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Hawkesbury Forest Experiment

What do trees do at night? Up until now the standard reply has been “not much”. However assumptions about nocturnal sap flow are being tested by UTS Plant Functional Biology and Climate Change Cluster (C3) researchers who are trying to get an accurate measure of how much water trees actually use. The results have important implications for how Australia manages its scarce water resources under the predicted impacts of climate change and is part of a much broader collaborative project called the Hawkesbury Forest Experiment (HFE).
“It has generally been assumed that at night the stomata on leaves are shut, because there is no light, and that, therefore, minimal water is lost via transpiration. Over the past few years the technology has evolved that allows us to use sensors that can measure very low sap flow and get a better understanding of what happens to trees at night,” said C3 postdoctoral fellow Dr. Melanie Zeppel.

Dr. Zeppel is a member of the C3 Terrestrial Ecohydrology Research Group lead by Professor Derek Eamus. A report, co-authored by Eamus, for the Department of Environment and Water Resources (FACE 2007), was a catalyst for establishing the unique HFE research facilities at the University of Western Sydney. Twelve whole- tree chambers , provided by the Swedish University of Agricultural Sciences, are being used to grow Eucalyptus saligna – a major Eucalyptus species native to the Hawkesbury region – under controlled CO2 and temperature conditions. The trees are being grown under elevated CO2 levels in order to mimic future predicted climate change scenarios. Half of the chambers are also being subjected to drought conditions to provide insight into possible interactions between elevated CO2 and drought.

Scientists have previously observed that plants respond to high CO2 levels by increasing photosynthetic activity which makes more fuel available for root mass growth and can lead to greater forest productivity. In addition high CO2 levels can cause the leaf stomata to close thereby reducing the plant’s requirements for water. This could impact the amount of water that flows into streams and rivers and groundwater recharge. However, climate change also involves a rise in temperatures and this will affect weather patterns including rainfall and humidity, which will also impact tree growth. According to HFE researchers there is an urgent need to study these effects on Australia’s native plant species ”to underpin Australia’s environmental and catchment management strategies for the 21st century”.

For Dr Zeppel it has been an opportunity to be a part of some very novel science that will help answer some very fundamental questions
“Nocturnal sap flows have been measured before but here we get to accurately measure low flow rates under elevated CO2 levels for the first time. We can then compare this information to what’s happening at ambient CO2 levels and this gives us some very powerful data to use to model the impact of climate change on Australian forest ecosystems. We will also be able to model the impact of drought on the trees and see if the effect is additive or synergistic [with elevated CO2 levels],” she said.
Ultimately it is hoped the information can be used by irrigators, farmers and resource managers.
“Tree physiology can help us understand climate change. We need to know if there will be less water in rivers and streams, we need to be able to quantify water loss so plantations can be put in the best locations and so that consideration can be given to farmers and irrigators downstream,” she said.

In the May Federal Budget the Government announced that the HFE site will be dramatically expanded under a $40m grant from the Higher Education Infrastructure Fund.
The UTS researchers expect to publish data from their nocturnal sap flow experiments later in 2009 along with information concerning the impacts of climate change on catchment water budgets.

 Photo of Whole- tree chambers courtesy of Dr Craig Barton

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