Bark Beetles: Cause for Concern in Snowy Western Watersheds?

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

With respect to bark beetle infestations, however, there is reason to believe that impacts would be much less severe than those seen from clear-cutting. Remaining live trees, especially young trees, are still capable of taking up nitrate. In addition, standing dead trees can help anchor soil and reduce erosion and mobilization of minerals and other chemicals into streams.

Recent research generally agrees with this line of thinking, although with some cautionary notes. One study of streams in Colorado found no significant changes in total nitrate concentrations when comparing watersheds with significant beetle kill against those without. Another study, however, did see greater levels of regulated carcinogenic trihalomethanes in water treatment plants located downstream of significant beetle kill compared to nearby plants in watersheds with less mortality. Again, as with streamflow, the effects of bark beetle infestations on water quality are likely much less impressive than seen in clear-cutting, and may not be easily detectable at large scales.

In the end, should water managers whose utilities depend on watersheds with significant bark beetle infestations be concerned about the quantity or quality of raw water supplies? As of now, prudence warrants monitoring the scope of the infestation and keeping an eye on water supplies. To date we have seen plenty of information demonstrating that bark beetle infestations do increase snow accumulation, speed up snowmelt, and lead to modest additional streamflow, but a variety of mitigating factors may make such changes hard to detect. At the watershed and basin scales that are most often of concern to water managers, it may be that the slow progression of tree death and decomposition, along with the patchwork nature of beetle infestations and the effects of rapid growth from young trees, will keep effects from being particularly great, although more significant changes are certainly possible as forests continue to respond to climate change.

It is worth remembering, however, that bark beetle infestations are just one of many land cover changes in western North America that can have serious impacts on water supplies and utility operations. Mechanical harvesting often results in greater streamflow, and road building or land clearing can increase overland flow rates and sedimentation. Dust from southwestern deserts can be deposited on mountain snowpacks, accelerating springtime melt and reducing overall streamflow volume. Finally, wildfires can have perhaps the most serious consequences, including high erodibility in fire-scarred areas, accelerated runoff from landscapes, and increased mobilization of metals and other pollutants into surface waters.

Eric Gordon is Managing Director of Western Water Assessment.

Researcher Evan Pugh is an expert on the impact of pine beetle tree deaths upon hydrology.

Research Scientist Ben Livneh is with Western Water Assessment.

For more information:
Pugh, E. T. and Gordon, E. S. (2013), A conceptual model of water yield effects from beetle-induced tree death in snow-dominated lodgepole pine forests, Hydrological Processes 27: 2048–2060. doi: 10.1002/hyp.9312

Rhoades, C. C., J. H. McCutchan, Jr., L. A. Cooper, D. Clow, T. M. Detmer, J. S. Briggs, J. D. Stednick, T. T. Veblen, R. M. Ertz, G. E. Likens, and W. M. Lewis, Jr. (2013). Biogeochemistry of beetle-killed forests: Explaining a weak nitrate response. Proceedings of the National Academy of Sciences, 10.1073/pnas.1221029110.

Mikkelson, K. M., E. R. V. Dickenson, R. M. Maxwell, J. E. McCray, and J. O. Sharp (2013). Water quality impacts from climate-induced forest die-off. Nature Climate Change 3: 218-222. doi:10.1038/nclimate1724

feature photo based on cc image from krossbow