Carbon dioxide fluxes in moist and dry arctic tundra during the snow-free season: responses to increases in summer temperature and winter snow accumulation

Climate-induced environmental changes are likely to have pronounced impacts on CO2 flux patterns in arctic ecosystems. We initiated a long-term experiment in 1994 in moist tussock and dry heath tundra in arctic Alaska in which we increased summer air temperature (ca. 20C) and increased winter snow accumulation (shortening the growing season approximately 4 wk). During the 1996 snow-free season, we measured ecosystem CO2 flux weekly in order to quantify net carbon gain or loss from these systems. Over the duration of the snow-free season, both dry heath and moist tussock tundra exhibited a net loss of carbon to the atmosphere, ranging from 12 to 81 g C m-2 depending upon experimental treatment. Elevated summer temperatures accelerated net CO2 loss rates over ambient temperatures in both deep and ambient snow treatments, and increased the total amount of carbon emitted during the snow-free season by 26 to 38% in ambient snow plots and by 112 to 326% in deep snow plots. Increased snow accumulation had less impact on CO2 flux than did warming, and snow effects on total carbon loss were not consistent between the two temperature regimes. Ecosystem respiration exceeded assimilation on most sampling dates throughout the season. These data, coupled with winter carbon losses recently demonstrated in the same ecosystems, indicate that the moist and dry arctic ecosystems we examined are currently net sources of atmospheric carbon on an annual basis, and that anticipated global warming may increase carbon losses from these systems.