Exchange of CO2 in Arctic tundra: impacts of meteorological variations and biological disturbance

An improvement in our process-based understanding of carbon (C) exchange in the Arctic and its climate sensitivity is critically needed for understanding the response of tundra ecosystems to a changing climate. In this context, we analysed the net ecosystem exchange (NEE) of CO2 in West Greenland tundra (64 degrees N) across eight snow-free periods in 8 consecutive years, and characterized the key processes of net ecosystem exchange and its two main modulating components: gross primary production (GPP) and ecosystem respiration (R-eco). Overall, the ecosystem acted as a consistent sink of CO2, accumulating 30 gCm(-2) on average (range of -17 to -41 gCm(-2)) during the years 2008-2015, except 2011 (source of 41 gCm(-2)), which was associated with a major pest outbreak. The results do not reveal a marked meteorological effect on the net CO2 uptake despite the high interannual variability in the timing of snowmelt and the start and duration of the growing season. The ranges in annual GPP (182 to 316 gCm(-2)) and R-eco (144 to 279 gCm(-2)) were > 5 fold larger than the range in NEE. Gross fluxes were also more variable (coefficients of variation are 3.6 and 4.1% respectively) than for NEE (0.7 %). GPP and R-eco were sensitive to insolation and temperature, and there was a tendency towards larger GPP and R-eco during warmer and wetter years. The relative lack of sensitivity of NEE to meteorology was a result of the correlated response of GPP and R-eco. During the snow-free season of the anomalous year of 2011, a biological disturbance related to a larvae outbreak reduced GPP more strongly than R-eco. With continued warming temperatures and longer growing seasons, tundra systems will increase rates of C cycling. However, shifts in sink strength will likely be triggered by factors such as biological disturbances, events that will challenge our forecasting of C states.