2 August 2016: Using the world's only Whole Tree Chambers, scientists at Western Sydney University's Hawkesbury Institute for the Environment examined the response of wholly-enclosed trees to see how closely the responses of native Australian trees matched the global prediction models in research published by New Phytologist.
Around the world, trees and plants absorb more than 120 gigatonnes of carbon out of the atmosphere every year and about half of that is returned back to the atmosphere as trees respire (by 'breathing out' carbon). As global carbon dioxide levels rise, scientists are determining how increasing average
temperatures and heatwave events affect this balance between absorption of carbon from the air and release of carbon through respiration in trees.
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 situation where rising temperatures promote increased carbon release, which then further promotes temperature increases.
Using the world's only Whole Tree Chambers, scientists at Western Sydney University's Hawkesbury Institute for the Environment examined the response of wholly-enclosed trees to see how closely the responses of native Australian trees matched the global prediction models.
Using Forest Red Gums (Eucalyptus tereticornis) in a warming experiment inside the twelve Whole Tree Chambers, scientist exposed six of the chambers to temperatures of 3 degrees Celsius above the outside temperature. The Whole Tree Chambers are able to precisely control internal temperatures to enable scientists
to grow trees in the ground up to nine metres high and measure every aspect of their functions such as carbon intake and output, and water usage.
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 C uptake and release via photosynthesis and respiration... These findings demonstrate that there is greater scope for trees
to adjust physiologically to warming except under heatwave conditions...
Forest Red Gums are a good species for exploring these responses to warming because they are found right up Australia's east coast from North Queensland to southern Victoria, so they naturally contain genes that might give them an advantage in warmer conditions and so make them better able to adapt
to warmer average temperatures.
Our current models would indicate that +3°C warming would increase tree C respiration more than photosynthesis, resulting in a net transfer of C from trees to the atmosphere, accelerating climate change. This research, however, indicates that warming of +3°C does tend to reduce photosynthesis,
but trees are able to change their physiology and maintain respiration at a lower level as well. This means that overall, warming does not result in a net transfer from trees to the atmosphere; the system remains in balance.
Heatwaves are an exception, however. 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 C to the atmosphere during heat waves, providing a positive feedback
to further climate warming.
This is a significant finding that adds new information to the prediction models and demonstrates that plants respond in a way that mitigates the negative effects of climate warming, a finding that is consistent with warming experiments undertaken in the northern United States.
The research clearly demonstrates that predicted increases in heatwave events will test the ability of trees to acclimate in this way and under extremely high temperatures. This contributes to a positive feedback loop where increased temperatures cause more carbon to be released and that increased
carbon contributes to extra 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 C uptake and release via photosynthesis and
respiration", explains Dr John Drake, lead researcher of this experiment.
"Field conditions expose trees to the natural variations in sunlight, heat conditions, activity of wood and stems, and shading from surrounding trees, giving us a representative picture of the performance of trees under real-world conditions", he explains.
"These findings demonstrate that there is greater scope for trees to adjust physiologically to warming except under heatwave conditions."
Being such a geographically widespread species, Eucalypts may contain more genetic ability to adapt to warming conditions which gives them a natural advantage in adjusting to changes brought about by increased temperatures.
The research was published in New Phytologist. (opens in a new window)
Prof Mark Tjoelker
Dr Mike Aspinwall
Prof Peter Reich
Dr Craig Barton
Prof Belinda Medlyn
Dr Remko Duursma