Carbon balance in tussock tundra under ambient and elevated atmospheric CO.

Whole ecosystem CO flux under ambient (340 μl/l) and elevated (680 μl/l) CO was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO resulted in greater carbon acquisition than control treatments and there was a net loss of CO under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO flux (sum of species' response) was apparent by the third week of exposure to elevated CO. Ecosystem dark respiration rates were not significantly higher at elevated CO levels. Rapid homeostatic adjustment to elevated CO may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO flux. For chambers exposed to ambient and elevated CO levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO flux at both ambient and elevated CO levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO levels. At any combination of physical conditions, flux of the elevated CO treatment was greater than that at ambient. In short-term manipulations of CO, tundra exposed to elevated CO had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO treatments. Elevated CO-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO.