Bark Beetles: Cause for Concern in Snowy Western Watersheds?

A five millimeter long insect triggers complex changes in the hydrology of entire watersheds.

To begin with, we can consider how beetle-killed affect the growth and melting of snowpacks. At the forest plot level (think of a “plot” as a square of roughly 100 yards on a side), tree death from bark beetle infestations can affect snowpack in two specific ways–by increasing total snow accumulation and by changing factors that control how the snow disappears through melt or sublimation, collectively known as ablation.

Overall changes in total snow accumulation are connected to interception, the primary mechanism that controls the amount of precipitation reaching the forest floor. Tree leaves and branches retain snow very effectively, preventing more than half of incoming snowfall from reaching the ground in some cases. Of that intercepted snow, nearly two-fifths can sublimate directly back into the atmosphere. However, in bark beetle-infested coniferous forests, trees drop their needles and develop drooping branches, reducing their ability to intercept snow and leaving more snow to accumulate on the ground. Calculations based on field observations conducted in northern Colorado by the authors demonstrate that this “grey phase” could result in roughly 5-15 percent more total snow accumulation.

Thinned canopies in dead conifer forests allow more precipitation, as well as more sunlight, to reach the ground.

Thinned canopies in dead conifer forests allow more precipitation, as well as more sunlight, to reach the ground.

Although this reduction in tree canopy material can lead to greater snowpack accumulation in forests, bark beetle-related changes in forests also generally result in an overall increase in the rate at which snow melts. Snowmelt is driven by a number of factors, but the primary mechanism is incoming shortwave radiation, otherwise known as visible sunlight. Just as trees shield the ground from greater snow accumulation, trees also reduce the amount of incoming sunlight reaching the snowpack. Thus as trees killed by beetle infestations lose needles and twigs, more sunlight can reach the ground, with the authors’ field research demonstrating an increase of roughly 10-15 percent. This additional sunlight leads to increased melting of snow. In addition, the dead needles and twigs that fall out of the tree canopies accumulate on the snowpack and reduce the reflectivity of the snow, increasing melt rates–in fact, studies in Colorado have shown up to a one week advance in the timing of snowmelt. Finally, dead trees do less to block the wind, resulting in higher rates of snow sublimation on the ground.