STREAMWATER NUTRIENT RESPONSES TO PINE BARK BEETLE INFESTATION AT THE FRASER EXPERIMENTAL FOREST, COLORADO

Mountain ecosystems of western North America are experiencing extensive tree death from a variety of insect species. Biogeochemical responses and ecosystem recovery from this episode of insect-caused mortality are linked to pre-disturbance forest and watershed conditions. The mountain pine bark beetle (Dendroctonus ponderosae) began to attack lodgepole pine (Pinus contorta) at the Fraser Experimental Forest in 2002 and spread throughout the research area by 2007. Here we compare streamwater nitrogen (N) concentrations and export from 2003 through 2012 with data from the previous two decades in four research watersheds with varying forest management history and watershed characteristics. During the infestation, flow-weighted streamwater nitrate concentrations were significantly higher than pre-infestation levels during the snowmelt and base flow seasons in two old-growth watersheds. Beetle infestation elevated nitrate export 43 and 74% in these two watersheds though the amounts of N released in streamwater (0.04 and 0.15 kg N ha^-1) were < 2% of annual atmospheric inputs. In contrast, nitrate concentrations were unaffected by beetle infestation in two watersheds comprised of a mixture of second-growth (30-60 year old) and old-growth (250-350 year old) forest stands where the density of residual live trees was 31% higher on average. Mortality of lodgepole pine trees has stimulated increased growth of understory and overstory trees with likely consequences for nutrient retention and water quality in recovering forests.