Bark beetle infestations affect water quality in the Rocky Mountains of North America

February 12th, 2013

Kristin Mikkelson, Dr. Eric Dickenson, Prof. Reed Maxwell, Prof. John McCray & Prof. Jonathan Sharp

Forest dieback related to climate-change associated stresses such as drought, heat, and insect outbreaks has emerged as a global concern and has been documented on all wooded continents and across diverse forest types.1,2 With warming temperatures, outbreaks are predicted to shift towards higher latitudes and higher elevations,3 impacting previously unscathed forests. In addition, as tree mortality within infested forests increases, the deforested zone shifts from a carbon sink to a carbon source, which may further contribute to climate change.4

A poignant example is the mountain pine beetle (Dendroctonus ponderosae; MPB) infestation in the Rocky Mountain west of North America. This native beetle’s range encompasses most of western North America; however, the current infestation has reached epidemic proportions throughout the Rocky Mountains with over 4 million acres of mortality in Colorado and Wyoming alone in the past decade (Figure 2).5,6 While the visual impact of dying forests is stunning, less visible hydrological and biogeochemical changes are taking place in impacted watersheds.

When beetles infect healthy trees they transport with them a blue-staining fungus that helps to restrict nutrient and water transport into the tree. This eventually can cause tree mortality that results in large canopy changes (see Figure 1) altering multiple components of the water and energy cycles, including interception of precipitation and radiation, canopy wind speed, snow accumulation and melt and evapotranspiration.7 Along with alterations in the water and energy budgets, bark beetle infestations have the potential to alter forest biogeochemistry, solute transport and hence water quality. One major water quality issue for local water treatment plants receiving their source waters from beetle-infested watersheds is changes in organic carbon loading.

Total organic carbon (TOC) is ubiquitous in surface and groundwaters and originates from both natural and man-made sources. Typically, spring runoff in mountainous watersheds mobilizes dissolved organic matter into surface waters8 creating a more pronounced seasonal release. Bark beetle infestations have the ability to influence TOC release and loading as decreases in canopy cover can increase runoff rates, alter hydrologic flow paths through more carbon rich soil layers and excess needle loss onto the forest floor can lead to increased decay and soil organic matter leaching.

These changes in TOC concentrations and properties can be problematic for water treatment facilities as chlorination of TOC-rich waters can lead to potentially carcinogenic disinfection byproducts (DBPs).9 Humic like substances typically found in dissolved organic matter, have been shown to be common DBP precursors when combined with chlorine disinfection. DBPs are heavily regulated by the United States Environmental Protection Agency (U.S. EPA).

Our study, based on water quality data sets from water treatment plants throughout the Colorado region, reveals on average about 300% more TOC and DBPS at water treatment facilities located in MPB-infested watersheds.10 We also saw an increasing trend in one particular class of DBPs since the initial onset of the bark beetle epidemic around 2004. This significant increase in DBP concentration was accompanied by only a modest increase in TOC concentrations. Our results revealed that at water treatment plants receiving their source waters from MPB-infested watersheds, trihalomethanes (THM) have significantly increased while haleoacetic acids (HAA) have not. THMs form from chlorination of the hydrophobic fraction of natural organic matter, while HAAs form from the chlorination of the hydrophilic fraction. Thus, our results indicate that while we are only seeing a slight increase in TOC levels due to the bark beetle epidemic, it appears that the composition of the TOC is changing and becoming more hydrophobic in nature.

Along with the bark beetle epidemic changing the composition of TOC, our results demonstrate that the seasonal coupling of high-flow rates, high TOC concentrations and high DBP concentrations has been altered. In beetle-impacted catchments we found the highest THM concentrations during the low flow months of July through September. What is interesting about this finding is that the increase in THM levels was not accompanied by an increase in TOC levels. The water treatment plants were still seeing the highest TOC levels in spring and early summer, as is typical in snow-dominated watersheds. However, the TOC reaching the treatment plants in the late summer and early fall seemed to be more reactive with the chlorine, despite its lower concentrations.

While the scientific significance of water quality impacts resulting from a climate-induced phenomenon is not fully understood, the practical and local implications of this study are of immediate concern. Individual water treatment plants have experienced quarterly THM concentrations that are more than 30 ug/l above the EPA’s regulatory limits. From an economic perspective, it is essential to determine if the change in TOC loading and composition is going to occur seasonally and for what duration in order for water treatment facilities to better prepare and possibly modify water treatment processes. It is unknown whether or not these trends are only going to occur in the mountainous watersheds of Colorado, or if other bark beetle infested watersheds throughout the world could experience similar changes in water quality.

References:

1. Williams, D. W. & Liebhold, A. M. Climate change and the outbreak ranges of two North American bark beetles. Agricultural and Forest Entomology 4, 87-99 (2002).

2. Allen, C. D. et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259, 660-684 (2010).

3. Dale, V. H. et al. Climate change and forest disturbances. BioScience 51, 723-734 (2001).

4. Kurz, W. A. et al. Mountain pine beetle and forest carbon feedback to climate change. Nature 452, 987-990 (2008).

5. Raffa, K. F. et al. Cross-scale drivers of natural disturbances prone to anthropogenic amplification: the dynamics of bark beetle eruptions. BioScience 58, 501-517 (2008).

6. Amman, G. D., McGregor, M. D. & Dolph, R. E. Mountain pine beetle. (USDA Forest Service, Pacific Northwest Region, Forest Insects and Diseases, 1997).

7. Mikkelson, K. M. et al. Mountain pine beetle infestation impacts: modeling water and energy budgets at the hill-slope scale. Ecohydrology, DOI: 10.1002/eco.1278 (2011).

8. Boyer, E. W., Hornberger, G. M., Bencala, K. E. & McKnight, D. M. Response characteristics of DOC flushing in an alpine catchment. Hydrological Processes 11, 1635-1647 (1997).

9. Nikolaou, A. D. & Lekkas, T. D. The Role of Natural Organic Matter during Formation of Chlorination By products: A Review. Acta Hydroch. Hydrob. 29, 63-77 (2001).

10. Mikkelson, K. M., Dickerson, E. R., McCray, J. E., Maxwell, R. M. & Sharp, J. O. Water quality impacts from climate-induced forest die-off. Nature Climate Change doi:10.1038/nclimate1724 (2012).

Kristin Mikkelson is a PhD candidate at the Colorado School of Mines (Colorado, USA). Her research primarily focuses on ecohydrology and climate-change related issues. Dr. Eric Dickenson is a project manager at Southern Nevada Water Authority (Nevada, USA) whose expertise lies in disinfection byproducts and water quality. Dr. Reed Maxwell is an associate professor at the Colorado School of Mines. His research interets include integrated watershed response to climate change, numerical model development and interactions between surface water, groundwater and the atmosphere. Dr. John McCray is a professor at the Colorado School of Mines whose research focuses on chemical transport in hydrologic systems. Dr. Jonathan Sharp is an assistant professor at the Colorado School of Mines and his research interests include the applications and implications of biological processes that relate to water quality and resuse. This article is based on an original piece of research published in Nature Climate Change, doi:10.1038/nclimate1724: “Water-quality impacts from climate-induced forest die-off”.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.