Fine‐scale fire patterns mediate forest structure in frequent‐fire ecosystems

Abstract

In frequent‐fire forests, wildland fire acts as a self‐regulating process creating forest structures that consist of a fine‐grained mosaic of isolated trees, tree groups of various sizes, and non‐treed openings. Though the self‐regulation of forest structure through repeated fires is acknowledged, few studies have investigated the role that fine‐scale pattern‐process linkages play in determining fire behavior and effects. To better understand the physical mechanisms driving these pattern‐process linkages, we used a three‐dimensional, physics‐based fire behavior model to investigate how the local arrangement of canopy fuels influences heat transfer from a surface fire to tree crowns and subsequent crown ignition and consumption. In particular, we were interested in the impacts of tree group size and crown separation distance on heat transfer. We found increased convective cooling for isolated individual trees and 3‐tree groups as compared to larger 7‐ and 19‐tree groups which resulted in a reduction of the net energy transferred from the surface fire to the tree crowns. Because isolated individuals and 3‐tree groups are exposed to less thermal energy, they require a greater surface fireline intensity to initiate torching and have less crown consumption than trees within larger groups. Similarly, we found that increased crown separation distance also reduced heat transfer and crown ignition. However, differences in crown ignition and consumption among various sized groups and separation distances depended upon the surface fireline intensity, suggesting that any change in crown consumption or tree mortality due to pattern‐process linkages may be best viewed as a conditional in nature. These findings identify the potential physical mechanisms responsible for supporting the complex forest structures typical of high‐frequency fire regimes, and the results may be useful for managers designing fuel hazard reduction and ecological restoration treatments.

Pyric tree spatial patterning interactions in historical and contemporary mixed conifer forests, California, USA

“…First, small tree dispersion provides numerous points for surface to crown fire transition to occur. Second, the increased stocking and presence of large tree groups reduces local convective cooling, facilitating both crown fire transition and spread (Ritter et al., 2020). These results suggest that the fire‐mediated patterns of tree mortality have been significantly altered since historical times and that these altered patterns are produced by more severe fires resulting from greater tree densities and altered tree arrangements.…”

“…However, relatively few studies have used physics-based fire modeling to explore how fire interacts with forest structure patterns within stands (e.g., Ritter et al, 2020). Furthermore, an explicit comparison of how pattern-process interactions may differ between historical and contemporary forests is lacking.…”

“…This line of inquiry has explored the feedback between heterogenous fuel arrangements and consequent fire behavior across stands (Hoffman et al., 2012; Linn et al., 2013; Parsons et al., 2017) and distance‐dependency of tree‐to‐tree crown fire spread (Contreras et al., 2012). However, relatively few studies have used physics‐based fire modeling to explore how fire interacts with forest structure patterns within stands (e.g., Ritter et al., 2020). Furthermore, an explicit comparison of how pattern–process interactions may differ between historical and contemporary forests is lacking.…”

Non-Destructive Fuel Volume Measurements Can Estimate Fine-Scale Biomass across Surface Fuel Types in a Frequently Burned Ecosystem

“…They assume a constant fuel bulk density regardless of fuel height, which results in a monotonic increase in fuel load with depth [22,24]. Hence, fuels have historically been viewed through a coarse lens, despite the importance of fine scale fuel variation to fire ecology [17,25,26]. New approaches are needed for better fine-scale estimates of mass, volume, and bulk density of various fuel types found in frequently burned surface fire regimes.…”

Crown fuel consumption in Canadian boreal forest fires

“…However, there are other fine-scale factors such as fuel structure that affect convective and radiative heat transfer and subsequent CFC (Pimont et al 2009;Hoffman et al 2012). Ritter et al (2020) noted that fine-scale patterns in crown spacing and tree group size affect heat transfer from a surface fire to tree crowns, influencing crown ignition and consumption. Individual and small groups of trees are exposed to less thermal energy so a higher level of surface fire intensity is required to initiate torching, resulting in less CFC.…”