Plant interspaces resulting from contrasting grazing management in northern mixed-grass prairie:Implications for ecosystem function

Authors: Derner, Justin; WHITMAN, ASHLEY - UNIVERSITY OF WYOMING

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2008
Publication Date: 1/28/2008
Citation: Derner, J.D., Whitman, A.J. 2008. Plant interspaces resulting from contrasting grazing management in northern mixed-grass prairie:Implications for ecosystem function. Rangeland Ecology and Management 62(1):83-88.

Interpretive Summary: Livestock grazing may modify the horizontal structure of vegetation at the soil surface by influencing the composition of bunchgrasses and rhizomatous grasses. Here, we sampled plant basal gaps (distances between stems of perennial plants) and litter cover in these gaps in pastures heavily grazed where blue grama (a bunchgrass) dominates the vegetation and ungrazed pastures where the rhizomatous grass western wheatgrass is the primary species. Litter was greater in the ungrazed pastures as was the size of plant basal gaps. The potential for water erosion from these larger gaps in the ungrazed area may be lessened by the higher litter cover, unless a disturbance such as fire removes this litter. Redistribution of soil and soil nutrients in the heavily grazed pastures likely occurs at small-scales and offsite transport is unlikely given the occurrence of smaller gaps.

Technical Abstract: We assessed plant basal gaps in July 2007 using continuous line intercepts in twice-replicated pastures of northern mixed-grass prairie with two grazing treatments: 1) long-term (25 years) heavily grazed, dominated by the bunchgrass blue grama (Bouteloua gracilis), and 2) ungrazed, dominated by the rhizomatous grass western wheatgrass (Pascopyrum smithii). In addition, we categorically estimated litter cover for each plant basal gap using a 0 to 4 scale where 0=0% cover, 1=1-25% cover, 1-2=26-50%, 2-3=51-75%, and 3-4=76-100%. The number of plant basal gaps longer than 2 cm was 26% higher with heavy grazing, but the amount of soil surface occupied by plant basal gaps was 27% greater without grazing. Plant basal gaps represented 87% and 68% of the total soil surface in the ungrazed and heavy grazed communities, respectively. The percentage of soil surface covered by plant basal gaps <20 cm was higher for the heavy grazed (94%) compared to the ungrazed (79%) community reflecting differences in the distribution of gap sizes. Litter cover scores in plant basal gaps were higher in the absence of grazing (3.7, in the 75-100% cover category) compared to heavy grazing (2.7, in the 51 to 75% category). Grazing-induced shifts in the vertical and horizontal structure of vegetation communities, accomplished through modifying composition of dominant grass species which possess contrasting plant morphologies, are reflected in these parameters of plant basal gaps and have implications for ecosystem functioning. Ecological consequences for erosion potential from larger plant basal gaps in the ungrazed community are likely offset by greater litter cover unless a disturbance such as fire removes this litter; conversely lower litter cover resulting from heavy grazing may increase erosion potential despite occurrence of smaller gaps and less proportion the soil surface covered by basal gaps.