Oak forests decouple photosynthesis from wood growth after July, per Science Advances study of 137 US sites

The study integrated MODIS satellite photosynthesis indices, automated dendrometers recording hourly trunk radius at 137 oak stands, canopy CO2 flux towers, and 1950-present tree-ring chronologies. Growth ceased after peak summer water availability declined, while leaf-level photosynthesis persisted, routing excess carbon into labile sugars, root exudates, and defense compounds rather than lignified biomass. This asynchrony was consistent across temperature gradients from New England to California oak woodlands.

Climate models that scale net primary productivity directly to woody biomass therefore overestimate long-term sequestration by 15-25%. The finding aligns with independent evidence from eddy-covariance networks showing late-season carbon uptake without corresponding stem increment and with isotopic studies tracing excess photosynthate into soil microbial pools. It also reframes why CO2 fertilization experiments often yield smaller biomass gains than predicted.

Revised dynamic global vegetation models incorporating explicit carbon allocation rules will be required before the next IPCC cycle. Field experiments that manipulate late-season water and nutrient supply while tracking non-structural carbohydrate pools could quantify the fraction ultimately returned to the atmosphere versus stabilized in soil organic matter.

Next steps include extending the sensor network to deciduous species outside Quercus and testing whether warming extends the photosynthesis window without extending the growth window.