Spatially-discrete disturbance overrides inherent environmental heterogeneity in grazed mixed-grass prairie

Authors: McGranahan, Devan

Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Forage production in rangelands is highly variable across the landscape, depending on soil, topography, and hydrology, and grazers tend to return to areas of preferred forage. Rangeland management often tries to prevent this site-specific selection with fencing or patch-burns, but there has yet to be a direct comparison of inherent heterogeneity and that created by disturbances such as prescribed fire. Using a dataset with forage production, forage nutritive value, and cattle site selection collected from within patches and ecological sites, this study compares the relative effects of soils vs burns as drivers of spatial heterogeneity. Without fire, ecological sites were the main drivers of forage production, but when patch-burning was used, the management patches were the important factor. Similarly, only patch-burning contributed to spatial heterogeneity in forage nutritive value and where cattle were found in pastures. These results suggest that patchy prescribed fire can be used to override the inherent variability in rangelands attributable to differences in ecological sites.

Technical Abstract: Spatial heterogeneity is described as patchiness, with patches constituting local aggregations of low variability in one or more ecosystem properties. Rangelands are heterogeneous natural ecosystems in which spatial heterogeneity can be attributed to inherent variability due to soils, topography, and hydrology, or disturbances such as fire and grazing. But little research has directly compared the relative contributions of inherent heterogeneity and disturbance to spatial variability. Here, variance partitioning is used to parse the relative contributions of inherent heterogeneity described in soil maps, and disturbance-driven heterogeneity imposed by patch burns, in grazed mixed-grass prairie of the US Northern Great Plains. Results support the hypothesis that disturbance-driven heterogeneity overrides inherent heterogeneity. Specifically, on pastures without patch burns, aboveground plant biomass depended on inherent variability in soil and topographic position; on patch-burned pastures, aboveground plant biomass was driven by time-since-fire and plant successional stages. Variability in forage nutritive composition and the spatial distribution of grazers were also driven by patchiness when imposed by spatially-discrete fire, while the role of inherent heterogeneity was weak for both. These results provide evidence for a default, often weak bottom-up control of spatial heterogeneity in the absence of spatially-discrete disturbance that can be overridden by top-down disturbance effects.