Spruce beetles reduce sublimation, causing increasing snowpack in the Wyoming mountains
The Glacier Lakes Ecosystem Experiment Site (GLEES) in southeastern Wyoming.
Sublimation of snow, the process of snow changing into water vapor in the air, is a major component of the annual water budget across the Colorado-Wyoming Front Range where recent bark beetle outbreaks have dramatically changed the forest canopy structure. In mountainous ecosystems one quarter of the annual precipitation can return to the atmosphere as snow sublimation. Yet, predicting the response of these ecosystems to beetle outbreaks is challenging. Conceptually, the loss of needles from the canopy and reduction of leaf area allows more radiant energy, wind, and turbulence to penetrate the understory which would increase sublimation. On the other hand, the loss of needles would decrease the canopy surface area to intercept snowfall and would thereby decrease sublimation.
USDA Forest Service and University of Wyoming researchers recently analyzed 17 years of eddy covariance data from the Glacier Lakes Ecosystem Experiments Site (GLEES) in Wyoming where a spruce beetle outbreak caused 75–85 percent basal area mortality. A Bayesian statistical analysis of the energy and mass balances of the canopy and snowpack was used to evaluate the effect of the spruce beetle outbreak on the process of sublimation. With its vast surface area, canopy intercepted snow allows greater sublimation rates than from the snowpack. The beetle outbreak reduced the ability of the canopy to intercept snow and subsequently led to a decline in total sublimation. Overall, the forest retained 6.1 percent more snowpack mass or equivalently 4.4 percent of the annual precipitation. Considering tree growth and ecological succession are slow in spruce-fir forests, this decrease in sublimation should persist as an increased snowpack for decades, with substantial impacts on catchment hydrologic processes and potentially streamflow.