Seasonal Patterns of Carbon, Water, and Energy Fluxes in Three Tundra Ecosystems in the Northern Foothills of the Brooks Range, Alaska

Understanding the carbon, water, and energy fluxes in the Arctic is essential for accurate assessment and prediction of the responses of these ecosystems to climate change, as well as their associated climate feedbacks. In the Arctic, the study of net CO₂, water, and energy exchange using micrometeorological methods has generally remained rather sporadic due to the difficulty of performing these measurements in cold, remote regions. Furthermore, when these measurements are performed, they are usually only collected during the growing season. We established eddy covariance flux stations in northern Alaska during the International Polar Year (2007-2009), and have now been continuously collecting carbon, water, and energy flux data year for over two years. These flux towers are located in a transect through three tundra ecosystems: heath tundra, tussock tundra, and wet sedge tundra. The peak of carbon uptake occurred during July, with these ecosystems accumulating between ~120 - 140 g C m-2 during the June - August months. However, these gains were generally lost through respiration during the snow covered months of September - May, with releases of ~100 - 180 g C m-2 during this time period. Analysis of the water fluxes indicates that the evaporative fraction (latent heat divided by the sum of sensible plus latent heat) is greatest in June and July in all three ecosystems, with a clear 'drying out' later in the year. Our results agree with other studies that have suggested that annually tundra ecosystems are either a net CO₂ source or CO₂ neutral due to winter respiration. Our results are also consistent with other tundra studies which have demonstrated that mid-summer evaporation is more similar between tundra types than net CO₂ exchange. These findings underline the importance of collecting data over the full annual cycle and across multiple types of tundra ecosystems in order to come to a more complete understanding of the carbon, water, and energy fluxes in the Arctic.