The impacts mountain pine beetle on forested snowpacks : accumulation and ablation

Abstract

The future of water resources in the west is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Associated with climate changes to the physical environment are changes to the biological environment that may impact snow dynamics; namely via the massive outbreaks of Mountain Pine Beetle (MPB; Dendroctonus ponderosae) that have devastated several million hectares of Lodgepole Pine forests in the western U.S. and Canada. If snow accumulation and melt are determined by the physical environment of the snowpack, and forest canopies define in part this physical environment, how might recent insect outbreaks alter the timing and intensity of snowmelt? MPB often attack in large numbers, and within a few years, the canopy of an infected forest will turn from green but dying, to red, to grey. As needles fall, impacts on the snow pack include changes to wind driven transport, temperature gradients, and snow interception. Additionally, the shifting canopy alters the radiated physics of the canopy, specifically the shortwave/longwave flux density. Combined with a corresponding decrease of snow reflectance (albedo) from litter fall, the dying canopy will provide more energy available to the surface and likely drive snowpacks to melt more rapidly. Conversely, the diminished canopy cover will presumably decrease net longwave radiation of the snowpack. Canopy interception of snow is expected to decrease, and an increase in accumulation will result. I investigate the impacts of MPB disturbance on snow melt through modeling and micrometeorological measurements in intact lodgepole pine and mixed coniferous forests, a MPB-infested forest in the red stage, and a clearcut stand. Albedo at the homogenous intact stand is found to be 16 and 34% higher than the red stand during the melt periods of 2011 and 2012, but no significant difference is found between the red stand and the more heterogeneous 'healthy' stand. Modeled sensible heat over-predicts sensible heat by over 300% during the melt period of 2012. Results highlight the role of beetle-infested and mixed stands on altering snow albedo, and additionally suggest that model formulations for turbulent exchange between snow and atmosphere below forest canopies require improvement.