A small percentage of leaves on trees in tropical forests may be approaching the maximum temperature threshold for photosynthesis to work, suggests a Nature paper.

An estimated 0.01% of all leaves currently surpass this critical temperature but there are uncertainties in the range of potentially critical temperatures in tropical trees.

Modelling suggests that tropical forests can withstand up to a 3.9 °C increase over current air temperatures before a potential tipping point, therefore action is needed to protect the fate of tropical forests under future climate change.

Tropical forests serve as critical carbon stores and host most of the world’s biodiversity and may be particularly sensitive to increasing temperatures. The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages at about 46.7 °C.

However, whether leaf temperatures experienced by tropical vegetation approach this threshold, or soon will under climate change, remains unclear.

Christopher Doughty and colleagues used high-resolution measurements of land surface temperatures at a global scale, including Brazil, Puerto Rico and Australia, to estimate peak tropical-forest canopy temperatures.

Measurements were taken from an instrument on board the International Space Station between 2018 and 2020.

The authors found that canopy temperatures peaked at around 34 °C on average, although a small proportion of those observed exceeded 40 °C.

Around 0.01% of the leaves in the upper canopies surpass the temperature at which photosynthesis starts to fail in a typical year.

Moreover, warming experiments predict this value will rise to 1.4% under future warming conditions.

A model incorporating these dynamics under future climate change scenarios suggests that large-scale leaf death and loss may start to occur if warming exceeds 3.9 °C.

The authors note that this temperature is within the worst-case scenario for climate change predictions; however, they propose that ambitious climate change mitigation goals and reduced deforestation are needed to help forests to stay below thermally critical thresholds.

Read more in the Nature paper

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