Bark beetle impacts on Colorado water supplies: Still waiting for the shoe to drop?
Presented by Jeff Lukas, Associate Scientist, Western Water Assessment
AWRA-Colorado January 2012 Meeting
Summarized by Steve Smith, AWRA-CO Publications Committee
Jeff Lukas gave a talk to the AWRA-Colorado Section regarding the impacts of the bark beetle on Colorado’s water supply. Jeff’s talk focused on the following two major premises regarding the mountain pine beetle epidemic in Colorado and Wyoming:
1. There were widespread expectations that the beetle would have significant impacts on water supplies, both to water quantity (e.g., higher peak flows) and water quality.
2. Actual impacts on water supply related to this epidemic in Colorado and Wyoming have been much more modest than originally anticipated.
The mountain pine beetle (MPB) epidemic, as well as effects from other native bark beetles, has dramatically affected Colorado’s landscape. Approximately four million acres in Colorado and Wyoming have been affected since 2003. It is hard to miss whole mountainsides filled with dead red and gray lodgepole pines as a result of the beetle epidemic. Although it appears that all of the trees are impacted within an affected area, the reality is that the maximum mortality rate is approximately 80 percent and is often less. Affected trees die almost immediately, and forest regeneration begins even as the dead trees are still in the “red phase.” The red phase occurs one to four years after the beetle infestation begins. Although lodgepole pine trees will still be an important part of the future forest, regeneration of aspen and other non-MPB host trees (spruce and fir) have been dominant in the wake of the infestation.
Original expectations for the impacts on water quantity as a result of the MPB epidemic were based on physical process studies regarding the potential influence on the water and energy budgets, in addition to a few previous studies of a bark beetle (spruce beetle) infestation in Colorado, and the results of forest harvest experiments. The basic expectation was that significant loss of overstory trees (i.e., canopy trees that stand over the rest of the forest plants) would likely cause greater overall runoff as well as higher and earlier peak flows, due to:
• Reduced sublimation of snowfall back into the atmosphere (20 to 40 percent of snowfall typically is intercepted by healthy lodgepole canopies, and is sublimated back to the atmosphere).
• Increased amount of snow (by about 20 percent) reaching the ground as a result of less interception by the canopy.
• Faster snowmelt of the increased snow as a result of more solar radiation on the forest floor (solar radiation drives snowmelt even more so than warm temperatures).
• Reduced transpiration losses of soil moisture as the dead trees no longer take up water.
Over time the actual effects of the MPB epidemic have become better understood. The current pine beetle infestation has not appeared to have as substantial of an effect on peak flow volume and timing as originally anticipated. Jeff presented data from three studies that examined annual runoff volumes or runoff efficiency (runoff-to-precipitation ratio) from a total of 25 MPB-impacted watersheds in western Colorado and southern Wyoming. None of the studies found consistent post-infestation changes in runoff volume or runoff efficiency across the watersheds. There have not been any studies completed regarding MPB impacts on the timing of runoff, though generally earlier runoff in the past two decades has been observed and attributed to regional warming and dust-on-snow effects. More substantial effects on water quantity were observed after the spruce beetle infestation in the 1940s in western Colorado. However, lodgepole pine forests affected by the MPB epidemic may respond differently, in part due to lower annual precipitation than in spruce forests. One of the likely reasons for a more subdued effect on water quantity than expected is the resurgence in growth of the forest understory combined with the number of trees that survived the infestation, offsetting the anticipated effects on the water balance.
The MPB epidemic has shown some effects on water quality in Colorado as a result of excess nutrients being exported to the system from the dead lodgepole pines, though as with runoff, the effects have been much smaller than anticipated. In particular, the observed increases in stream nitrate concentrations are not to levels of concern, and pale in comparison to those associated with forest harvest treatments such as clearcutting. Other effects on water quality associated with the MPB epidemic that have been seen in some parts of Colorado are:
• Elevated trihalomethanes in treatment plant effluent (dissolved organic carbon from dead trees is reacting with chlorine in the treatment process to produce the disinfection byproducts).
• Discolored water from dissolved organic carbon in water supplies.
• Increases in stream temperature of up to two degrees Celsius associated with reduced shading from beetle-killed riparian trees.
• Pesticides used to control the MPB epidemic have been observed in low concentrations in at least two locations in the water supply or effluent.
Although effects on water supplies associated with the MPB epidemic have been generally modest, it is still possible that more serious impacts could occur (the “shoe” that may yet drop.) In particular, large fires have not occurred in the MPB-affected areas in Colorado or Wyoming, but could still occur depending on how hot and dry future summers are. Jeff noted, though, that large and destructive fires have occurred and can occur in “green” forests, and the additional fire hazard due to the MPB infestation has often been overstated. Also, a beetle “signal” in runoff may yet emerge from the noise of year-to-year variability and other factors, such as climate change and dust-on-snow. A Western Water Assessment-funded study is currently trying to tease apart those three factors in an effort to help the National Oceanic and Atmospheric Administration (NOAA) improve runoff forecasts.
More information on Western Water Assessment’s efforts to bring together researchers and water and forest managers working with the MPB epidemic can be found on their website at the link below. Beetles, Water, and Climate is a multi-purpose web resource intended for both researchers and resource managers trying to understand and respond to impacts of the MPB epidemic. It includes a comprehensive list of references and presentations from recent research symposia and workshops. See their website at: http:/wwa.colorado.edu/ecology/beetle/.