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Pugh, Evan T ¹ ; Small, Eric E ²

¹ University of Colorado
² University of Colorado

More than 8,000 km2 of lodgepole pine forest in Colorado have been infested by mountain pine beetles since 1996. We examine the impact of beetle-induced tree death on snowpack accumulation and ablation. The data were collected one to two years after beetle infestation, when most needles remain on the trees but the trees are dead. During the winter of 2009, snowpack and meteorological properties were measured at eight pairs of dead and living lodgepole pine stands. All stands are located at an elevation of 2720 ± 32m, in a subalpine region along the headwaters of the Colorado River. Trees in living stands were generally smaller in diameter and more densely populated than trees in dead stands. Snowpack accumulated equally beneath living and dead tree stands. In addition, snow under all tree stands became isothermal on the same date regardless of mortality. However, the snow melted more rapidly under dead trees. As a result, the snow was depleted as much as one week earlier beneath dead stands. During the melt interval, the lower SWE in dead stands was due to shallower snow, not lower snow density. Canopy transmission of solar radiation was not consistently different between living and dead stands. We noted more ground litter in dead stands which would decrease snowpack albedo and lead to the snowmelt differences observed. Earlier snowmelt caused by tree mortality accentuates established one to four week advances in snowmelt due to global warming. However, as beetle-induced tree death progresses, earlier snowmelt under dead stands may no longer occur. Once dead trees are denuded of needles and small branches, more snow may accumulate under dead stands due to a reduction in canopy interception of snow and subsequent sublimation.