Modelling the effects of the mountain pine beetle on snowmelt in a subalpine forest

The recent mountain pine beetle epidemic in the Colorado River Basin has resulted in widespread tree mortality in pine stands across the Colorado Plateau. Because of complex micro-scale (i.e. tree well scale) interactions between vegetation and snow processes, one of the most significant issues resulting from this epidemic is the potential hydrologic impacts of the effects of changing forest structure. Using SNTHERM, we conducted a comparative modelling scenario analysis of the snowpack along a transect between two trees over the course of the snow ablation season (28 February–30 June) under four forest stand conditions to assess changes in snowpack characteristics because of loss of canopy biomass. We found that the red phase scenario (intermediate phase of tree death) exhibited a 4-day earlier snow disappearance date than the living stand scenario and grey phase scenario (advanced phase of tree death), although the timing of isothermal conditions at 0 °C was identical. The modelled clearcut scenario snowpack became isothermal at 0 °C 10 days earlier than the living, red phase, or grey phase scenarios. The clearcut modelling scenario also exhibited the greatest homogenization of snow properties, and the spatio-temporal distribution of snow disappearance at the tree well scale was 70% as variable as the living, red phase and grey phase modelling scenarios. These results provide insight to the processes responsible for changing hydrologic dynamics in snow-dominated forest ecosystems with the onset of vegetation stress and death and may help inform future forest management strategies.