Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2009
Publication Date: 8/20/2009
Citation: Davies, K.W., Svejcar, A.J., Bates, J.D. 2009. Interaction of Historical and Nonhistorical Disturbances Maintains Native plant Communities. Ecological Applications. 19(6):1536-1545.
Interpretive Summary: There is considerable controversy over the use of both livestock grazing and fire on sagebrush steppe rangeland. Some groups advocate removing livestock and allowing fires to occur at more frequent intervals. We evaluated the impacts of fire on sagebrush rangeland, which had either been grazed up until the year of burning (1993), or which had been excluded from grazing since 1937. Vegetation characteristics were measured in the 12th through 14th years after burning. We found that burning caused a huge increase in cheatgrass (an invasive annual) in the ungrazed burn, but not in the grazed burn. The increase in cheatgrass coincided with mortality of the native perennial bunchgrasses. We suspect that the accumulation of plant litter in the ungrazed treatment resulted in a hotter fire and thus bunchgrass mortality. That might not have been a problem prior to the introduction of annual invaders such as cheatgrass, but under current conditions it might influence the ability of the bunchgrass community to recover. This information should be of interest to scientists and land managers interested in fire and grazing interactions.
Technical Abstract: Historical disturbance regimes are often considered a critical element in maintaining native plant communities. However, the response of plant communities to disturbance may be fundamentally altered as a consequence of invasive plants, climate change, or prior disturbances. The appropriateness of historical disturbance patterns under modern conditions and the interactions among disturbances are issues that ecologists must address to protect and restore native plant communities. We evaluated the response of Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh plant communities to their historical disturbance regime compared to other disturbance regimes. The historical disturbance regime of these plant communities was periodic fires with minimal grazing by large herbivores. We also investigated the influence of prior disturbance (grazing) on the response of these communities to subsequent disturbance (burning). Treatments were: 1) ungrazed (livestock grazing excluded since 1936) unburned, 2) grazed unburned, 3) ungrazed burned (burned in 1993), and 4) grazed burned. The ungrazed burned treatment emulated the historical disturbance regime. Vegetation cover, density, and biomass production were measured the 12th, 13th, and 14th year post-burning. Prior to burning the presence of Bromus tectorum L., an exotic annual grass, was minimal (< 0.5% cover) and vegetation characteristics were similar between grazed and ungrazed treatments. However, litter accumulation was almost 2-fold greater in ungrazed than grazed treatments. Long-term grazing exclusion followed by burning resulted in a substantial B. tectorum invasion, but burning the grazed areas did not produce an invasion. The ungrazed burned treatment also had less perennial vegetation than other treatments. The accumulation of litter (fuel) in ungrazed treatments may have resulted in greater fire-induced mortality of perennial vegetation in ungrazed compared to grazed treatments. Our results demonstrate that prior disturbances exert a strong influence on the response of plant communities to subsequent disturbances and suggest that low-severity disturbances may be needed in some plant communities to increase their resilience to more severe disturbances. Modern deviations from historical conditions can alter ecosystem response to disturbances, thus restoring the historical disturbance regime may not be an appropriate strategy for all ecosystems.