Boreal and temperate deciduous forests at northern mid-latitudes play an important role in the global carbon cycle. We analyze 3 years (1996-1998) of eddy-covariance carbon dioxide flux measurements from two contrasting deciduous forest ecosystems in the boreal and temperate regions of central Canada. The two forest stands have similar ages, heights, and leaf area indices but differ in species composition and climate. Mean annual net ecosystem productivity (NEP) was similar for the two ecosystems, varying between 0.7 and 2.7 t C·ha-1 (boreal) and 0.6 and 2.4 t C·ha-1 (temperate). In the boreal ecosystem, interannual differences in NEP were primarily controlled by early spring temperature. The warm spring of 1998 caused early leaf out and increased photosynthesis but had little effect on respiration. In the temperate ecosystem, the same warm spring not only caused early leaf out but also increased respiration and drought stress. The contrasting impact of the warm spring on annual NEP at the two sites illustrates the complexity of interpreting climatic impacts on the forest carbon balance. It also illustrates two competing influences of climate change on NEP: spring warming, which promotes photosynthesis and increases NEP, and increased soil temperature and drought, which promote respiration and reduce photosynthesis, thus reducing NEP. We discuss the need for a consistent data post-processing methodology in ecosystem intercomparisons. We also compare our results with a recent synthesis of data from European forests.