Grazing intensity and spatial heterogeneity in bare soil in a grazing-resistant grassland

Authors: Augustine, David; Booth, D; Cox, Samuel; Derner, Justin

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2011
Publication Date: 1/10/2012
Citation: Augustine, D.J., Booth, D.T., Cox, S.E., Derner, J.D. 2012. Grazing intensity and spatial heterogeneity in bare soil in a grazing-resistant grassland. Rangeland Ecology and Management. 65:39-46.

Interpretive Summary: Variation in the distribution of vegetation can be an important indicator of rangeland degradation, and can also be an important component of wildlife habitat. We illustrate the use of very large scale aerial (VLSA) photography to measure bare soil of the northeastern Colorado shortgrass steppe. Using three pairs of pastures stocked at moderate versus very heavy rates, we detected greater bare soil under very heavy (mean = 22.5%) versus moderate stocking (mean = 13.5%). In addition, variability in bare soil across pastures was lower under very heavy versus moderate stocking. Bare soil was patchily distributed across distances of 2 yards under both stocking rates. Bare soil was also patchily distributed across distances of 60 – 120 yards under moderate grazing, but this patch structure was eliminated under very heavy grazing. Patterns for bare soil were similar when restricted to a single dominant ecological site (Loamy Plains), indicating similar variation among versus within ecological sites. Under very heavy grazing, bare soil increased primarily at the scale of individual plant clusters, both through increases in the density of small bare patches, and increases in patch size. Our approach demonstrates the utility of VLSA for analyzing interactions between grazing and other landscape features, and highlights the importance of spatially explicit sampling across pastures while still testing for potential shifts in patchiness of bare soil at the scale of individual grass clusters.

Technical Abstract: Spatial patterns in rangeland vegetation serve as indicators of rangeland condition and are an important component of wildlife habitat. We illustrate the use of very-large-scale aerial photography (VLSA) to quantify spatial patterns in bare soil of the northeastern Colorado shortgrass steppe. Using three pairs of pastures stocked at moderate versus very heavy rates, we detected higher bare soil under very heavy (mean = 22.5%) versus moderate stocking (mean = 13.5%; P = 0.053), while the coefficient of variation across pastures was lower under very heavy (mean = 0.48) versus moderate stocking (mean = 0.75; P =.032). The magnitude of spatial autocorrelation was greatest at a 2-m separation distance under both stocking rates (Moran’s I = 0.48 – 0.58). Bare soil still exhibited significant positive spatial autocorrelation across distances of 60 – 120 m under moderate stocking (mean Moran’s I = 0.14), while patchiness at this scale was eliminated under very heavy grazing (mean Moran’s I = -0.05). Across separation distances of 120 – 480 m, we observed no spatial autocorrelation under either grazing intensity. Means and spatial patterns for bare soil were similar when analyses were restricted to a single dominant ecological site (Loamy Plains), indicating similar variation among versus within ecological sites. Thus, very heavy grazing did not generate spatial contagion in bare soil at spatial scales of 2 – 480 m. Rather, bare soil increased primarily at the scale of individual plant clusters. Our approach demonstrates the utility of VLSA for analyzing interactions between grazing and other landscape features, and highlights the importance of spatially explicit sampling across broad scales (pastures) while still testing for potential shifts in patchiness of bare soil at the scale of plant interspaces.