Trees tolerate extreme heat by 'sweating', but they also stop absorbing carbon from air: Study

In an effort to see how trees respond to extreme heat, a team of Australian researchers conducted a unique year-long study, and found that leaves "sweat" to cope with ever-rising average temperatures and severe, long-lasting heatwaves.

The study, published in the journal Global Change Biology this week, revealed that trees continue to release water through their leaves -- a process that keeps trees cool -- during extreme heatwaves. At the same time, the trees also temporarily stop absorbing carbon from the air, leaving scientists worried about the planet's future as the trend of global warming continues.

The latest findings are deemed significant because it was previously believed that photosynthesis and transpiration -- the process of water release -- were linked. But, the new study showed that both processes can occur independently, helping trees survive critically high temperatures.

If the current predictions are to be believed, these extreme heatwaves will become more frequent and severe in the future. So, does this mean that trees could one day stop capturing carbon for once and for all?

"If heatwaves occur over a large surface area ... clearly the trees and native forests in that area would take up less carbon," Prof Mark Tjoelker, an author of the study from the University of Western Sydney's Hawkesbury Institute for the Environment, told the Guardian. "And if there is an increased frequency of heatwaves that obviously impacts their ability to serve as carbon sinks."

As part of the study, the researchers used what are called Whole Tree Chambers, located at the university campus. The chambers, that can accurately measure the trees' rates of photosynthesis and water use, helped the researchers impose a year of warming and then a four-day high-intensity heatwave on tress local to the Sydney region.

To simulate the impacts of higher average temperatures in the Sydney region, the researchers imposed an additional heat of 3 degrees Celsius on Parramatta Gums (Eucalyptus parramattensis). After 12 months, when the trees grew to more than 6 metres, the researchers imposed four days of heat at 43 degrees Celsius.

The results revealed that the trees used a range of strategies to cope with the heatwave:

1. Trees stopped their leaves from reaching critically high temperatures by evaporating large quantities of water.
2. To maintain the high rates of transpiration, the trees sourced water from throughout the soil profile, to depths of 1.5 metres and below.
3. Within 24 hours of the start if the heatwave, the trees increased their threshold temperature by 2 degrees Celsius.

"What normally happens is that a tree's use of water and its rate of photosynthesis are closely related and this process is the basis of how scientists predict what the effects of a warmer Australia on trees and forests will be," Tjoelker said in a statement.

"Under these extreme temperatures, this relationship changes completely – the trees can no longer photosynthesise, but they continue to use a lot of water to keep their leaves from reaching damagingly high temperatures. In addition, the ability to increase the high-temperature tolerance of their leaves helps to explain how eucalypts cope with heatwaves that would burn the leaves of other species," Tjoelker added.