The Effects of Disturbance Size and Frequency on a Shortgrass Plant Community

Abstract

We examined the effects of small, patch—producing disturbances on a shortgrass plant community dominated by the perennial grass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths). Size distributions and rates of occurrence were determined from field data and from the literature for B. gracilis plants and for three disturbance types: cattle fecal pats, western harvester ant mounds, and small animal burrows. A Spatially explicit simulation model was then used to determine the effects of each disturbance. The average turnover rates (percent affected per year) of the basal cover and density of B. gracilis plants in the simulated plots were calculated for three topographic positions and three grazing intensities. Disturbance size relative to the size of B. gracilis plants distinguished two classes of disturbances. Ant mounds and animal burrows were significantly larger than B. gracilis plants, while fecal pats were comparable in size to B. gracilis plants. The frequency of occurrence of a disturbance was found to be a good predictor of the amount of basal cover and the number of pants killed each year by each disturbance type. The fastest turnover rates for basal cover were associated with the smallest and most frequent disturbance type (fecal pats). The effects of these disturbances were found to be dependent on topographic position and grazing intensity, especially the former. Slopes and lowlands had faster turnover rates than uplands. Our results suggest that the recovery of B. gracilis is dependent on the spatial scale of the disturbance.

Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe

“…vidual plants depend on spatial patterns of biomass, which influence litter inputs, soil movement, microclimate, and interactions between nutrient uptake and SOM quality. Spatial patterns of biomass are influenced by the lifeform of the dominant species, resource availability, disturbance, and herbivory (Coffin and Lauenroth, 1988;). The small-scale heterogeneity we observed at the CPER probably results from the morphology of the dominant grass species and climatic constraints on plant cover rather than from direct effects of grazing on community structure.…”

Habitats of native and exotic plants in Colorado shortgrass steppe: a comparative approach

“…76, 1998belowground competition in the native-dominated CPER grassland is severe (Milchunas et al 1992;Hook et al 1994;Aguilera and Lauenroth 1995), but the addition of water or nutrients ameliorates belowground conditions and allows aliens to invade (Milchunas et al , 1990Milchunas and Lauenroth 1995). Physical disturbance may have a similar effect, reducing belowground competition by breaking up the nearly continuous root network (Coffin and Lauenroth 1988;Hook et al 1994;Aguilera and Lauenroth 1995). In addition, physical disturbance may directly favour exotics with ruderal life histories (dispersal, dormancy, phenology, etc.)…”

Cross-system comparisons elucidate disturbance complexities and generalities

“…Although a number of definitions, conceptual frameworks, and theories have been developed (e.g., Clements 1916, Grime 1977, Sousa 1984, the most commonly used definition by ecologists is: ''any relatively discrete event in time that disrupts ecosystem, community or population structure and changes resource, substrate availability, or the physical environment'' (White and Pickett 1985:7). This definition, with minor modifications, has been applied extensively to both terrestrial and aquatic ecosystems (e.g., Collins 1987, Coffin and Lauenroth 1988, Resh et al 1988, Palmer et al 1996, Reich et al 2001, Fraterrigo and Rusak 2008.…”