Biomass and production of tundra vegetation under three eddy covariance towers at Imnavait Creek, Alaska

Understanding the magnitude of, and controls over, CO2 and water fluxes in arctic ecosystems is essential for accurate assessment and prediction of their responses to climate change. There have been relatively few year-round studies of net CO2, water, and energy exchange using micrometeorological methods in the Arctic. We established eddy covariance flux towers in three representative Alaska tundra ecosystems along a toposequence (a ridge site composed of heath tundra and moist non-acidic tundra, a mid-slope site composed of moist acidic tussock tundra, and a valley bottom fen site composed of wet sedge tundra and moist acidic tundra), and have collected CO2, water, and energy flux data continuously for six years. Our ridge site shows a cumulative loss of CO2 over these six years, while the fen site is closer to being in balance. We have also compared net CO2 fluxes measured in chambers and scaled to the footprint of the tower with those measured by the towers. There was good agreement between chamber level fluxes and tower fluxes for overall net ecosystem exchange (NEE), but less agreement for ecosystem respiration (ER) and gross ecosystem production (GEP). In order to improve our assessments of ER and GEP, in 2013 we harvested biomass and soils from the most common community types within the source area for each flux tower during the growing season. We measured above- and belowground biomass and net primary production of the vegetation, soil C and N stocks, and plant-available soil N. Plant biomass and production were greatest in the tussock tundra at the mid-slope tower, and least in the wet sedge community at the fen tower, while plant diversity was greatest in the communities at the ridge site. Soil carbon stocks were greatest at the fen tower, which had the least cumulative CO2 loss over the six years that we have been measuring. These data will be used to improve our calculations of gross ecosystem production and ecosystem respiration, and to parameterize a terrestrial ecosystem model and simulate the response of ecosystem carbon balance to historical and future climate warming in arctic tundra.