Recoupling fire and grazing reduces wildland fuel loads on rangelands

Abstract

Fire suppression and exclusion, the historically dominant paradigm of fire management, has resulted in major modifications of fire-dependent ecosystems worldwide. These changes are partially credited with a recent increase in wildfire number and extent, as well as more extreme fire behavior. Fire and herbivory historically interacted, and research has shown that the interaction creates a unique mosaic of vegetation heterogeneity that each disturbance alone does not create. Because fire and grazing have largely been decoupled in modern times, the degree to which the interaction affects fuels and fire regimes has not yet been quantified. We evaluated effects of fire-only and pyric herbivory on rangeland fuels and fire behavior simulated using BehavePlus at four sites across the southern Great Plains. We predicted patches managed via pyric herbivory would maintain lower fuel loads, and less intense simulated fire behavior than fire alone. We found that time since fire was a significant predictor of fuel loads and simulated fire behavior characteristics at all sites. Fuel loads and simulated fire behavior characteristics (flame length and rate of spread) increased with increasing time since fire in all simulated weather scenarios. Pyric herbivory mediated fuel accumulation at all sites. Mean fuel loads in fire-only treatments exceeded 5000 kg/ha within 24 months, but pyric herbivory treatments remained below 5000 kg/ha for approximately 36 months. Simulated flame lengths in fire-only treatments were consistently higher (up to 3 9 ) than in pyric herbivory treatments. Similarly, fire spread rates were higher in fire-only than in pyric herbivory treatments in all simulated weather conditions. Although all sites had potential to burn in the most extreme weather conditions, pyric herbivory reduced fuel accumulations, flame lengths, and rates of spread across all weather patterns simulated. These reductions extended the amount of time standard wildland firefighting techniques remain effective. Therefore, incorporating pyric herbivory into fuel management practices, in areas of high herbaceous productivity, increases the effectiveness of fuel treatments.

Synergistic effects of long‐term herbivory and previous fire on fine‐scale heterogeneity of prescribed grassland burns

“…Interactions among herbaceous vegetation, woody vegetation, and consumers play out dramatically in savannas, which are defined by the coexistence of trees and grasses, and characterized by their potential for alternative states of either grass‐ or tree‐dominated vegetation reinforced by fire and herbivory (Sankaran et al 2008, Touboul et al 2018). There have been an increasing number of studies of how interactions among fire, herbivores, and vegetation in these ecosystems influence the landscape‐scale heterogeneity of tree‐grass ratios (Higgins et al 2000, Sankaran et al 2008, Holdo et al 2009, Hempson et al 2019, Johansson et al 2020) and grass fuel loads (Bielski et al 2018, Starns et al 2019). Fewer examine how these interactions affect fine‐scale landscape heterogeneity (Blackhall et al 2017), despite the assertion that spatial heterogeneity in turn engenders landscape‐level diversity (Pickett and Cadenasso 1995, Stein et al 2014) and resilience to future disturbance (Koontz et al 2019).…”

Post‐Soviet shifts in grazing and fire regimes changed the functional plant community composition on the Eurasian steppe

“…In contrast, where grassland vegetation is shaped by grazing, fires are suppressed, as grazers preferentially feed on grasses (Elson & Hartnett, 2017) and reduce overall fuel load. This results in a patchy vegetation structure that inhibits fire spread (Fuhlendorf et al., 2009; Hempson et al., 2015; Starns et al., 2019). Whereas grazers select plants based on nutritional value, fire favours flammable plants while suppressing fire‐sensitive competitors (Bond & Midgley, 1995).…”

A Device for Instantaneously Estimating Duff Moisture Content Is also Effective for Grassland Fuels

“…While a recent nominal standard temperature for oven-drying fuel is 105 • C [30], errors associated with 60 • C were minimal as the effect of the differences between the two temperatures is realized at low fuel moisture contents (e.g., <10%). Furthermore, 60 • C as used here lies between higher temperatures (95-145 • C) used to determine moisture content in forest fuels [30,31] and lower temperates (45-55 • C) used in other rangeland fuel moisture measurements [32][33][34]. Water content is expressed on a dry-weight basis.…”