Long Term Effect of Surface Warming on CO2 Flux Components from Wet and Dry Sites at Two Locations in the Alaskan Coastal Plain

Historically arctic ecosystems have been carbon sinks. Waterlogged soils, low temperatures and shallow permafrost have contributed to the accumulation of close to 14% of the world's soil organic carbon. Expected global changes in temperature and hydrology are likely to be magnified in the Arctic, potentially shifting the balance of photosynthesis and respiration toward greater CO₂ losses to the atmosphere. Arctic CO₂ losses could represent a significant positive feedback to the global greenhouse gas effect, further affecting the arctic climate conditions. We analyzed peak season CO₂ fluxes collected using static chamber techniques over a period spanning 9 years (2000–09) from tundra sites established in Barrow (Alaskan coastal plain) and Atqasuk (100 km south of Barrow), Alaska as part of the International Tundra Experiment. At each site we used open-top chambers (OTCs) to test the effect of warming on the CO₂ flux components at two ends of a hydrological gradient: wet sedge and dry heath tundra. We found that warming increases both Gross Primary Productivity (GPP) and Ecosystem Respiration (ER); however, during hot and dry seasons ER had a greater response to warming resulting in net CO₂ losses, especially in the dry site at Barrow. In Atqasuk, warming increased both GPP and ER, but different from Barrow, the ER response to warming was not as strong resulting in lower CO₂ losses. At each location the dry sites were the most negatively affected by warming, but the effect was particularly strong at Barrow. During typical seasons, high soil moisture in wet sites negatively affected ER resulting in low CO₂ losses and strong sink activity on the control plots. However, during two recent warm dry years (2007 and 2009), dry soils at the wet sedge sites resulted in net carbon losses on both control and warmed plots at the time of year when GPP is usually highest. During such seasons when the ecosystem experiences net losses at peak season, the seasonal CO₂ losses are likely to be much larger. Therefore, as result of interannual variability of weather conditions, in particular changes in water availability and temperature, and the fast response of the ecosystem to these changes, some areas in the Arctic could be sources of CO₂ over the growing season in warm and dry years.