Summer aridity decouples growth from carbon assimilation in temperate oaks

Forest biomass resulting from tree radial growth can remain on the landscape over decadal to centennial timescales and plays a critical role in forest carbon cycling. However, visually green vegetation may not be a good proxy for carbon allocation to growth as the phenology and environmental sensitivity of photosynthesis may be different from radial growth. Here we investigate the decoupling between photosynthesis and tree radial growth across intra to interannual timescales for seven North American oak species (Quercus spp.) at four sites (Lamont Sanctuary, NY; Morton Arboretum, IL; Pace Forest, VA; & Tonzi Vaira, CA, USA). Using point dendrometers and wood anatomy, we find that oak trees generally commenced radial growth (cell division and expansion) one month prior to full canopy development and peak carbon assimilation estimated using eddy covariance, satellite and in-situ remote sensing, and leaf-level chlorophyll fluorescence. Further, radial growth was essentially completed by early summer, two to three months prior to the early autumn end of the photosynthetic activity, and before the annual peak in temperature and vapour pressure deficit (VPD) and lowest soil moisture. This suggests that high summer aridity limits carbon allocation to growth more strongly than assimilation. Tree-ring width chronologies for these species across North America further supports that results that earlywood growth depends on prior season climate and assimilated carbon while latewood growth ends by early-summer and responds primarily to current year climate variability. In summary, temporal decoupling between radial growth and photosynthesis and the stronger constraint of summer aridity on growth than photosynthesis appears to be widespread among multiple North American temperate oak species. As summers continue to warm and dry under climate change, this source-sink (or photosynthesis-growth) decoupling needs to be better resolved to constrain forest carbon cycling, as increasing aridity will likely influence the ability of trees to allocate carbon to long-term storage as woody biomass.