Forested, mountain landscapes in the Pacific Northwest (PNW) are changing at an unprecedented rate, largely due to shifts in the regional climate regime. Documented climatic trends across the PNW include increasing wildfire frequency and intensity and an increasingly ephemeral snowpack, especially at moderate elevations. One relationship that has yet to be studied thoroughly is the influence of snow cover on post-wildfire forest regeneration. This research presents two approaches that aim to characterize the relationship between snowpack and post-wildfire greening by analyzing 24 recent severe wildfires across the PNW within four distinct mountainous sub-regions. Fires were binned together for analysis in each region with the goal of including a spectrum of post-fire climatic conditions. Winter snow water equivalent is proxied using a new Snow Cover Frequency (SCF) metric derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) daily snow cover product. Post-wildfire vegetation productivity is measured using the maximum summer Enhanced Vegetation Index (EVI) value, also derived from daily MODIS satellite imagery. To assess the importance of snow cover for regenerating vegetation compared to other variables relating to climate, topography, and burn severity, binary regression tree models were constructed for the pre-fire dominant conifer species within each of the four PNW sub-regions.
Summer precipitation was shown consistently to be the most important variable driving post-fire regrowth. Snow cover variables, along with elevation, were shown to be secondary but significantly influential explanatory variables for revegetation in the Oregon and Washington Cascades. Revegetation was also assessed across 200 m elevation bands using a time series of linear regressions measuring correlations between SCF and maximum summer EVI. 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. Post-fire correlations were weak in the Washington Cascades, and were more ambiguous in the Oregon Cascades, as negative correlations were observed for grand fir and western hemlock pixels while positive correlations were observed for mountain hemlock. Results support the hypothesis that winter snowpack plays an important role for regenerating vegetation, likely as an important source of summer soil moisture following spring snowmelt. Considering trends of increasing wildfire activity, lower snowpacks, and earlier snow disappearance dates across the PNW, forests will likely experience more frequent scenarios of post-wildfire regrowth during drought-stressed conditions. This research offers evidence that future PNW forests may be characterized by lower forest densities and may become populated with increasing proportions of drought-tolerant tree species.