Assessing the Role of Snow Cover for Post-Wildfire Revegetation Across the Pacific Northwest

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.