Vegetation’s influence on fire behavior goes beyond just being fuel

Abstract

Background The structure and function of fire-prone ecosystems are influenced by many interacting processes that develop over varying time scales. Fire creates both instantaneous and long-term changes in vegetation (defined as live, dead, and decomposing plant material) through combustion, heat transfer to living tissues, and subsequent patterns of recovery. While fuel available for combustion may be relative to the amount of vegetation, it is equally instructive to evaluate how the physical structure and other characteristics of vegetation influence fire dynamics, and how these interactions change between fire events. This paper presents a conceptual framework for how vegetation not only embodies the legacy of previous fires but creates the physical environment that drives fire behavior beyond its combustion as a fuel source. Results While many environmental factors affect both the post-fire vegetation trajectory and fire dynamics themselves, we present a conceptual framework describing how vegetation’s structural characteristics control the local microclimate and fluid dynamics of fire-induced flows, and how that is influenced by ecosystem and atmospheric processes. Shifting our focus from fuels to vegetation allows us to integrate spatial and temporal feedbacks between fire, vegetation, soil, and the atmosphere across scales. This approach synthesizes the combustion and flammability science, the physical influence on fire behavior, and the ecosystem dynamics and processes that occur between fires and within a fire regime. Conclusions We conclude that fire behavior, including its prediction and ecological effects, should be broadened to include the dynamic processes that interact with vegetation, beyond its role as fuel. Our conceptual framework illustrates the crucial feedbacks across scales that link the finer details of vegetation and fire behavior processes that occur within a fire and have additive effects that feedback into the coarser scale processes and functions within an ecosystem. Shifting the fuels paradigm to integrate the combustion, physical, and ecological roles of vegetation as complex drivers of fire behavior and outcomes will broaden discovery within wildland fire science and ecology.

Terrestrial laser scan metrics predict surface vegetation biomass and consumption in a frequently burned southeastern U.S. ecosystem

“…These data are particularly useful in ecosystems dominated by fine scale variation in surface fuels (Hiers et al 2009) and for inputs into high resolution coupled fire-atmosphere models (Linn et al 2001, Mell et al 2009, Linn et al 2020) that operate in three dimensions with the ability to represent withinstand variability (Hiers et al 2020). Such high-resolution representation of fuel variation will improve our prediction of heterogeneity in fire effects and the underlying physical influences of vegetation on the fire environment [13]. In this study, we found that the variability in surface mass across these two burn units (658 ha total) was considerable (mean: 841 g m -2 , stdev: 350 g m -2 from the 'Total 0-30' model (Figure 2), highlighting the need to think beyond stand level averages [66].…”

“…In southeastern U.S. ecosystems where production and turnover rates are high [19,20], repeated burning provides a continuum of fine surface fuels and live vegetation that enable a broad range of fire behavior throughout the year [21,22]. As such, the low (often <1m height) vegetation (shrubs, grasses, forbs, leaf litter, soil organic layer, and small coarse wood) continuously changes with each fire and ecological response [13], making estimates of surface vegetation structure and loading, notwithstanding fuel moisture dynamics [23], highly variable at fine temporal (<hr) and spatial scales (<1m). Furthermore, large diameter coarse woody debris (>20 cm) have little opportunity to accumulate and hence contribute little to combustion during prescribed burns because of their high moisture retention and fast decay rates [24,25].…”

“…Though these approaches have been successfully used by managers in a variety of situations including large-scale fire management, they do not provide information on the fine scale variability in fuels that drive fire behavior and effects in prescribed fires and in frequently burned ecosystems. There are no operational products that provide information on finer-scale attributes of surface vegetation composition and architecture, despite research in laboratory, field, and modeling environments that have identified these biophysical characteristics as drivers of variation in fire behavior and effects from surface fires [13-17]. Thus, improving estimation of the spatial resolution of fuels and including information about the vertical dimensionality of the fuelbed architecture or 3D characteristics of surface live and dead vegetation are critical to improving fire behavior predictions for surface fires.…”

Terrestrial Laser Scan Metrics Predict Surface Vegetation Biomass and Consumption in a Frequently Burned Southeastern U.S. Ecosystem

“…Though these approaches have been successfully used by managers in a variety of situations, including large-scale fire management, they do not provide information on the fine-scale variability in fuels that drive fire behavior and effects in prescribed fires and in frequently burned ecosystems. There are no operational products that provide information on the finer scale attributes of surface vegetation composition and architecture, despite research in laboratory, field, and modeling environments that have identified these biophysical characteristics as drivers of variation in fire behavior and in the effects of surface fires [13][14][15][16][17]. Thus, improving estimation of the spatial resolution of fuels, and including information about the vertical dimensionality of the fuelbed architecture or the 3D characteristics of surface living and dead vegetation, are critical to improving the prediction of low-intensity surface fire behavior.…”

“…In southeastern U.S. ecosystems, where production and turnover rates are high [19,20], repeated burning provides a continuum of fine surface fuels and living vegetation that enable a broad range of fire behavior throughout the year [21,22]. As such, the low (often <1 m height) vegetation (shrubs, grasses, forbs, leaf litter, soil organic layer, and small coarse wood) continuously changes with each fire and ecological response [13], making estimates of surface vegetation structure and loading, notwithstanding fuel moisture dynamics [23], highly variable at fine temporal (<hr) and spatial scales (<1 m). Furthermore, large diameter coarse woody debris (>20 cm) have little opportunity to accumulate, and hence contribute little to combustion during prescribed burns because of their high moisture retention and fast decay rates [24,25].…”