Spatial heterogeneity across five rangelands managed with pyric‐herbivory

Abstract

Summary1. Many rangelands evolved under an interactive disturbance regime in which grazers respond to the spatial pattern of fire and create a patchy, heterogeneous landscape. Spatially heterogeneous fire and grazing create heterogeneity in vegetation structure at the landscape level (patch contrast) and increase rangeland biodiversity. We analysed five experiments comparing spatially heterogeneous fire treatments to spatially homogeneous fire treatments on grazed rangeland along a precipitation gradient in the North American Great Plains. 2. We predicted that, across the precipitation gradient, management for heterogeneity increases both patch contrast and variance in the composition of plant functional groups. Furthermore, we predicted that patch contrast is positively correlated with variance in plant functional group composition. Because fire spread is important to the fire-grazing interaction, we discuss factors that reduce fire spread and reduce patch contrast despite management for heterogeneity. 3. We compared patch contrast across pastures managed for heterogeneity and pastures managed for homogeneity with a linear mixed effect (LME) regression model. We used the LME model to partition variation in vegetation structure to each sampled scale so that a higher proportion of variation at the patch scale among pastures managed for heterogeneity indicates patch contrast. To examine the relationship between vegetation structure and plant community composition, we used constrained ordination to measure variation in functional group composition along the vegetation structure gradient. We used the meta-analytical statistic, Cohen's d, to compare effect sizes for patch contrast and plant functional group composition. 4. Management for heterogeneity increased patch contrast and increased the range of plant functional group composition at three of the five experimental locations. 5. Plant functional group composition varied in proportion to the amount of spatial heterogeneity in vegetation structure on pastures managed for heterogeneity. 6. Synthesis and applications. Pyric-herbivory management for heterogeneity created patch contrast in vegetation across a broad range of precipitation and plant community types, provided that fire was the primary driver of grazer site selection. Management for heterogeneity did not universally create patch contrast. Stocking rate and invasive plant species are key regulators of heterogeneity, as they determine the influence of fire on the spatial pattern of fuel, vegetation structure and herbivore patch selection, and therefore also require careful management.

Inconsistent outcomes of heterogeneity-based management underscore importance of matching evaluation to conservation objectives

“…Ordination is effective in calculating the range of variation in composition, a measure of contrast in plant functional group composition (McGranahan et al, 2012a). Range of variation was measured using site scores along NMDS axis 1, the gradient of greatest variation in plant functional group composition.…”

“…We used an existing dataset of five rangeland experiments in Oklahoma and Iowa, USA (McGranahan et al, 2012a Knopf and Miller (1994) Ant community composition affected by changes in bare ground Graham et al (2008) Litter cover Litter cover < 25% doubled success rate of Greater Prairie-chicken nests McKee et al (1998) Altered litter cover associated with altered ant community composition Bestelmeyer and Wiens (1996) Winter cover, greater soil moisture increase survival of grassland obligate butterflies Vogel et al (2010) sizes, and management schemes (Table 2). Although established independently, the basic structure of each experiment was consistent: each experiment consisted of a replicated treatment in which fire was applied in spatially discrete patches, and a replicated control reflecting conventional management with homogeneous fire regimes.…”

“…We calculate an effect size for each variable at each location to compare the level of patch contrast-"the degree of difference between patches" (Kotliar and Wiens, 1990)-created by heterogeneity-based management versus homogeneity-based management. Although we do not expect all study locations to universally respond to heterogeneity-based management (McGranahan et al, 2012a), we predict that habitat variables should respond consistently within each location. We discuss these results with respect to conservation goal-setting and the evaluation of management outcomes.…”

“…In managed rangeland, prescribed fire is applied in discrete patches to replicate the spatially-and temporally-shifting mosaic of pre-European landscapes Engle, 2004, Fuhlendorf et al, 2009). Known as patch burn-grazing (McGranahan et al, 2012a), such heterogeneity-based management creates a landscape mosaic to support greater biodiversity than conventional, homogeneity-based management (Coppedge et al, 2008, Doxon et al, 2011, Fuhlendorf et al, 2006. As such, managers are often encouraged to promote landscape heterogeneity to conserve rangeland fauna (Toombs et al, 2010).…”

Topoedaphic Variability and Patch Burning in Sand Sagebrush Shrubland

“…This practice capitalizes on the profound influence of fire on the distribution and behavior of grazing animals to alter vegetation heterogeneity across landscapes Vermeire et al 2004;Fuhlendorf et al 2006;Coppedge et al 2008;Allred et al 2011;McGranahan et al 2012;Winter et al 2012). Because application of the fire grazing interaction has been the subject of focused research for a relatively short time, land managers accustomed to using fire alone or grazing alone may have uncertainty regarding its use.…”