In contrast to prevailing theories claiming that canopy leaves can always maintain their temperatures within an optimal range for photosynthesis, a new study led by Oregon State University (OSU) has found that, in fact, leaves in forest canopies are often not able to cool themselves below the ambient air temperature. These findings suggest that trees’ ability to avoid harmful temperature rises and to pull carbon from the atmosphere will likely be compromised in a warmer, drier climate.
“A hypothesis known as limited leaf homeothermy argues that through a combination of functional traits and physiological responses, leaves can keep their daytime temperature close to the best temperature for photosynthesis and below what is damaging for them,” said study lead author Chris Still, a professor of Forest Ecosystems and Society. “Specifically, leaves should cool below air temperature at higher temperatures, typically greater than 25 or 30 degrees Celsius. That theory also implies that the impact of climate warming on forests will be partially mitigated by the leaves’ cooling response.”
By using thermal imaging to measure canopy-leaf temperature at various sites in North and Central America, the scientists discovered that canopy leaves do not consistently cool below daytime air temperatures or remain within a narrow temperature range, as the limited leaf homeothermy theory would predict.
The investigations revealed that canopy leaves warm faster than air, are warmer than air during most of the day, and only start cooling off in mid- to late-afternoon. Since global warming will most likely lead to even higher canopy leaf temperatures, forest carbon cycling may be disrupted, and forest mortality risk will spike.
“Leaf temperature has long been recognized as important for plant function because of its influence on carbon metabolism and water and energy exchanges,” Professor Still explained. “If canopy photosynthesis declines with increasing temperature, the ability of forests to act as a carbon sink will be reduced.”
Leaf temperature in different regions is affected by how leaf size varies with climate and canopy structure. For instance, large leaves can be found mainly in warm and wet climates, while leaf traits such as higher reflectance and smaller sizes – features which increase the ability to shed heat – occur mostly in plants from hot, dry areas. Thus, in much of the warm, wet tropics, leaf temperatures are already approaching or even surpassing thresholds needed for positive net photosynthesis.
“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,” Professor Still concluded.
The study is published in the journal Proceedings of the National Academy of Sciences.