Trees do not necessarily keep growing for as long as they keep photosynthesizing, according to a new study published in Science Advances. Researchers found that oak trees continue absorbing carbon dioxide well after their annual growth has ended, suggesting forests may store less carbon in wood than many climate models currently predict.
The discovery challenges a long standing assumption that higher rates of photosynthesis naturally lead to greater tree growth. If trees continue taking in carbon without turning much of it into new wood, less carbon may remain locked away over the long term.
Trees Keep Capturing Carbon After Growth Stops
Forests play a major role in slowing climate change because trees remove carbon dioxide (CO2) from the atmosphere and store much of it in their trunks, branches, and roots. Scientists have generally expected that rising atmospheric CO2 levels would boost photosynthesis, leading to faster growth and increased long term carbon storage.
The new findings suggest the relationship is more complicated. While trees may continue absorbing additional carbon, much of it does not necessarily become new wood. Instead, that carbon may be used to produce leaves, fuel short lived metabolic processes, or serve other functions, reducing the amount of carbon stored in forests compared with previous expectations.
The results could have important implications for climate forecasting.
“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” says lead author Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.”
Why Photosynthesis and Growth Are Different
During photosynthesis, plants use sunlight to convert CO2 and water into sugars while releasing oxygen back into the atmosphere. The captured carbon remains inside the plant, but it is not all used to build wood.
Some of that carbon becomes woody tissue in the trunk, branches, and roots, where it can remain stored for decades, centuries, or even millennia. The rest supports the production of leaves and fruit, is temporarily stored as starch, or is converted into compounds released into the soil to nourish microbial communities, improve nutrient uptake, and help defend the tree against disease.
Because wood stores carbon for such long periods, understanding how much of the carbon captured through photosynthesis ultimately becomes woody biomass is critical for estimating how forests help slow climate change.
“Understanding how photosynthesis and growth are linked is very important from the perspective of understanding how forests will store carbon over long time scales,” says Rao.
Tracking Trees Across the United States
Scientists had previously suspected that carbon uptake and tree growth were not always synchronized, but there had been too few detailed observations to fully understand why.
To investigate, Rao and his colleagues combined several sources of data. They analyzed satellite imagery capable of detecting photosynthesis at 137 oak forest sites across the eastern United States and California. They also used instruments that measured CO2 levels in tree canopies every hour and sensors attached to tree trunks that tracked tiny changes in trunk size throughout the day. (Trees tend to expand at night as roots take up water, then shrink slightly in daytime as they transpire water, with the long-term trajectory adding up to growth.) The team also incorporated tree ring records and temperature data spanning 1950 through the present.
Together, these datasets provided daily measurements of photosynthesis, carbon uptake, and tree growth.
Trees Stop Growing Months Before Photosynthesis Ends
The researchers found a clear separation between growth and photosynthesis.
At eastern U.S. sites, oak trees typically grew from May through July but continued photosynthesizing into October. About 36 percent of their annual carbon assimilation occurred after growth had already stopped in late summer.
California oaks showed a different seasonal schedule but the same overall pattern. Growth generally occurred between December and April, then slowed during mid summer and ended by August even though photosynthesis continued. Roughly 26 percent of the trees’ yearly carbon uptake happened after growth had ceased.
According to Rao, the explanation is straightforward. Tree growth depends on internal water pressure, and that pressure drops quickly during hot, dry conditions.
“The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” says Rao.
What Happens to the Extra Carbon?
Some of the carbon captured after growth ends is saved to help fuel growth when the next growing season begins. The remainder is used to produce new roots and leaves or is oxidized to keep living cells functioning through the winter.
Researchers still do not know exactly how much of that carbon eventually becomes long term woody biomass versus how much returns to the atmosphere over shorter time periods. However, the findings suggest that projections of forests growing larger and storing substantially more carbon in a warmer, CO2 rich world may need to be reconsidered.
The team also found that the disconnect between photosynthesis and growth became even stronger during years when local weather swung between unusually wet and unusually dry conditions. Because climate change is expected to increase this kind of variability in many regions, the pattern could become more common in the future.
Rao and his colleagues are now investigating whether similar patterns occur in other tree species, forest ecosystems, and climates. He expects the degree of separation between photosynthesis and growth will vary across different forests, but says many questions remain unanswered.
“I don’t really have answers yet,” he says. “There are many questions still left to address.”