First-entry wildfires can create opening and tree clump patterns characteristic of resilient forests

Abstract A century of firesuppression has left fire-dependent forests of the western United States increasingly vulnerable to wildfire, drought, and insects. Forest managers are trying to improve resilience using treatments such as mechanical thinning and prescribed fire; however, operational and resource constraints limit treatments to a fraction of the needed area each year. An alternative is to let wildfires burn under less-than-extreme fireweather where human lives and infrastructure are not at risk. We examined post- fireforest structure using airborne lidar data to determine whether a single wildland firefollowing an extended fire-free period could produce forest structures resembling fire-resilient historical conditions. We studied forest structures resulting from these “first-entry” firesin a forest with a history of timber management (2008 American River Complex Fires, Tahoe National Forest) and in a wilderness area(2009 Big Meadow Fire, Yosemite National Park). We compared the results of these first-entry fireswith nearby reference areas that had experienced 2+ firesthat burned predominately at low- and moderate-severity. We identified visible overstory trees from the lidar data and examined their patterns in terms of individuals, tree clumps, and openings. We found that moderate-severity fireeffects in these first-entry firesproduced similar patterns to the reference areas with area in openings at approximately 40% and trees predominately in small (2 to 4 trees) and medium (5–9 trees) clumps High-severity fireproduced mortality likely to lead to large canopy openings that were historically uncharacteristic in these forests. As burn severity increased, the amount of the residual canopy area represented by taller trees (>16 m and especially >32 m) decreased, which could result from firespreferentially killing taller trees or from locations with taller trees more commonly experiencing lower burn severities. Our study suggests that first-entry firesallowed to burn under less-than-extreme conditions can reproduce spatial patterns resembling historical conditions resilient to firesand drought but possibly at the disproportionate expense of larger trees.