Ar-Rehman

Trees growth stops but still keep absorbing carbon

A tree that is photosynthesizing is not necessarily a tree that is growing. It sounds counterintuitive, the two processes seem like they should go hand in hand. But a new study of oak trees has found that growth stops in mid-summer, even as the trees continue to absorb carbon well into autumn. Oak trees may keep capturing carbon long after they stop growing, raising new questions about how much carbon forests can really store. Oak trees has revealed that photosynthesis and wood production are not as closely linked as scientists once believed. The finding could reshape forecasts of how much carbon forests will be able to store in a warmer future. New research have challenged a key assumption about how forests store carbon. Trees do not necessarily keep growing for as long as they keep photosynthesizing. Researchers found that oak trees continue absorbing CO2 well after their annual growth has ended, suggesting forests may store less carbon in wood than many climate models currently predict.

The finding has real consequences for how much carbon forests can be expected to store as the climate warms. The research was led by Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, part of the Columbia Climate School. The team combined satellite imagery, treetop CO2 sensors, trunk-mounted growth monitors, tree ring records and temperature data stretching back to 1950. This helped them pull together daily readings of photosynthesis, carbon uptake and actual wood growth across 137 sites in the eastern US and California. 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 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." The carbon absorbed after growth stops doesn’t disappear. Some of it is stored as starch and used to kick-start growth the following spring, while some goes into new leaves and roots. Moreover, carbon is also burned through cellular respiration to keep the tree alive through winter. Some is released into the soil, feeding microbial communities. What it mostly isn’t doing is being converted into woody biomass, the durable, long-lived form of carbon storage that makes forests valuable as carbon sinks. Carbon stored in wood can remain there for decades, centuries or even millennia depending on conditions. Carbon used to grow leaves or feed soil microbes cycles back into the atmosphere much faster. “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. Exactly how much of the post-growth carbon ends up sequestered long-term versus released relatively quickly is still unknown. But the implication is clear: projections that assume a direct line from increased photosynthesis to increased carbon storage are probably too optimistic.

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. 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. 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. 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. Together, these datasets provided daily measurements of photosynthesis, carbon uptake, and tree growth. The researchers found a clear separation between growth and photosynthesis.

Climate models have generally assumed that more photosynthesis means more tree growth, and more tree growth means more carbon locked away in wood for decades or centuries. Rising CO2 levels, act as a fertilizer, with trees absorbing more, growing faster, storing more carbon and partially offsetting what humans are pumping into the atmosphere. “Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” Rao said. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.” At eastern US sites, oak trees typically grew from May through July but continued photosynthesizing into October. About 36% 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% 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.

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. Wood growth requires water pressure inside the tree. When conditions turn hot and dry, that pressure drops and growth stops almost immediately. Photosynthesis is more resilient, it keeps running, at a slightly reduced rate, even when the tree can no longer convert the carbon it’s capturing into new wood. “The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” Rao said. 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." The research also found that the gap between photosynthesis and growth was most pronounced in years when local climates swung between wet and dry extremes. Highly variable conditions, periods of abundance followed by drought, widened the decoupling. Those conditions are expected to become more common as climate change intensifies. The bigger question hanging over all of it is how much forests can actually be relied upon to absorb the carbon humans are producing. The answer, this research suggests, is probably less than the models have been assuming, and the gap between photosynthesis and growth is a significant reason why.