Some tropical trees cool their leaves to survive the heat — but not all species have ways to cope

How do you cool yourself on a hot day? Perhaps you find shade, switch on a fan or retreat to air conditioning? But spare a thought for tropical forest trees. As the climate warms, they must either adjust to the heat, adapt over generations, or begin a slow decline toward death.

In full sun, tropical leaves can become much hotter than the surrounding air – hot enough to slow – or even stop – the life-sustaining process of photosynthesis.

So how can trees keep their leaves within safe temperature limits? And are some species better at this than others? Our new research examined that question.

We found some tropical tree species have ways to cope with damaging temperatures in warmer parts of their range. This could give them an advantage over competitors as the climate continues to warm.

A thermal image showing high daytime leaf temperatures in tropical trees. Author provided

The cooling strategies of leaves

In warmer climates, plants may cool their leaves and avoid heat damage by evaporating water through their “stomata” – tiny pores on the surfaces of leaves and stems.

Or they might develop narrower, smaller leaves. These can shed heat more effectively than large leaves because wind passes closer to the leaf surface, breaking up the thin layer of still air that insulates the leaf.

Leaves may also change their orientation to absorb less radiation from the Sun.

Yet we don’t know what species are best at making these shifts, which are collectively known as “thermoregulation”. We also don’t know if this ability evolved over generations or if trees have adjusted during their lifetime. Our new study sought to shed light on these questions.

What the study involved

The researchers trekked into remote forests then used a giant slingshot to knock down branches from high in the canopy. Kali Middleby

First, we tested how various characteristics of leaves influence how hot they get. To do this, we sampled trees from 16 forest sites across the Wet Tropics of Queensland, from hot lowlands to cool mountaintops.

The sampling involved three species: Darlingia darlingiana (silky oak), Elaeocarpus grandis (blue quandong) and Cardwellia sublimis (bull oak).

We trekked into remote forests to locate our study species. Then we used a giant slingshot to knock down branches from high in the canopy.

We measured the leaves according to factors that influence how hot they become: width and thickness, chemical composition, the use of stomata to expel water for cooling, and colour and reflectivity.

These field measurements were entered into a computer model. We asked the model to predict how the temperature difference between leaves and air changed across the habitats where the species grew.

The modelling showed two of the three studied species – silky oak and blue quandong – were clearly able to “self cool” in hotter environments. They did this by increasing the activity of their “stomata” and by having narrower, smaller leaves.

The researchers measured the leaves according to factors that influence how hot they become. Alexander Cheesman

Was this evidence of climate adaptation?

But why were some tree populations able to avoid damaging temperatures? Did the genes of those populations evolve from one generation to the next to become better suited to a warmer world? Or was another factor at play?

The study involved a glasshouse experiment using blue quandong seedlings. Kali Middleby

To answer these questions, we examined the DNA of the varying populations of all three species. We were looking for small differences linked to the climates in which individual trees grew.

We found signals in all three species associated with both temperature and rainfall. This suggests climate history has shaped their genetic responses – but not always with the same outcome.

For example, although bull oak showed signs of adaptation, this may not help with temperature regulation, but instead influence the plant’s function in other ways.

To test the idea, we ran a glasshouse experiment using common garden plantings of blue quandong seedlings, collected from different populations. The plants were exposed to warmer or cooler temperatures in separate glasshouse chambers to mimic the current conditions of the uplands and lowlands.

Seedlings of blue quandong, grown from populations originating in different climates, showed the same variation in leaf-to-air temperature differences that we observed in the field. This occurred regardless of whether they were grown in the cooler or warmer glasshouse chambers.

It suggests genetic adaptation is helping some tree populations keep their leaves cooler. This could guide conservation managers when choosing where to collect seeds for rainforest restoration in a warming world.

Different species, different strategies

Tropical rainforests are vital for biodiversity, and for tackling climate change by absorbing carbon dioxide from the atmosphere. But heatwaves and droughts are pushing many tree species to their limits.

Our study shows variation within species that can buffer some risk from rising temperatures. But not all tree species have these strategies to cope with heat.

As we’ve shown, some tropical trees may be more vulnerable to a warming world. As heatwaves become more frequent and intense, trees that can’t adjust their leaf temperatures may face higher risks of tissue damage, reduced growth or even local extinction.

Understanding how tropical trees have adapted to temperature rises is crucial for evaluating their resilience to global warming – and helping to protect them.