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

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.