Surface fuel treatments in young, regenerating stands affect wildfire severity in a mixed conifer forest, eastside Cascade Range, Washington, USA

Previous studies have debated the flammability of young regenerating stands, especially those in a matrix of mature forest, and no consensus has emerged as to whether young stands are inherently prone to highseverity wildfire. This topic has recently been addressed using spatial imagery, and weak inferences were made given the scale mismatch between the coarse resolution of spatial imagery and the fine resolution of mechanisms driving fire severity. We collected empirical stand and fire-severity data from 44 regenerating stands that are interspersed in mature, mid-elevation forests in the Okanogan-Wenatchee National Forest (OWNF) on the eastside of the Cascade Range in Washington, USA. These stands are mixed-species plantations that were established to promote regeneration of seral to late-seral tree species (Douglas-fir, subalpine fir, Engelmann spruce, western larch) in small patches within a larger lodgepole pine forest. In 2006, the 70,925 ha Tripod Fire burned through all the plantations and the surrounding lodgepole pine matrix. To understand what drives fire effects in plantations, especially those that exist in spatially heterogeneous forests, we compared fire severity in plantations with and without fuels-reducing site preparation (i.e., fuel treatments), using three metrics to quantify severity: mortality (%), exposed mineral soil (%), and char height (m). We built generalized linear models for each severity metric and tested for a difference in all severity metrics between treated and untreated units using Permutational Multivariate Analysis of Variance. Units without fuel treatments have more severe fire effects: mortality is 77% in untreated units and 37% in treated units (p = 0.0005). Other variables contribute to differences in fire severity, including species composition, canopy closure, and canopy base height. Canopy base height and canopy closure both exhibit a reverse relationship with mortality from what was expected: the higher the canopy closure and the lower the canopy base height, the lower the mortality. In other words, stands that have trees closer together with crowns near the ground are more likely to have lower mortality. Overall, the results suggest that young stands in some dry mixed conifer forests can be resilient to wildfire if surface fuel loading is low upon stand establishment