We analyzed 13 years (1992-2004) of CO2 flux data, biometry, and meteorology from a mixed deciduous forest in central Massachusetts. Annual net uptake of CO2 ranged from 1.0 to 4.7 Mg-C ha-1y-1, with an average of 2.5 Mg-C ha-1y-1. Uptake rates increased systematically, nearly doubling over the period despite forest age of 75-110 yrs; there were parallel increases in mid-summer photosynthetic capacity at high light level (21.5 - 31.5 ?mole m-2s-1), woody biomass (101 - 115 Mg-C ha-1 from 1993-2005, mostly due to growth of one species, red oak), and total leaf area index (4 - 5.5 from 1998-2005). The long term trends were interrupted in 1998 by sharp declines in photosynthetic capacity, Net Ecosystem Exchange (NEE) of CO2 and other parameters, with recovery over the next 3 years. The observations were compared to empirical functions giving the mean responses to temperature and light, and to a terrestrial ecosystem model (IBIS2). Variations in GEE and NEE on hourly to monthly time scales were represented well as prompt responses to the environment, but inter-annual variations and long-term trends were not. IBIS2 simulated mean annual NEE, but greatly over-predicted the amplitude of the seasonal cycle and did not predict the decadal trend. The drivers of interannual and decadal changes in NEE are long-term increases in tree biomass, successional change in forest composition, and disturbance events, processes not well represented in current models.