Abstract: Exclusion of large grazers from rangelands that evolved with significant grazing pressure can alter natural processes and may have legacy effects by changing magnitude or direction of community responses to subsequent disturbance. Three moderately grazed pastures were paired with 12-ha areas that had livestock excluded for 15 years. Six levels of disturbance were assigned to each in a 2 x 3 factorial arrangement of fire (fall fire or no fire) and grazing utilization (0, 50, or 75% biomass removal) to determine grazing history effects on rangeland response to subsequent disturbance. Livestock exclusion increased C3 perennial grass (1232 vs. 980 ± 50 kg ha-1) and forb biomass (173 vs. 62 ± 19 kg ha-1) and reduced C4 perennial grass (36 vs. 180 ± 25 kg ha-1) with no effect on total current-year biomass. Diversity was greater in grazed pasture than exclosures (H’ = 1.5400 vs. 1.3823 ± 0.0431). Every biomass, cover and diversity measure was affected by fire, grazing utilization, or both except sub-shrub biomass. Contrary to expectations and despite numerous treatment effects, grazing history only interacted with fire effects for old standing dead material and interactions with grazing utilization were limited to old dead, bare ground, richness and dominance. The only fire by grazing utilization interaction was for bare ground. Fire reduced annual grass (64 vs. 137 ± 29 kg ha-1), forbs (84 vs. 133 ± 29 kg ha-1) and diversity (H’ = 1.3260 vs. 1.5005 ± 0.0537) with no differences in total current-year biomass (1557 vs. 1594 ± 66 kg ha-1). Grazing to 75% utilization reduced total current-year biomass (1467 vs. 1656 ± 66 kg ha-1) and dominance (0.4824 vs. 0.5584 ± 0.0279). Limited interaction among disturbances indicates effects of livestock exclusion were not sufficient to alter change in biomass or diversity in response to fire or grazing utilization.