Assessing the Role of Snow Cover for Post‐Wildfire Revegetation Across the Pacific Northwest. Issue 11 (16th November 2021) - Wilson, Andrew C.
Abstract: Forested, mountain landscapes in the Pacific Northwest (PNW) are changing at an unprecedented rate, largely due to shifts in the regional climate regime. Documented climate warming trends across the PNW include increasing wildfire frequency and severity and an increasingly ephemeral snowpack, especially at moderate elevations. We analyzed 24 high severity wildfires across four distinct PNW mountainous subregions, examining snow‐vegetation relationships for two years pre‐fire and four years post‐fire. To assess the importance of snow cover for revegetation compared to other climatic, topographic, and burn severity‐related variables, binary regression tree models were constructed for the dominant pre‐fire conifer species within each of the four PNW subregions. Summer precipitation consistently appeared as the most important variable driving post‐fire revegetation across all four subregions. Snow cover variables (snow cover frequency and snow disappearance date), along with elevation, were shown to be secondary but significantly influential explanatory variables for revegetation in the Oregon and Washington Cascades. Revegetation was also analyzed using a time series of linear regressions across 200‐m elevation bands by measuring correlations between winter snow cover and summer vegetation greenness. Results showed strong positive post‐fire correlations at moderate elevations in the western Montana Rockies and at the lowest elevation band in the Idaho Rockies. Considering trends of increasing wildfire activity, lower snowpacks, and earlier snow disappearance dates across the PNW, forests will likely experience more frequent drought conditions that will impact post‐wildfire vegetation regrowth. Plain Language Summary: Wildfires continue to burn more area each year across many regions of the planet, including the Pacific Northwest. There are many short‐ and long‐term effects from these fires, including erosion, debris flows, and water quality issues, which can affect the health of aquatic ecosystems and downstream community water supply. The persistence of these impacts is strongly related to reestablishment and recovery of vegetation in burned areas. However, post‐wildfire revegetation in forested, mountainous landscapes is complex and depends on many factors, including fire severity, pre‐fire vegetation, elevation, slope, aspect, rain, and snow accumulation and melt. Little is known about the linkages between these drivers and revegetation across the PNW. Our study illustrated that summer precipitation, snow cover, and elevation were all important drivers of revegetation success. In particular, we found that snow cover was a critical explanatory variable for revegetation in the Oregon and Washington Cascades. Given the trends of increasing wildfire activity, lower snowpacks, and earlier snow disappearance dates across the PNW, forests will likely experience more frequent drought conditions, which will impact the success of post‐wildfire vegetation recovery. Key Points: Summer precipitation, snow cover, and elevation are important drivers of post‐fire revegetation success Snow cover was a critical explanatory variable for post‐fire revegetation in the Oregon and Washington Cascades Strong correlations exist between winter snow cover and summer vegetation greenness in the lower snow zone in Montana and Idaho Rockies