A new study published in Science Advances indicates that oak trees continue to absorb carbon dioxide after their annual growth has concluded. The findings challenge prior assumptions that linked photosynthesis directly with wood development in trees.

The study suggests that forests may store less carbon in wood than climate models currently estimate. Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, said, "Right now, most models assume that if you have photosynthesis, you have growth. We find that's not the case."

Researchers analyzed satellite imagery that could detect photosynthesis at 137 oak forest sites across the eastern United States and California. The team also used instruments to measure CO2 levels hourly in tree canopies and sensors attached to tree trunks to track changes in their size. The study incorporated tree ring records and temperature data from 1950 to the present.

In the eastern U.S. sites, oak trees typically grew from May through July, but they continued photosynthesizing into October. Approximately 36% of the annual carbon assimilation in these eastern U.S. sites occurred after growth ceased in late summer. For California oaks, growth generally happened between December and April, slowed during mid-summer, and concluded by August, with photosynthesis continuing after growth ended. Roughly 26% of the yearly carbon uptake for California oaks occurred after growth ceased.

Rao noted that the disconnect between photosynthesis and growth became more pronounced in years with fluctuating wet and dry weather conditions. He explained, "The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate."

Some of the carbon captured after growth stops is used to fuel growth during the next growing season. The remaining carbon is allocated to produce new roots and leaves or is oxidized to maintain the function of living cells through the winter. Researchers currently do not know how much of this post-growth carbon becomes long-term woody biomass or how much returns to the atmosphere in shorter periods.

Rao said, "Just because there is more photosynthesis might not necessarily mean more tree growth in the future." He added, "Understanding how photosynthesis and growth are linked is very important from the perspective of understanding how forests will store carbon over long time scales." Rao concluded, "I don't really have answers yet. There are many questions still left to address."