Bark Beetles: Cause for Concern in Snowy Western Watersheds?

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

Sub-canopy snowpack under dead conifers has reduced surface reflectivity due to fallen needles, twigs, and branches. This "dirty" snow surface causes the snow to melt more rapidly.

Sub-canopy snowpack under dead conifers has reduced surface reflectivity due to fallen needles, twigs, and branches. This “dirty” snow surface causes the snow to melt more rapidly.

With more snow and faster melt, what might we see in terms of water yield, especially at scales that are of interest to water managers? The authors and their colleagues have used computer models to assess how sensitive streamflow is to beetle impacts at catchment scales, roughly the size of a small western creek watershed. Results showed that the amount of water coming from the catchment could rise up to 10 percent just due to forest canopy loss due from bark beetle kill, as compared with expected flow in the same catchment prior to the infestation.

A number of factors, however, are likely to make real-world changes less impressive. First, growth of the surviving younger understory trees can accelerate dramatically after beetles kill older trees–up to three times their previous growth rates in some cases, due to a new abundance of root-zone water and sunlight. Our modeling suggests that this alone could reduce the increase in water yield sensitivities by half. Second, the patchwork nature of beetle infestations means that we rarely see entire watersheds completely killed at the same time, which can moderate impacts. Third, year-to-year variability in snowfall (up to 300 percent change is not uncommon in Colorado) can essentially disguise most of the contribution of changes in water yield due to beetle infestations.

In addition to the water quantity changes described, forest hydrology research has demonstrated that removal of large sections of forested watersheds may also result in significant impacts on water quality. In particular, researchers found that removal of forest canopy from clear-cutting resulted in significant short-term spikes in nitrate concentrations in surface waters. A lack of vegetative cover also contributes to erosion during significant precipitation events, and both the decomposition of organic material and the mobilization of minerals in soils can negatively impact water quality.