Fluxes of energy, water, and carbon dioxide from mountain ecosystems at Niwot Ridge, Colorado

Background: The eddy covariance (EC) technique provides a direct measure of water vapour and carbon dioxide fluxes between ecosystems and the atmosphere. Aims: This review article highlights the findings of various studies that have integrated EC observations into basic meteorological, hydrological and ecological research questions in two ecosystems near Niwot Ridge, Colorado, and synthesises these studies into a catchment-scale model of water and carbon cycling, within the context of regional disturbance and environmental change. Methods: EC was implemented continuously over subalpine forest and alpine tundra vegetation at Niwot Ridge, and resulting data were compared with discrete measurements and modelling studies. Results: Sensible heat fluxes were generally in excess of latent heat fluxes, indicating that the forest and tundra ecosystems were moisture limited. Snow cover regulated the annual sum of primary productivity in the forest, and beneath-snow respiration represented a significant portion of ecosystem respiration at both locations. Conclusions: Changes in the magnitude, timing, or spatial distribution of snow are likely to have the greatest impact on ecological processes in these semi-arid mountain catchments, but possibly in compensatory ways. Ultimately, the degree to which net carbon losses from alpine tundra offset forest carbon sequestration will determine the future magnitude of the Western United States carbon sink.