Oak study finds carbon uptake can outlast seasonal growth
Research in Science Advances suggests oaks can keep photosynthesizing for months after wood growth stops, complicating forest carbon forecasts.
By Lucas Ferreira · Science & Environment Writer
3 min read
Oak trees can continue drawing carbon dioxide from the air months after their seasonal growth has ended, according to research published in Science Advances. The finding matters for climate projections because it suggests that more photosynthesis does not automatically mean more long-lived carbon stored in wood.
Columbia Climate School said the study challenges a common assumption in many climate models: that higher carbon uptake by trees translates into greater growth. Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory and the study’s lead author, said the work shows photosynthesis and growth are not as tightly linked as scientists have often treated them.
Forests slow climate change by removing carbon dioxide from the atmosphere and storing some of that carbon in trunks, branches and roots. Carbon held in wood can remain there for decades or far longer, while carbon used for leaves, fruit, root turnover, metabolism or soil compounds may cycle back more quickly, according to Columbia Climate School.
The researchers examined oak forests at 137 sites in the eastern United States and California. Columbia Climate School said the team combined satellite observations of photosynthesis with hourly carbon dioxide readings from tree canopies, trunk sensors that detected small changes in stem size, tree-ring data and temperature records dating from 1950 to the present.
Those measurements allowed the team to compare daily carbon uptake with tree growth. Trunk sensors helped capture the pattern behind growth measurements: trees can swell at night as roots take up water and shrink during the day as they lose water, while longer-term changes reveal whether the trunk is adding wood.
In eastern U.S. oak forests, the study found that growth usually took place from May through July, while photosynthesis continued into October. Columbia Climate School said about 36% of annual carbon assimilation at those sites happened after growth had already stopped in late summer.
California oaks followed a different calendar but showed the same split between uptake and growth, according to the researchers. There, growth generally ran from December through April, slowed during midsummer and had ended by August, while photosynthesis continued; roughly 26% of yearly carbon uptake occurred after growth stopped.
Rao attributed the pattern to water stress. “The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” he said, according to Columbia Climate School.
The extra carbon taken in after growth ends does not all become wood, the researchers reported. Some may be stored for the next growing season, while some may support new roots and leaves or be used to keep living cells active through winter.
The study does not settle how much late-season carbon eventually becomes long-term woody biomass. Columbia Climate School said that uncertainty could affect forecasts that assume forests will grow larger and store substantially more carbon as atmospheric carbon dioxide rises.
The researchers also found that the separation between photosynthesis and growth was stronger in years marked by swings between unusually wet and unusually dry local conditions. Because climate change is expected to increase such variability in many regions, Columbia Climate School said the pattern may become more relevant to future forest carbon estimates.
Rao and his colleagues are now testing whether similar patterns appear in other tree species, ecosystems and climates. He said the strength of the split between photosynthesis and growth is likely to differ across forests, and that many questions remain unresolved.
This story draws on original reporting from ScienceDaily.