Forest’s ability to absorb carbon will be reduced by climate crisis

A new study suggests we should not rely on forests alone to fight the climate crisis, since when temperatures climb, a tree’s ability to absorb carbon is compromised.

The research led by Oregon State University used thermal imaging to study canopy-leaf temperature, by measuring carbon, water and energy ‘fluxes’, at sites in North America and Central America.

Contrasting from the prevailing theory that canopy leaves keep their temperature within an optimal range of photosynthesis, they found canopy leaves do not consistently cool below daytime air temperatures.

Instead, the leaves warmed faster than air, were warmer than air during most of the day and only cooled below air temperature in the afternoon.

Canopy leaves could reach even higher temperatures as the climate crisis worsens, which would negatively impact forest’s ability to absorb carbon and enhance forest mortality risk.

Lead author of the study, Chris Still, from the OSU College of Forestry, said: ‘Leaf temperature has long been recognized as important for plant function because of its influence on carbon metabolism and water and energy exchanges. If canopy photosynthesis declines with increasing temperature, the ability of forests to act as a carbon sink will be reduced.’

Different habitats cause leaf temperatures to vary, as leaf size depends on climate, latitude and canopy structure.

Large leaves occur more often in warm and wet climates, while small leaves, with a better ability to shed heat, show up more often in hot, dry areas.

In warmer and wet tropics, research has shown that leaf temperature is already approaching or surpassing the threshold for positive net photosynthesis.

‘If leaves are generally warmer than the surrounding air, as our findings suggest, trees may be approaching critical thresholds of temperature stress faster than we expect,’ said Andrew Richardson, professor at Northern Arizona University and co-author of the study.

Still added: ‘Our results have big implications for understating how plants acclimate to warming, and they suggest a limited ability for canopy leaves to regulate their temperature. Our data and analyses suggest a warming climate will result in even higher canopy leaf temperatures, likely leading to reduction of carbon assimilation capacity and eventually heat damage.’

Photo by Irina Iriser