A long-standing goal of ecology has been to understand the cycling of carbon in forests. This has taken on new urgency with the need to address a rapidly changing climate. Forests serve as long-term stores for atmospheric CO2, but their continued ability to take up new carbon is dependent on future changes in climate and other factors such as age. We have been measuring many aspects of carbon cycling at an unmanaged evergreen forest in central Maine, USA, for over 25 years. Here we use these data to address questions about the magnitude and control of carbon fluxes and quantify flows and uncertainties between the different pools. A key issue was to assess whether recent climate change and an aging tree population were reducing annual C storage. Total ecosystem C stocks determined from inventory and quantitative soil pits were about 23,300 g C m-2 with 46% in live trees, and 48% in the soil. Annual biomass increment in trees at Howland Forest averaged 161±23 g C m-2 yr-1, not significantly different from annual net ecosystem production (NEP = -NEE) of 211 ± 40 g C m-2 y-1 measured by eddy covariance. Unexpectedly, there was a small but significant trend of increasing C uptake through time in the eddy flux data. This was despite the period of record including some of the most climate-extreme years in the last 125. We find a surprising lack of influence of climate variability on annual carbon storage in this mature forest.