Heat waves and rising global temperatures are increasingly pushing tropical forests beyond the temperature limits required for photosynthesis reducing their ability to absorb carbon dioxide and increasing the risk of tree mortality according to a new study.
Published in the Proceedings of the National Academy of Sciences (PNAS), the study found that around 57 million hectares of tropical forests experienced treetop temperatures above the average critical threshold between 2001 and 2020. Researchers warn this area could expand to more than 93 million hectares by 2050 and nearly 160 million hectares by 2100 an area larger than South Africa.
Researchers from the École Polytechnique Fédérale de Lausanne (EPFL) compared the heat tolerance of 200 plant species with satellite derived temperature data collected across tropical forests over two decades.
The study found that plants typically have a thermal safety margin of about 15 degrees Celsius for efficient photosynthesis. However extreme heat, prolonged droughts and global warming are shrinking this margin making it increasingly difficult for trees to carry out the process that enables them to absorb carbon dioxide, release oxygen and produce nutrients.
Scientists explained that once leaf temperatures exceed a critical threshold the proteins responsible for photosynthesis begin to break down, slowing plant growth and increasing the likelihood of tree death.
The researchers noted that prolonged thermal stress could alter forest composition reduce biodiversity and weaken ecosystem resilience to future heat waves and droughts. They warned that declining carbon absorption by tropical forests could accelerate global warming, while reduced water vapour released by forests may increase the risk of droughts and other extreme weather events.
Lead researcher Charlotte Grossiord said the findings help identify forest areas most vulnerable to heat stress enabling conservation efforts to focus on landscapes facing the greatest climate risks.
Although the study focused on tropical forests, researchers noted that recent heat waves have also pushed some tree species and crops in Switzerland beyond their thermal tolerance suggesting that heat stress is becoming a growing concern across different ecosystems.
The study also indicted that while some heat tolerant plant species may gradually replace more vulnerable species, it remains uncertain whether forests can adapt quickly enough to keep pace with rising global temperatures.