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Spruce beetles reduce sublimation, causing increasing snowpack in the Wyoming mountains

Date: July 16, 2019

Recent spruce beetle outbreaks have changed forest canopy structure, reducing winter snow sublimation and increasing snowpack


Background

Sublimation of snow 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.

Research

USDA Forest Service and the University of Wyoming researchers recently analyzed seventeen 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.

An analysis of seventeen years of eddy covariance data using a Bayesian statistical model based on the two-source energy and canopy snow mass balance showed a decrease in sublimation in a subalpine forest following a spruce beetle outbreak.
An analysis of seventeen years of eddy covariance data using a Bayesian statistical model based on the two-source energy and canopy snow mass balance showed a decrease in sublimation in a subalpine forest following a spruce beetle outbreak

Key Findings

Following the loss of canopy, the forest could only intercept a third of the snow as it could prior to the outbreak.
Although the loss of canopy increased the available energy transmitted to the snowpack and led to a modest increase in snowpack sublimation, the loss of intercepted snow overwhelmingly decreased canopy sublimation, leading to one-quarter less sublimation from the ecosystem.
This is equivalent to 6.1 percent of the snowfall or 4.4 percent of the total annual precipitation being retained in the snowpack.

 

 

Featured Publications

Frank, John M. ; Massman Jr, William J. ; Ewers, Brent E. ; Williams, David G. , 2019


Principal Investigators: 
External Partners: 
Brent E. Ewers, Co-Investigator, University of Wyoming
David G. Williams, Co-Investigator, University of Wyoming
Research Location: 
Glacier Lakes Ecosystem Experiments Site, Wyoming