Forests are a vital defence against climate breakdown, but their power depends in part on how much carbon dioxide they can convert into wood. A new study suggests trees may store less planet-heating carbon than hoped, as photosynthesis does not always result in wood growth.
Study Findings
Scientists studied 137 sites across the United States and found that trees stopped growing months before the point in the year at which photosynthesis stopped. This decoupling between photosynthesis and wood growth could have significant implications for climate models.
Mukund Palat Rao, a carbon cycle scientist at the Lamont-Doherty Earth Observatory at Columbia University and lead author of the study, stated: “Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case.” He added that just because there is more photosynthesis might not necessarily mean more tree growth in the future.
Regional Variations
At sites in the eastern US, the researchers found that about 36% of yearly carbon uptake occurred after tree growth stopped in late summer. In California, it was about 26%. More detailed measurements at four sites showed that wood growth was restricted to periods of low aridity and temperature, which are becoming rarer as global temperatures rise.
Rao explained: “The moment you have dry and hot conditions, growth activity stops pretty instantly, while photosynthesis seems to continue at a slightly decreased rate.”
Implications for Climate Models
As fossil fuel emissions leave more carbon dioxide in the atmosphere for trees to absorb, climate scientists expect the land-based carbon sink to remain stable or grow over the 21st century. However, many models estimate uptake using levels of photosynthesis rather than actual wood growth.
The researchers warned: “Earth system models that assume consistently tight coupling between photosynthesis and growth may therefore overestimate future forest carbon sequestration under rising atmospheric moisture demand.”
Broader Context
Last week, a report found that humanity must suck carbon out of the atmosphere with new technologies even faster than the speed with which it has deployed solar panels. Land-based actions such as planting trees make up the overwhelming majority of human efforts to remove carbon dioxide, with machines and chemical processes responsible for just 0.1% of the 2.2 billion tonnes of CO2 removed globally each year.
The researchers are now studying whether the decoupling of photosynthesis and wood growth can be observed in other tree species and regions. They said their results showed that the capacity of forests to store carbon over long periods depended on how much carbon was absorbed and then directed toward wood growth. If more absorbed carbon were to flow toward transient uses – such as foliage and internal processes – the power of forests as carbon sinks would fall.
