Abstract
The magnitude of the terrestrial carbon sink remains a key uncertainty in future climate projections, in part due to poorly understood links between carbon uptake and its allocation to woody biomass in vegetation. Here, we show that photosynthesis and above-ground growth occur asynchronously across diel to seasonal scales in eight North American oak species. Across 137 tree-ring sites, current-year annual growth was insensitive to climate variability after mid-summer, despite 26-36% of annual gross primary productivity (GPP) occurring during this period. Hourly GPP flux and growth measurements at four sites spanning seven site-years further demonstrate that wood formation ceases earlier than photosynthesis and is restricted to periods of low atmospheric aridity and temperature. This photosynthesis-growth decoupling intensifies with inter-annual variability in vapour pressure deficit (r=0.86, p<0.05), suggesting that by assuming tight coupling between photosynthesis and woody biomass, current earth system models may overestimate long-term carbon sequestration in forests.