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

Abstract

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.

Keywords

• crown fire
• crown fire transition
• forest ecology
• forest management
• pattern‐process feedback
• restoration