Biogeochemical and Hydrologic Changes During the Incipient Stage of Mountain Pine Beetle Outbreak in Managed and Unmanaged Subalpine Watersheds at the Fraser Experimental Forest

Chuck Rhoades, Kelly Elder, Rob Hubbard and Mark Dixon

USDA Forest Service Rocky Mountain Research Station

It is estimated that more than 30 million ha of pine forest are at risk of bark beetle attack in the United States.  Mountain pine beetle (Dendroctonus ponderosae; MPB) killed nearly 1 million trees on national and private forestland in the Rocky Mountain region between 1995 and 1999.  There is little information to assist resource managers to evaluate the extent to which bark beetles threaten watershed functions.  The USFS Fraser Experimental Forest (FEF) is located near the epicenter of the current outbreak in the central Rockies of Colorado.  Extensive logging within the region followed by wildfires in the early 1900s established lodgepole pine forests that have now reached an age and stand structure that put them at risk for bark beetle attack.  MPB mortality was first observed at FEF in 2003 following several years of below average precipitation; MPB mortality now exceeds 90% of the overstory in several large areas of the Arapaho-Roosevelt National Forests that adjoin FEF.  The Fraser Experimental Forest supports a network of long-term streamflow (> 50 yrs) and biogeochemical (~ 20 yrs) information that offer a unique opportunity to assess how this disturbance agent will influence managed and unmanaged subalpine forest ecosystems.  As part of separate paired watershed studies aimed at assessing forest harvesting effects on watershed hydrology, portions of the Fool Creek (50%) and Deadhorse Creek (~33%)watersheds were clear-felled in the mid-1950s and 1980s.  Research at FEF has shown that forest structure regulates the water balance and nutrient budgets of harvested and unharvested watersheds; we hypothesize that residual differences between FEF basins will determine basin-scale response to bark beetle mortality.  This preliminary assessment will characterize the incipient stage of this large-scale disruption and will evaluate how the responses of nutrient export, vegetation water use and basin hydrology differ between managed and unmanaged watersheds.

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