Trees do not necessarily keep growing for as long as they keep photosynthesizing. This was what a new study published in Science Advances is trying to establish. 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.
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 LamontDoherty 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.” 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. 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. 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