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

How do you cool with a hot day? Maybe you find the shadow, activate a fan or remove in air conditioning? But save a thought for tropical forest trees. As the climate warms up, they must either adapt to heat, adapt to generations, or start a slow drop towards death.

In full sun, tropical leaves can become much warmer than the surrounding air – enough to slow down – or even stop – the survival process of photosynthesis.

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

We found that certain tropical trees have means to deal with damaging temperatures in warmer parts of their distribution area. This could give them an advantage over competitors because the climate continues to warm up.

Leaves cooling strategies

In warmer climates, plants can cool their leaves and avoid thermal lesions by evaporating water through their “stomates” – dull pores on the surfaces of leaves and stems.

Or they can develop narrower and smaller leaves. These can eliminate heat more effectively than large leaves because the wind approaches the surface of the leaves, breaking the thin layer of motionless air which isolates the sheet.

The leaves can also change their orientation to absorb less sun radiation.

However, we do not know which species are the best for making these changes, which are collectively called “thermoregulation”. Nor do we know if this capacity has evolved during generations or if the trees have been adjusted during their lifetime. Our new study sought to shed light on these questions.

What the study involved

First of all, we tested how the different characteristics of the leaves influence the heat that they become hot. To do this, we sampered trees from 16 forest sites through the humid tropics of Queensland, from low warm land to fresh summits.

The sampling involved three species: Darlingia Darlingana (Silky Oak), Elaeocarpus Grandis (Blue Encreong) and Cardwellia Sublimis (bull oak).

We have traveled distant forests to locate our study species. Then we used a giant sling to shoot the branches of the canopy in the canopy.

We have measured the leaves according to the factors that influence their warm: width and thickness, chemical composition, use of stomata to expel water for cooling, color and reflectivity.

These measures in the field have entered a computer model. We asked the model to predict how the temperature difference between the leaves and the air has changed through the habitats where the species developed.

Modeling has shown two of the three species studied – Silky oak and Bleu – were clearly capable of “self -cool” in warmer environments. They did so by increasing the activity of their “stomates” and having narrower and smaller leaves.

Was it proof of climate adaptation?

But why have some populations of trees have avoided damaged temperatures? Have the genes of these populations evolved from one generation to another to become better suited to a warmer world? Where was another factor in play?

To answer these questions, we have examined the DNA of the variable populations of the three species. We were looking for small differences related to climates in which individual trees have developed.

We found signals in the three species associated with temperature and precipitation. This suggests that the history of the climate has shaped their genetic responses, but not always with the same result.

For example, although the bull oak has shown signs of adaptation, this may not help the regulation of temperature, but rather influence the function of the plant by other means.

To test the idea, we organized a glassware experience using common garden plantations of Sowing of Bleu, collected in different populations. The plants were exposed to warmer or cool temperatures in separate greenhouse rooms to imitate the current conditions of highlands and plains.

The Blue Entis Sowing, from populations from different climates, showed the same variation in temperature differences in air sheets that we have observed on the ground. It happened, whether they were cultivated in the cooler or hotter rooms.

This suggests that genetic adaptation helps certain populations of trees to keep their leaves cooler. This could guide conservation managers when choosing seeds for the restoration of tropical forests in a warming world.

Different species, different strategies

Tropical tropical forests are vital for biodiversity and to fight climate change by absorbing carbon dioxide from the atmosphere. But the waves of heat and droughts push many species of trees to their limits.

Our study shows a variation within species that can stamp a certain risk from the increase in temperatures. But all the species of trees do not have these strategies to deal with heat.

As we have shown, some tropical trees can be more vulnerable to a warming world. As heat waves become more frequent and more intense, trees that cannot adjust the temperatures in their leaf can face higher risks of tissue damage, reduced growth or even local extinction.

Understanding how tropical trees have adapted to temperature increases is crucial to assess their resilience to global warming and help protect them.

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