Ecological Thresholds in Alpine Ecosystems

J.S. Baron (1), K.R. Nydick (2), B.M. LaFrancois (3), A.M. Wolfe (4), H.M. Rueth (5). 1 U.S. Geological Survey, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins CO. 2 Aquatic, Watershed, and Earth Resources, Utah State University, Logan UT. 3 St. Croix Watershed Research Station, National Park Service, Marine-on-St.Croix MN. 4 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton AB. 5 Ecosystems Center, Woods Hole, MA.

Ecological Thresholds Meetin
November 4-5, 2002



Remote high elevation ecosystems are not obvious places to look for ecological thresholds crossed in response to disturbance; yet there appear to be strong rapid responses to slight increases in atmospheric nitrogen deposition. Nitrogen deposition to the Colorado Front Range has ranged 3-6 kg N ha-1 yr-1since about 1980, and many lakes have elevated NO3 concentrations over expected values. Paleolimnological reconstructions of diatom assemblages from alpine lakes show pronounced changes in species composition about 1950-1970, coincident with increasing N emissions from the South Platte River basin to the east. Experimental work in lakes with low NO3 concentrations showed rapid responses in species assemblages and productivity similar to the paleolimnological record, confirming the role of NO3 in affecting lake algae. There have been concurrent changes in alpine tundra vegetation, and in subalpine forest soil microbial activity and C:N ratios, suggesting an entire ecosystem response. Modeled ecosystem responses to climate change suggest warming will promote earlier snowmelt, enhancing soil and vegetation N uptake, possibly reducing the NO3flux to aquatic ecosystems. The response of aquatic algal assemblages to reduced N availability is unknown.