The relationship between forests, carbon and climate change

According to a recent study by Oregon State UniversityLeaves in forest canopies cannot lower indoor temperatures below ambient temperature. This suggests that the trees’ ability to prevent harmful increases in temperature and absorb carbon dioxide will be affected in warmer and drier climates.

Jungle Canopy, HJ Andrews Experimental Forest, Cascade Series, Oregon. Image Credit: Theresa Hogg, Ohio State University

Findings from international efforts by scientists from many universities and government agencies conflict with the prevailing scientific notion that canopy leaves can maintain their temperature in an optimal range for photosynthesis – the process that makes green plants their food from sunlight and carbon dioxide.

According to lead author Chris Steele of The Ohio State University College of Forestry, whose research was published today in Proceedings of the National Academy of Sciences, many of the world’s forests may have approached the thermal carbon uptake limit. This research is necessary to understand and anticipate how plants respond to climate change.

A hypothesis known as the limited household heat of leaves says that through a combination of functional traits and physiological responses, leaves can maintain a daytime temperature close to the best temperature for photosynthesis and below what is harmful to it..

Chris Steele, lead study author, College of Forestry, Oregon State University

Specifically, the leaves must cool below air temperature at higher temperatures, usually greater than 25 or 30 degrees Celsius. This theory also suggests that the effect of climate warming on forests will be partially mitigated by the leaf cooling response‘ he still says.

From the rainforests of Panama to the high treeline of Colorado, Steele and colleagues used thermal imaging to examine the temperature of the canopy leaf. They discovered that, contrary to what the home heat theory predicted for bound leaves, canopy leaves do not cool consistently below daytime air temperatures or remain within a narrow range of temperatures.

Thermal cameras were installed on the towers with instruments to measure various environmental factors, including carbon, water, and energy “flows” – exchanges between the forest and the atmosphere.

Using continuous high-frequency thermal imaging to monitor forest canopies is changing what we can learn about how forests deal with the stress of rising temperatures.said Andrew Richardson, a professor at Northern Arizona University and co-author of the study.

Before thermal cameras, if you wanted to measure the temperature of the canopy, you had to tape the thermocouples to the sheets with bandages and wait for the wind to pull it off. But these cameras allow us to measure change 24 hours a day, seven days a week, across many seasons and years.. “

The study revealed that canopy leaves heat up more quickly than ambient air, stay warmer most of the day, and cool only below air temperature in the mid to late afternoon.

According to the scientists, increased leaf canopy temperatures due to future global warming are expected to have a detrimental effect on the carbon cycle in forests and will increase the likelihood of forest death.

Leaf temperature has long been recognized as important to plant function because of its influence on carbon metabolism and water and energy exchange. If canopy photosynthesis decreases with increasing temperature, the ability of forests to act as a carbon sink will decrease‘ he still says.

According to Still, how leaf size fluctuates with climate, latitude, and canopy structure affects leaf temperature in different ecosystems. Large leaves are more common in warm and humid places, while smaller leaves and high reflectivity, which improve heat shedding capacity and increase cooling, are more common in plants growing in hot and dry conditions.

Positive net photosynthesis parameters – the rate of carbon fixation minus the amount of carbon dioxide released during plant respiration – have already been approached or exceeded in much of the warm, humid tropics.

If leaves are generally warmer than ambient air, as our findings suggest, trees may approach critical temperature pressure thresholds faster than we expect.Richardson thinks.

Our results have major implications for understanding how plants adapt to warming, and they indicate a limited ability of canopy leaves to regulate their temperature. Our data and analyzes indicate that climate warming will lead to higher canopy leaf temperatures, likely resulting in reduced carbon uptake capacity and ultimately heat damage.Still added.

The National Science Foundation funded this investigation.

magazine reference

Still, CJ, and others. (2022) No evidence for canopy-scale leaf thermoregulation of leaf cooling below air temperature across a range of forest ecosystems. Proceedings of the National Academy of Sciences.