Moderate patchiness optimizes heterogeneity, stability, and beta diversity in mesic grassland

Abstract

Heterogeneous disturbance patterns are fundamental to rangeland conservation and management because heterogeneity creates patchy vegetation, broadens niche availability, increases compositional dissimilarity, and enhances temporal stability of aboveground biomass production. Pyrodiversity is a popular concept for how variability in fire as an ecological disturbance can enhance heterogeneity, but mechanistic understanding of factors that drive heterogeneity is lacking. Mesic grasslands are examples of ecosystems in which pyrodiversity is linked strongly to broad ecological processes such as trophic interactions because grazers are attracted to recently burned areas, creating a unique ecological disturbance referred to as the fire–grazing interaction, or pyric herbivory. But several questions about the application of pyric herbivory remain: What proportion of a grazed landscape must burn, or how many patches are required, to create sufficient spatial heterogeneity and reduce temporal variability? How frequently should patches burn? Does season of fire matter? To bring theory into applied practice, we studied a gradient of grazed tallgrass prairie landscapes created by different sizes, seasons, and frequencies of fire, and used analyses sensitive to nonlinear trends. The greatest spatial heterogeneity and lowest temporal variability in aboveground plant biomass, and greatest plant functional group beta diversity, occurred in landscapes with three to four patches (25%–33% of area burned) and three‐ to four‐year fire return intervals. Beta diversity had a positive association with spatial heterogeneity and negative relationship with temporal variability. Rather than prescribing that these results constitute best management practices, we emphasize the flexibility offered by interactions between patch number and fire frequency for matching rangeland productivity and offtake to specific management goals. As we observed no differences across season of fire, we recommend future research focus on fire frequency within a moderate proportion of the landscape burned, and consider a wider seasonal burn window.

Structural and compositional heterogeneity influences the thermal environment across multiple scales

“…To estimate the amount of thermal variability that could be attributed to site-level differences in each grassland landscape, we used a variance partitioning approach with mixed-effects models (McGranahan et al 2016(McGranahan et al , 2018. Using the lme4 package in program R (Bates et al 2019), we calculated separate mixed-effect model for each grassland landscape with I-button temperatures as the response variable and a random effect variable for I-button site in a grassland landscape.…”

Examining the assumptions of heterogeneity-based management for promoting plant diversity in a disturbance-prone ecosystem

“…Also at our same study site, two studies on avian communities found that some metrics of landscape scale bird diversity were slightly but significantly higher in pach-burn treatments while other metrics showed no change (Coppedge et al, 2008; Hovick et al, 2015). McGranahan et al (2018) examined a range of different patch-burn management designs at our study site and found that spatial and temporal heterogeneity in plant functional groups and vegetation structure was highest in pastures with intermediate numbers of patches. Gibson & Hulbert (1987) also examined the relative importance of fire, topography, and climate on tallgrass prairie vegetation but in contrast to our results they found time since fire to be the most important variable followed by topography.…”

Moderate Grazer Density Stabilizes Forage Availability More Than Patch Burning in Low-Stature Grassland

“…We used visual obstruction readings to non-destructively sample aboveground plant biomass, which incorporates both vegetation height and density [34] and correlates strongly with vegetation biomass determined by clipping irrespective of burn treatment in tallgrass prairie [35]. Used widely in grassland wildlife habitat monitoring, visual obstruction is also useful in estimating grassland standing crop for the purposes of modeling spatial heterogeneity [7,18,20].…”

“…Basically, patchy landscapes always contain enough long-unburned, ungrazed vegetation to ensure at least some amount of aboveground biomass each year. The diversity of patches with respect to time-sincefire increases plant compositional diversity, as well, which can be expressed through conventional plant ecology concepts such as β diversity [19,20].…”