CO2 flux in Arctic and alpine dry tundra: Comparative field responses under ambient and experimentally warmed conditions

We compared growing season CO2 flux patterns between botanically similar ar ctic and alpine dry tundra ecosystems in Alaska and Colorado under ambient and experimentally warmed conditions. Measurements were taken during the 19 97 growing season, 3 yr after the warming treatments were begun. Under ambi ent weather conditions, arctic dry tundra at Toolik Lake, Alaska was a net source (4 g CO2-C m(-2)) of CO2 to the atmosphere, while alpine dry tundra at Niwot Ridge, Colorado, was a net CO2 sink (7 g CO2-C m(-2)) during the g rowing season. Experimental warming of arctic tundra by 1 to 3 degrees C, r esulted in a sevenfold (32 g CO2-C m(-2)) increase in this ecosystem's carb on source activity. Similar warming in alpine tundra changed this ecosystem from a net carbon sink to a net carbon source of 8 g CO2-C m(-2) over the growing season. In the Arctic, increased CO2 efflux with warming was largel y the result of increased rates of ecosystem respiration throughout the ent ire growing season, while in the alpine ecosystem respiration increased onl y early in the growing season. Rates of photosynthesis were generally not a ffected by experimental warming at either site. These data suggest that glo bal warming will accentuate the carbon source activity of dry tundra in the northern foothills of Alaska and will change the net CO2 exchange of alpin e dry tundra in the northern Rocky Mountains from a net CO2 sink to a sourc e.