For decades, forests have occupied a central place in discussions about climate change. Trees remove carbon dioxide from the atmosphere, convert it into organic matter and store part of that carbon in their trunks, branches and roots. The process seems straightforward enough. More photosynthesis should mean more growth, and more growth should mean more carbon locked away for years to come.A new study suggests the reality is less direct. Looking closely at oak forests across North America, scientists found that trees continue drawing carbon from the atmosphere long after they have stopped producing new wood. The discovery points to a gap between carbon uptake and tree growth that has largely been overlooked in many projections of future forest carbon storage.
Why trees keep absorbing carbon long after growth stops
The recent research, published in Science Advances, titled “Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks”, the research combined evidence from satellite observations, carbon flux measurements, growth sensors attached to tree trunks and decades of tree-ring records collected across 137 oak forest sites.What emerged was a surprisingly consistent pattern. In the eastern United States, oak trees carry out most of their annual wood growth between spring and midsummer. By around July, the formation of new woody tissue had largely finished. Yet the trees remained active for months afterwards, continuing to photosynthesise and absorb carbon until autumn.The situation was similar in California’s oak woodlands, although the timing reflected the region’s different seasonal cycle. Growth slowed and ended well before photosynthesis did, leaving a lengthy period during which trees were still taking carbon from the atmosphere despite adding little or no new wood.
Study finds carbon uptake continues after tree growth ends
Many climate models assume a fairly close relationship between photosynthesis and growth. Under that view, rising carbon dioxide concentrations can stimulate photosynthesis, allowing forests to grow more rapidly and store larger quantities of carbon in wood.The new findings indicate that the connection is not always so simple. According to the study, between 26 and 36 per cent of annual carbon uptake in the oak forests occurred after the year’s wood growth had effectively ended. That means a substantial share of the carbon captured by these trees was not immediately being turned into new trunks or branches.“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” said Rao. Just because there is more photosynthesis might not necessarily mean more tree growth in the future.The distinction matters because wood represents one of the longest-lasting forms of carbon storage in forests. Carbon incorporated into woody biomass can remain locked away for decades or even centuries, while carbon used elsewhere often returns to the atmosphere much sooner.
Hot and dry conditions stop tree growth, but not carbon uptake
The study suggests that growth is far more sensitive to hot and dry conditions than photosynthesis. Producing new wood depends on water pressure within a tree’s tissues. When temperatures rise and the air becomes drier, the pressure falls. The cells responsible for building new wood become less able to expand, and growth can stop rapidly.Photosynthesis responds differently. Leaves can continue capturing carbon even when conditions are no longer favourable for producing new woody tissue. The rate may decline somewhat, but the process does not halt as abruptly.As per the study, the moment you have dry and hot conditions, growth activity stops pretty instantly, while photosynthesis seems to continue at a slightly decreased rate.Measurements collected at high-frequency monitoring sites showed that growth tended to occur under cooler and more humid conditions than photosynthesis. The environmental window suitable for building wood was considerably narrower than the window suitable for capturing carbon.
What happens to the carbon absorbed after tree growth stops
The carbon absorbed after growth ends does not simply vanish. Some of it is stored inside the tree as reserves, often in the form of non-structural carbohydrates such as starches and sugars. These reserves can help support the next year’s growth before fresh leaves are fully active.A portion is directed towards roots, reproduction or maintenance processes that keep the tree functioning. Some enter the soil through root activity and become available to microbes. Some is eventually released again through respiration.What remains uncertain is exactly how much of this late-season carbon ultimately becomes long-term storage and how much cycles back into the atmosphere relatively quickly.That uncertainty is important because it affects estimates of how effectively forests can offset human greenhouse gas emissions over long periods.“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 said.
