A regional study of the controls on water vapor and CO2 exchange in arctic tundra

Water vapor and CO2 exchanges were measured by the eddy covariance method in 24 ecosystems along a transect from the Arctic coast to the latitudinal tree line in northern Alaska during three growing seasons. Variations in net ecosystem exchange across the region were controlled by differences in the net uptake of CO2 due to photosynthesis, rather than by differences in ecosystem respiration. Daytime CO2 uptake was related mainly to site differences in leaf area index, whereas nighttime CO2 efflux was related to leaf area index and soil moisture. Temperature had no effect on regional patterns of ecosystem respiration during the growing season. Regional variations in surface conductance were largely explained by the water content of the moss-soil layer. Water vapor and CO2 fluxes were poorly coupled because water vapor exchange was determined largely by evaporation from mosses, whereas CO2 exchange was controlled by vascular plant activity. This is in contrast to the close correlation between maximum rates of surface conductance and CO2 assimilation that is observed at the global scale. The results also suggest that the relationship between the maximum rates of surface conductance and stomatal conductance in arctic tundra departs markedly from the global pattern, because the moss layer dried more readily than a moist soil. These spatially distributed eddy covariance measurements revealed new functional relationships among water vapor and CO2 exchanges and their environmental controls in arctic tundra; however, there remains a need for winter flux observations and long-term measurements to understand the net effect of these processes on the annual carbon balance of the region.