Ecosystems in topographically complex (mountainous) terrain are responsible for a majority of land-atmosphere CO2 exchange (net ecosystem exchange; NEE) across the western United States due to high inputs of winter precipitation as snowfall. NEE in these regions has been historically difficult to quantify using the eddy covariance (EC) method, however, due to complexities in surface terrain that lead to irregularities in streamline air flow, particularly advective fluxes during periods of low turbulent mixing. This research evaluated the applicability of the EC method at an alpine tundra site in conjunction with the Niwot Ridge Long Term Ecological Research Project (LTER), and subsequently investigated meteorological factors controlling observed NEE. Persistent high wind speeds, a relatively short turbulent flux footprint, and the nearly flat ridge-top location of the study site resulted in 85% mean annual energy balance closure and general fulfillment of basic EC methodological requirements. In all, the alpine tundra was a net source of C to the atmosphere during 2008. Peak rates of summertime CO2 sequestration and wintertime CO2 respiration were of comparable magnitude. Rates of CO2 sequestration during the growing season increased non-linearly with increasing soil temperature, in agreement with results from other alpine tundra studies. Net CO2 sequestration was 22% less during the relatively longer 2007 growing season, highlighting a possible inverse relationship between alpine growing season length and net CO2 sequestration on Niwot Ridge.