Australian researchers have made an interesting new observation
Researchers from New Western Sydney University, based within the Hawkesbury Institute for the Environment, observed trees local to the Sydney region for a whole year, imposing them to heatwave conditions. Australia is intensely affected from climate change and had the third-hottest year ever in 2017, while seven of the 10 hottest years recorded in the continent have occurred since 2005. For the needs of the research, the unique Whole Tree Chambers was used, located at the University’s Hawkesbury campus. According to the findings, published in Global Change Biology, the trees managed to stay functional under such conditions, by evaporating large volumes of water through their leaves -in a process similar to sweating- and temporarily stopped capturing carbon. Up to now, scientists believed that photosynthesis and transpiration – the process of releasing water – were linked, meaning one would not occur without the other.
The new observation worries the researchers about what will happen as global warming escalates. “If heatwaves occur over a large surface area … clearly the trees and native forests in that area would take up less carbon,” says Prof Mark Tjoelker from the University of Western Sydney’s Hawkesbury Institute for the Environment and one of the authors of the study. “And if there is an increased frequency of heatwaves, that obviously impacts their ability to serve as carbon sinks”, he adds.
At the Whole Tree Chambers installation, researchers can grow trees up to a height of 9 m (29.5 ft) in a fully controlled environment in terms of air temperature, soil moisture, irrigation, CO2 levels and humidity, being able to precisely and accurately measure the trees’ rates of photosynthesis and water use. In this experiment, the performance of Australian eucalypts was measured, with six Parramatta Gums (Eucalyptus parramattensis) being grown in the chambers under conditions simulated at an average of 3° C (5.4° F) warmer than average, while the other six capsules were set to reflect the ambient temperature. The trees were left to grow for 12 months to more than 6 metres and were then imposed to four days of heat at 43°C.
The results showed that the trees used various strategies to avoid being damaged by the heat, like evaporating large quantities of water in order to protect their leaves from burning, sourcing water from throughout the soil profile, to depths of 1.5 metres and below, and increasing their high-temperature tolerance. Within 24 hours of the start of the heatwave, the threshold temperature at which leaves start to become damaged had increased by 2 °C.
“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,” says Professor Mark Tjoelker. “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”.
Although the results seem promising, there is a limit to each plants’ ability to adjust to heat. “We were surprised how well these eucalypts acclimated to the heatwaves and maintained their function,” says Dr John Drake, formerly of the Hawkesbury Institute for the Environment and now a researcher at the College of Environmental Science and Forestry in the US. “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”, he says.
Image credits: Western Sydney University - Hawkesbury Institute for the Environment