It's intuitive to think that if a tree is photosynthesizing, it's also growing. But that's not necessarily so—and a new study of oak trees, published in the journal Science Advances, found that even as they photosynthesize late into the year, their growth stops by mid-summer.
Much of the long-term carbon storage that forests provide depends on trees converting the carbon they absorb through photosynthesis into new wood. Many researchers have predicted that rising atmospheric carbon dioxide (CO2) levels will enhance photosynthesis and stimulate tree growth, putting some of that planet-warming carbon into long-term storage inside wood. However, the observed decoupling of photosynthesis from growth suggests that increased carbon uptake does not necessarily translate into greater wood production. Instead, some of the absorbed carbon may be used to produce foliage or used in short-lived metabolic processes rather than being locked away long term, reducing the amount of carbon stored in forests compared with previous expectations.
The finding has climate implications.
"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."
During photosynthesis, plants absorb CO2 from the air and then use sunlight to power the conversion of CO2 and water into sugars. Oxygen is released back into the atmosphere while the carbon stays in the plant. In a tree, some of that carbon goes into the woody biomass of trunk, branches and roots. The rest goes into foliage and fruits and is temporarily stored as starch, or is converted into compounds that are released into the soil to feed microbial communities, make nutrients available for uptake and defend against pathogens.
Carbon stored in woody biomass may take decades, centuries or even millennia—depending on conditions—to re-enter the atmosphere, making it an important carbon sink. That also makes it important to understand the relationship between photosynthesis and tree growth. "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.
Earlier research has suggested that carbon uptake and tree growth might not be synonymous, but detailed measurements were in short supply and the mechanisms unclear. To study the question, Rao and his colleagues used photosynthesis-detecting satellite imagery of trees at 137 sites across the eastern United States and California; readings from instruments that provided hour-by-hour measurements of treetop CO2 levels; and trunk-borne sensors that yielded real-time measurements of minute fluctuations in tree size. (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.) They also drew on growth ring records and temperature data from 1950 to the present.
All this produced daily recordings of photosynthesis, carbon uptake and tree growth—and the researchers found that oak growth in their eastern sites generally took place from May through July, even though trees continued to photosynthesize well into October. Roughly 36 percent of all carbon assimilation through photosynthesis occurred after growth had stopped in late summer. At the California sites, oak grew from December through April, but growth slowed in mid-summer and ceased by August even as photosynthesis continued. About 26 percent of those trees' annual carbon uptake occurred after growth ceased.
This makes mechanistic sense: when water is scarce, trees lose the internal water pressure they need to grow. "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 fraction of that post-growth carbon is used to kick-start growth the following year, says Rao. The rest is used to grow new leaves and roots or is oxidized to keep cells alive through winter. Exactly how much is sequestered long-term in woody biomass and how much is released at shorter time scales is unknown, but it seems likely that projections of trees growing larger and storing more carbon in a warmer, CO2-saturated world will need to be revisited.
The researchers also observed that the decoupling between photosynthesis and growth was especially pronounced in years when local climates were most variable, oscillating between extremes of wet and dry. This pattern is expected to become more common as the climate changes.
Rao and his colleagues are now studying whether the decoupling of photosynthesis and growth is taking place in other tree species, ecosystems and regions. Rao expects that decoupling will be found to varying degrees in different forest types and climates, but "I don't really have answers yet," he says. "There are many questions still left to address."
