Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 3/10/2004
Publication Date: 8/1/2004
Citation: Derner, J.D., Hickman, K.R., Hart, R.H. 2004. Vegetation states in a semi-arid grassland: grazing-induced transitions between states. pp. 122-123. In: Ecological Society of America Abstracts. Interpretive Summary:
Technical Abstract: Conceptual state-and-transition models are being developed for rangelands, but empirical evidence is lacking for explicit testing of these models. A long-term (24 year) grazing study evaluating the effects of light, moderate, heavy and no grazing on northern mixed-grass prairie, with a history of no grazing prior to initiation of treatments, has been conducted on the High Plains Grasslands Research Station (Cheyenne, Wyoming). Plant species basal and canopy cover, along with litter and bare ground, were visually estimated using a Daubenmire technique in mid-July 2003 on 50, 0.2 m X 0.5 m quadrats in each of two replicated pastures. Heavily grazed pastures had the highest vegetative basal cover, canopy cover, amount of bare ground and density of Bouteloua gracilis, the dominant C4 perennial grass, but the lowest levels of litter and density of Pascopyrum smithii, the dominant C3 perennial grass. Light and ungrazed pastures were similar for all measured plant community parameters and exhibited the lowest vegetative basal cover, canopy cover, amount of bare ground and density of B. gracilis. In contrast, those pastures had the highest levels of litter and density of P. smithii. The moderately-grazed pastures were intermediate for all plant community parameters. Neither species richness nor diversity differed with grazing treatment. These results indicate that 1) heavy grazing has induced transitions in vegetation states for this previously ungrazed NMP as the plant community became C4 grass dominated with greater canopy and basal cover, and exhibited plant structural changes, 2) light grazing did not induce a transition, and 3) moderate grazing has at least induced a phase change in the vegetation to an "intermediate" plant community.