Australasian Science: Australia's authority on science since 1938

Can Trees Cope with the Third Degree?

Scientists at Western Sydney University have used the world’s only “whole tree chambers” to measure the respiration of trees under proposed climate change conditions.

The world’s trees and plants absorb more than 120 Gt of carbon from the atmosphere every year, and about half of that is returned back to the atmosphere as trees respire. Current models predict that rising temperatures will increase the amount of carbon that trees respire for every unit of carbon they absorb, leading to a positive feedback loop where rising temperatures promote increased carbon release, leading to further temperature increases.

Using the world’s only “whole tree chambers” in which trees can be grown in the ground up to 9 metres high under precisely controlled temperatures, a team led by Dr John Drake measured carbon exchange and water usage of wholly-enclosed forest red gums (Eucalyptus tereticornis) at temperatures 3°C above the outside temperature.

Current models indicate that warming of 3°C would see respiration of CO2 exceed uptake of CO2 by photosynthesis, resulting in a net transfer of carbon from trees to the atmosphere, thus accelerating climate change. However, Drake’s research, published in New Phytologist, indicates that warming of 3°C tends to reduce photosynthesis, but trees are able to change their physiology and maintain respiration at a lower level as well. This means that warming does not result in a net transfer from trees to the atmosphere; the system remains in balance.

However, this isn’t true for heat waves. During uncommonly hot conditions of approximately 35°C, a further warming of 3°C leads to a strong increase in tree respiration and a reduction in photosynthesis. This causes trees to release carbon to the atmosphere during heat waves, providing a positive feedback event that promotes climate warming.

“This is the first experiment where we have been able to subject a widely distributed Australian tree species to whole-canopy warming in field conditions and measure whole-canopy carbon uptake and release via photosynthesis and respiration,” Drake says. “These findings demonstrate that there is greater scope for trees to adjust physiologically to warming except under heat wave conditions.”