Trees won’t continue to grow as long as they continue photosynthesizing, according to new research published in . scientific progress. Researchers found that oak trees continue to absorb carbon dioxide even after they have finished their annual growth, suggesting that forests may store less carbon in wood than many climate models currently show.
This finding challenges the long-held assumption that high rates of photosynthesis naturally promote tree growth. If trees continue to capture carbon without converting much of it into new wood, less carbon may remain locked up in the long term.
Trees continue to absorb carbon even when they stop growing
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 had generally expected that higher levels of carbon dioxide in the atmosphere would boost photosynthesis, boost growth, and increase long-term carbon storage.
New findings suggest the relationship is more complex. Trees may continue to absorb additional carbon, but much of it won’t necessarily become new wood. Instead, that carbon is used to produce leaves, fuel short-lived metabolic processes, and perform other functions, potentially reducing the amount of carbon stored in forests compared to previously expected.
This result could have important implications for climate prediction.
“Right now, most models think that photosynthesis leads to growth, but we now know that’s not the case,” says lead author Mukund Parat Rao, an ecoclimatologist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School. “Just because photosynthesis increases doesn’t necessarily mean that trees will grow more in the future.”
Why are photosynthesis and growth different?
During photosynthesis, plants use sunlight to convert CO2 and water into sugars while releasing oxygen into the atmosphere. The captured carbon remains in the factory, but not all of it is used in wood construction.
Some of that carbon becomes the woody tissue of trunks, branches, and roots, where it can remain stored for decades, centuries, or even millennia. The remainder supports leaf and fruit production and is temporarily stored as starch or converted into compounds released into the soil to feed the microbial community, improve nutrient uptake, and protect the tree from disease.
Because wood stores carbon for very long periods of time, understanding how much of the carbon captured through photosynthesis ends up in woody biomass is important for estimating how forests can help slow climate change.
“Understanding how photosynthesis and growth are linked is critical in terms of understanding how forests store carbon on long-term scales,” Rao said.
Tracking trees across the United States
Scientists had previously suspected that carbon uptake and tree growth were not necessarily synchronized, but there were too few detailed observations to fully understand why.
To investigate, Rao and colleagues combined several data sources. They analyzed satellite images where photosynthesis could be detected in 137 oak forest locations across the eastern United States and California. They also used instruments that measured CO2 levels in the tree canopy hourly, and sensors attached to the tree trunks to track small changes in trunk size throughout the day. (Trees tend to expand at night as their roots absorb water and contract slightly during the day to transpire water, adding to the long-term trajectory of growth.) The researchers also incorporated tree-ring records and temperature data spanning from 1950 to the present.
Combining these datasets provided daily measurements of photosynthesis, carbon uptake, and tree growth.
Trees stop growing several months before photosynthesis ends
The researchers found that growth and photosynthesis are clearly distinct.
In locations in the eastern United States, oak trees typically grow from May to July, but continued photosynthesizing into October. About 36 percent of their annual carbon assimilation occurred after growth had already stopped in late summer.
California oak showed a different seasonal schedule, but the overall pattern was the same. Growth typically occurred between December and April, then slowed during midsummer and ceased by August, although photosynthesis continued. About 26 percent of the trees’ annual carbon uptake occurred after they stopped growing.
According to Rao, the explanation is simple. Tree growth depends on internal water pressure, which drops rapidly in hot, dry conditions.
“When conditions become dry and hot, growth activity immediately stops, but photosynthesis appears to continue at a slightly reduced rate,” Rao says.
What happens to the extra carbon?
Some of the carbon captured after growth ends is stored to facilitate growth when the next growing season begins. The rest is used to generate new roots and leaves, or oxidized to keep living cells functioning through the winter.
Researchers still don’t know exactly how much of that carbon ends up in woody biomass in the long term, and how much returns to the atmosphere in the short term. But the findings suggest that predictions that forests will grow and store substantially more carbon in a warmer, carbon-rich world may need to be reconsidered.
The researchers also found that the disconnect between photosynthesis and growth became even stronger during years when local weather oscillated between abnormally wet and abnormally dry conditions. This pattern may become more common in the future, as climate change is expected to increase this type of variability in many regions.
Rao and his colleagues are now investigating whether similar patterns occur in other tree species, forest ecosystems, and climates. He expects the degree of decoupling between photosynthesis and growth will vary from forest to forest, but says many questions remain unanswered.
“We don’t have the answers yet,” he says. “There are still many problems left to solve.”
