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

Abstract. 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° 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 g C m⁻² on average (range of −17 to −41 g C m⁻²) during the years 2008–2015, except 2011 (source of 41 g C m⁻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 g C m⁻²) and R_eco (144 to 279 g C m⁻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.