Our understanding of the terrestrial carbon cycle is strongly focussed on C fixation in the leaf, the process of photosynthesis, while the process whereby trees actually sequester C in durable form, i.e., tissue growth, has received much less attention and is neglected in global vegetation models 1,2. Significant uncertainty exists in estimating the response of terrestrial C sequester to climatic change if photosynthesis and growth respond differently to environmental drivers 3. Here we analysed the long-term (1984-2010) annual aboveground growth biomass response (AABIper_area) across North America to temperature and precipitation by combining tree-ring observations and satellite-based tree stem-density variations. We discover a contrasting spatio-temporal pattern of climatic responses between photosynthesis and AABIper_area, resulting in a weak correlation of their inter-annual variations. Moreover, AABIper_area is significantly more strongly limited by climatic variability than is predicted by dynamic global vegetation models (DGVMs), with significant consequences for the predicted response to terrestrial C uptake projections. Our findings question current understanding of controls on terrestrial carbon cycling with major implications for future atmospheric CO 2 concentrations.