The effects of wildfire on tree temperatures and tree well development in the seasonal snow zone

Abstract

The western United States is experiencing unprecedented increases in wildfire activity in mountainous regions. Some of these fires are burning high elevation watersheds of the Rocky Mountains, impacting downstream water quality and availability for both ecosystems and human use. Wildfires can affect the magnitude of snow accumulation and timing of snow melt in these ecosystems, causing earlier peak runoff and extended periods of drought. Prior research on energy budget changes after a wildfire have predominantly focused on altered shortwave radiation and the resulting consequences for the snowpack, but little is known about how the darker tree surfaces of burned trees alter radiation and snow processes in a wildfire-impacted forest. This study quantified how temperatures of burned trees compared to those of unburned, live trees during the snow season in the Cache la Poudre River Watershed in Northern Colorado. This study also analyzed the tree well development and resulting snow melt rates around burned trees compared to open regions. My results showed that burned trees had surface temperatures 2.9–7.3 °C warmer, on average, than live trees during daytime hours. At night, burned trees' surface temperatures were 1.8–4.7 °C cooler than live trees. Tree wells developed and persisted primarily during the peak ablation period in the spring, when the average tree surface temperature remained above freezing and snow depth and snow water equivalent (SWE) decreased from day to day. For a given date, the size of the tree trunk was strongly correlated with tree well diameter (mean R2 value of 0.78), and over the peak ablation season, the cumulative tree surface temperature strongly correlated with well diameter (mean R2 value of 0.82). Snow melt volume calculations revealed that the area directly surrounding burned trees had a daily mean of 38% less snow than an open area of the same size during peak ablation. The snow melted 64% faster, on average, around trees than in open areas, resulting in a mean snow-free date 13 days earlier directly next to burned trees compared to open regions. These results indicate that tree surface temperature plays a critical role in the total radiation budget of a burned forest. The lack of canopy and increased shortwave radiation coupled with char on burned tree surfaces drives the increase in surface temperature, resulting in more longwave radiation emitted from burned tree trunks during the day. This longwave radiation melts the snow surrounding the trees to create tree wells. Further analyses of wildfire's impact on snowpack and water availability should consider the cumulative influence of longwave radiation from burned trees on the snow energy budget and resulting melt rates. This information will help to improve snow energy budget and snowmelt runoff models, providing critical improvements in water planning and management throughout regions of the west that rely on snow as a primary water source.