Climate Change Research Spotlight

Bark Beetles Cause Big Tree Die-Offs, but Streams Flow Steadily

Recent beetle epidemics have driven tree die-offs across North America, and previous studies predicted an increase in annual streamflow would follow—but a new study shows this may not be the case.

Source: Water Resources Research

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The native bark beetle is an important player in healthy forest ecosystems across western North America, cycling nutrients, shaping habitats for insects and fungi, and altering the vegetative landscape. Recently, however, booming bark beetle populations have devastated forests across the western United States and Canada.

Warmer temperatures, drought, and previous management practices have contributed to a bark beetle epidemic, which started in the mid-1990s and continues to drive extensive tree mortality. Following early studies of forest hydrology, scientists predicted an increase in streamflow due to massive declines in snow evaporation from deteriorated forest canopy and water use by trees. In a new study, however, Biederman et al. assess 50 years of streamflow observations before and after beetle impact in the headwaters of the Colorado River to compare these predictions with actual stream behavior.

Billions of trees have died across western North America in a decade-long bark beetle epidemic. Researchers are trying to learn how forest die-off impacts water supplies. Credit: Juan Fernandez, NCALM
An aerial view of a forest north of Laramie, Wyo., showing trees affected by a decades-long bark beetle epidemic. The blight has killed billions of trees across western North America, but this die-off does not seem to be changing annual streamflow.  Credit: Juan Fernandez, NCALM

The researchers used streamflow data from the U.S. Geological Survey National Water Information System and multiple empirical and modeling approaches to assess how streams responded during a decade following extreme tree die-off. They performed a double mass analysis comparing cumulative annual streamflow in eight beetle-infested catchments to a nearby control catchment that was not infested by bark beetles. They also compared the amount of precipitation that became runoff before and after die-off and used a climate-driven model to analyze the beetle’s impact on streamflow over time.

This comprehensive analysis yielded a surprising result: Rather than the increase predicted in earlier research, the team found little significant change in annual streamflow—and in one catchment, streamflow even declined. They attribute this major deviation to increased evaporation of snowpack and soil moisture as dying forests expose the land surface to solar radiation and wind, a topic examined in Biederman et al. (Water Resour. Res., 2014, doi:10.1002/2013WR014994). The authors use these results to highlight the challenges faced by land and water managers during a period of changing climate and widespread ecosystem disturbance. (Water Resources Research, doi:10.1002/2015WR017401, 2015)

—Lily Strelich, Freelance Writer

Citation: Strelich, L. (2016), Bark beetles cause big tree die-offs, but streams flow steadily, Eos, 97, doi:10.1029/2016EO047593. Published on 9 March 2016.

© 2016. The authors. CC BY-NC 3.0