Effects of the mountain pine beetle on forest hydrology and chemistry in the southern Rocky Mountains

The mountain pine beetle is the main cause of insect-induced tree mortality in western North America.  Over the past fifteen years, pine forests in the southern Rockies experienced the most severe mountain pine beetle epidemic in recorded history, with up to 50% mortality of mature pine trees.  Contributing factors include an abundance of mature trees in dense lodgepole forests, drought stress, and warming temperatures.  These factors have allowed the mountain pine beetle to expand its elevation and latitudinal ranges into areas formerly too cold for its survival.

The death of large numbers of pine trees represents a profound change in the pine forest landscape of the western U.S., which covers millions of acres. USGS and university scientists are working together to assess possible effects of the mountain pine beetle epidemic on the hydrologic cycle, nutrient and carbon cycles, and water quality. The results of three recent studies are presented here.

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In a study in Rocky Mountain National Park, scientists used chemical and isotopic tracers of water to determine whether beetle-induced tree mortality influences the hydrologic cycle.  Results indicate that forest transpiration, which can be thought of as evaporation from plant leaves, decreased after trees infested by the mountain pine beetle died.  Soil moisture and ground evaporation increased below dead trees, and the net effect was increased groundwater recharge and groundwater contributions to stream flow.

In two related studies, scientists looked at changes in the characteristics of nutrients, water, and carbon associated with soil and streams within pine-beetle affected forests to determine how the epidemic affected forest and aquatic ecosystems.  Soil nutrients and soil moisture increased in affected areas as uptake by dying trees declined.  Despite these changes in soil fertility, little change has been seen in nutrient or carbon concentrations in streams, leaving aquatic ecosystems mostly unaffected.  It appears that small, young trees, which formerly were shadowed by the large, mature trees that were killed by the pine beetles, have been able to take advantage of the increase in soil moisture, nutrients, and light.  The young trees are now growing rapidly, rebuilding the forest in a natural succession process.

Results from these studies indicate that pine forests can be surprisingly resilient, even in the face of large-scale insect outbreaks.  Hydrologic effects in insect-infested forests were modest, and changes in water quality were minimal.  Selective clearing of dead trees may help reduce visual impacts and fire hazard; however, more aggressive forest management techniques, such as clear cutting, can cause ground disturbance that might be detrimental to ecosystem health.

The papers, published in Nature Climate Change, Applied Geochemistry, and Proceedings of the National Academy of Sciences, are available at: https://pubs.er.usgs.gov/publication/70141031, https://pubs.er.usgs.gov/publication/70190172, and https://pubs.er.usgs.gov/publication/70204833.

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