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

Understanding the carbon and water fluxes in the Arctic is essential for accurate assessment and prediction of the responses of these ecosystems to climate change. In the Arctic, there have been relatively few studies of net CO2, water, and energy exchange using micrometeorological methods due to the difficulty of performing these measurements in cold, remote regions. Furthermore, when these measurements are performed, they are usually collected only during the short summer growing season. We established three eddy covariance flux towers in northern Alaska during the International Polar Year (2007 -2009), and have collected carbon (C), water, and energy flux data continuously for three years. These flux towers are located in a transect through three tundra ecosystems (heath tundra, tussock tundra, and wet sedge tundra) at in northern Alaska. In all ecosystems the peak of C uptake occurred during July, with accumulations of ~90 - 150 g C m-2 during late May - August. These gains were generally lost through respiration during the snow covered months of September - mid-May, with releases of ~50 - 140 g C m-2 during this time period. All three ecosystems switched from source to sink (positive to negative NEE) several weeks after the snow melted, and switched from sink to source well before the snow reappeared. The springtime switch from source to sink corresponded to the timing of leaf-out, with the ecosystems switching from source to sink approximately two weeks following leaf-out each year. While the ecosystems continually lost carbon during the snow season, there was some evidence of degassing events in the spring in conjunction with snowmelt and strong spikes of positive values of NEE, indicating a release of carbon to the atmosphere. These findings underline the importance of collecting data over the full annual cycle, and in particular, during the shoulder seasons, as well as across multiple types of tundra ecosystems in order to come to a more complete understanding of the carbon and water fluxes in the Arctic.