Spatial and temporal variability in forest-atmosphere CO2 exchange

Seven years of carbon dioxide flux measurements indicate that a ~ 90-year-old spruce dominated forest in Maine, USA, has been sequestering 174±46 gCm-2 yr-1 (mean±1 standard deviation, nocturnal friction velocity (u*) threshold >0.25ms-1). An analysis of monthly flux anomalies showed that above-average spring and fall temperatures were significantly correlated with greater monthly C uptake while above-average summer temperatures were correlated with decreased net C uptake. Summer months with significantly drier or wetter soils than normal were also characterized by lower rates of C uptake. Years with above-average C storage were thus typically characterized by warmer than average spring and fall temperatures and adequate summer soil moisture.