TREES BEAT THE HEAT
06 Feb 2018
Researchers have uncovered the novel strategies Australian eucalyptus trees use to survive extreme heatwaves, one which involves the tree "sweating" large volumes of water through its leaves.
The study involved the University of Western Sydney's novel Whole Tree Chambers, which give researchers the ability to grow trees up to a height of 9m in fully enclosed and controllable environments that allow air temperature, soil moisture, irrigation, CO2 levels and humidity to be manipulated. Six Parramatta Gums (Eucalyptus parramattensis) were grown in the chambers for 12 months under conditions simulating an average of 3°C warmer than average, while six were grown under ambient temperature conditions.
After the trees reached a height of over six metres the researchers paused irrigation for one month, drying out the surface soils, before subjecting all 12 trees to four consecutive days of extreme heatwave temperatures above 43°C. The trees responded to the extreme heat by activating a process called transpiration, whereby large volumes of water are transported to the leaves and subsequently evaporate to avoid the leaf being damaged from the heat. 
"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," explains Professor Mark Tjoelker, one of the authors on the study. "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."
The responses to the extreme heat did not vary between the trees raised at ambient temperatures and the trees raised at the warmer temperatures. This indicated that growth in a warmer temperature did not contribute to any extra tolerance of heatwaves. 
Other strategies the trees deployed to manage the heatwave included rapidly increasing the thermal tolerance of their leaves to stop them from burning, and sourcing water from depths of greater than 1.5m below the ground.
"We were surprised how well these eucalypts acclimated to the heatwaves and maintained their function," says John Drake, another author on the study. "This indicates that eucalypts can tolerate elevated temperatures and significant heatwave events as long as they have access to water. If heat and drought combine then we may see more damage occurring and the potential for tree mortality." 
More research will be done by the team examining other tree species to identify if this response if unique to eucalypts or if other trees utilise similar dynamic tactics to survive heatwaves.
