July 12: Oak trees continue absorbing carbon dioxide for months after their annual growth ends, challenging long held assumptions about how forests store carbon and raising new questions about future climate projections according to a new study published in Science Advances.
Researchers from the Columbia Climate School found that photosynthesis and tree growth are not as closely linked as previously believed. While trees continue capturing carbon dioxide well into late summer and autumn, much of the absorbed carbon is not converted into new wood, suggesting forests may store less long-term carbon than many climate models currently estimate.
Forests play a crucial role in mitigating climate change by removing carbon dioxide from the atmosphere and storing it in trunks, branches and roots. Scientists have generally assumed that rising atmospheric carbon dioxide levels would increase photosynthesis, leading to faster tree growth and greater carbon storage.
However, the new study indicates that additional carbon captured through photosynthesis is often diverted to producing leaves and roots, supporting metabolic processes, storing energy as starch, or nourishing soil microbes, rather than increasing woody biomass that locks away carbon for decades or centuries.
“Our findings show that just because there is more photosynthesis does not necessarily mean there will be more tree growth,” said lead author Mukund Palat Rao, an ecoclimatologist at Columbia Climate School’s Lamont Doherty Earth Observatory.
The research combined satellite observations, hourly canopy carbon dioxide measurements, trunk growth sensors, tree ring records and temperature data from 137 oak forest sites across the eastern United States and California.
Scientists found that eastern U.S. oak trees generally grew between May and July but continued photosynthesizing until October. Approximately 36% of their annual carbon uptake occurred after growth had already stopped. California oaks displayed a similar pattern, with around 26% of yearly carbon absorption taking place after growth had ceased.
Researchers attribute the separation between photosynthesis and growth largely to hot and dry conditions. While photosynthesis continues at a reduced rate, tree growth slows rapidly because declining water pressure inside the tree limits wood formation.
The study also found that the disconnect between carbon uptake and growth became more pronounced during years with alternating wet and dry weather conditions. As climate change is expected to increase weather variability in many regions, researchers say this pattern could become increasingly common.
The findings suggest current climate models may overestimate the amount of carbon forests can store through increased tree growth under rising atmospheric carbon dioxide levels. Researchers plan to investigate whether similar patterns occur in other tree species and forest ecosystems to improve future carbon cycle projections.