Process‐Oriented Modeling of a High Arctic Tundra Ecosystem: Long‐Term Carbon Budget and Ecosystem Responses to Interannual Variations of Climate

Abstract Terrestrial carbon (C) cycling in high Arctic tundra depends on ecosystem responses to climatic warming and concurrent changes in environmental conditions. There are very few studies to quantify long‐term C budget in high Arctic tundra due to lack of sufficient measurements. Here based on well‐established multiyear measurements, we calibrated a process‐oriented model (CoupModel) to quantify various components of the C budget at a Cassiope tetragona heath ecosystem in northeast Greenland. Net ecosystem exchange of CO 2 (NEE) for 2000–2014 was estimated at −15 ± 10 g C m −2 yr −1 . Ecosystem respiration (ER) for nongrowing seasons was estimated at 10.3 ± 5.3 g C m −2 yr −1 , representing around 13% of the annual ER. Significant trends for interannual variations of aboveground and belowground C fluxes and stocks were found for the subperiods (i.e., 2000–2008 and 2008–2014) but not for the entire period. Interannual variations of NEE largely relied on the response of gross primary production (GPP) and ER to seasonal changes in climate. Moreover, the model showed that interannual variations of GPP, ER, and NEE had a much higher linear correlation with July temperature and annual maximum thawing depth (ALD max ) than other climatic and site characteristics. ALD max had the highest correlation with the decomposition rate of humus C. Overall, this modeling study suggests that a sink‐source transition of the studied ecosystem depends on ecosystem responses to interannual variations of climate and that the net C balance may be sensitive to summer warmth and active layer thickness.