Assessing spatial heterogeneity outcomes in five rangelands experimentally managed with pyric-herbivory

Background/Question/Methods

Many rangelands evolved under an interactive disturbance regime in which herbivores respond to the spatial pattern of fire and create a patchy, heterogeneous landscape. Several studies report that spatially-heterogeneous fire and grazing create landscape-level vegetation heterogeneity (patch contrast) and increase rangeland biodiversity versus grazing under spatially-homogeneous fire regimes. In this meta-analysis of five experiments comparing spatially heterogeneous fire treatments to spatially homogeneous fire treatments on grazed rangeland in the North American Great Plains, we compared patch contrast across pastures managed for heterogeneity and pastures managed for homogeneity. We used a linear mixed-effect regression model that partitioned variation in each of three response variables: vegetation structure, litter cover, and bare ground. We also use a meta-analytical statistic to calculate an effect size for patch contrast for each response variable at each location.

Results/Conclusions

Heterogeneity-based treatment increased the range of plant functional group composition and increased spatial contrast between patches in vegetation structure at three of the five experimental locations. However, heterogeneity-based treatment increased patch contrast in litter cover and bare ground at four of the five locations. Management for heterogeneity created landscape-level heterogeneity in vegetation across a broad range of precipitation and plant community types, but management for heterogeneity did not universally create patch contrast: among locations, effect sizes varied for each response variable, and within locations, effect sizes varied among vegetation structure, litter cover, and bare ground. Stocking rate and invasive plant species are key regulators of heterogeneity-based management, as they determine the influence of fire on the spatial pattern of fuel, vegetation structure and patch selectivity by large herbivores. Although fire spread is important to coupling the fire-grazing interaction, the observed success of heterogeneity-based treatment can still vary independent of fire spread depending on which response variables are measured.