LB 23-199 - Fire refugia and fire severity in forests of the Pacific Northwest: Models, drivers, and applications

Recent stand-replacing wildfires in old forests of the Pacific Northwest (PNW) of the United States have increased land manager and scientific interest in fire refugia that can provide important ecosystem services during a time of rapid change. Fire refugia are locations that burn less severely or less frequently than the surrounding landscape, and contribute critical heterogeneity to forest ecosystems. The overall goal of this project is to model, map, and share information on fire refugia essential for conservation and adaptation of mature and old forest ecosystems in the PNW across western Washington, western Oregon, and northern California. We showcase a suite of fire refugia models and products. We developed boosted regression tree models trained on remotely-sensed contemporary (2002-2017) fire refugia, intermediate fire severity, and high-severity fire effects in forests, using topography, fuels/vegetation, fire weather, fire behavior, and climate as explanatory variables. This framework enabled us to represent normal versus extreme fire growth events – the latter exemplified by the 2020 Labor Day fire events. We used these models to evaluate drivers of fire refugia and severity, to produce probability surface maps for fire refugia, intermediate-severity fire, and high-severity fire, and to examine how probability shifts under low, moderate, and extreme fire weather and fire growth scenarios. We tested the predictive capacity of the models on the 2020 and 2021 fire footprints in the region, showing the value of the modelling tools as durable predictors of forest persistence within contemporary and future fire environments.Our results show there is interpretable predictability in the locations of fire refugia the PNW. Vegetation and topographic template were key predictors. Live tree biomass was consistently a top predictor suggesting biomass characteristic of old growth forest is strongly related to capacity to persist as fire refugia. There is a topographic template that supports fire refugia: low slopes, valley bottoms, northwest aspect are all important terrain features. However higher temperatures and lower relative humidity decrease the likelihood of fire refugia, as do blowup events. Our model results integrate complex, multivariable features of forests and their context that contribute to forest persistence as fire refugia.The fire refugia model suite aims to inform regional conservation planning efforts, project-level planning where old forests and habitat for old-forest associated species are high priorities, and to facilitate critical discussions about the ecology and geography of fire and old forests, and their coexistence, as fire activity accelerates with climate warming.