1203rd Ordinary General Meeting

"Climate change, regional drought and forest mortality: are we seeing a new global phenomenon?"

Professor Derek Eamus, University of Technology, Sydney

Wednesday 5 September 2012 at 6.30 pm

Union, University and Schools Club, 25 Bent Street, Sydney

Meeting report by Donald Hector

Around the world, forests seem to be under stress. At the 1203rd OGM, Professor Derek Eamus, a plant physiologist at University of Technology Sydney, gave a fascinating talk on what is causing the major problems in the world's forests and the implications if there is a substantial increase in global temperatures. In every continent there are numerous examples of forest die-back in recent years. Understanding the background of this is critical given the importance of forests in the global ecosystem. Forests are large repositories of carbon, have a large influence on the way in which water moves through the environment, are important for biodiversity, have a major impact on the absorption of energy from the sun and have high amenity value.

There are two theories to explain die-back of forest during drought conditions. The first of these is carbon starvation. This is an important factor in forest health particularly with isohydric trees species (isohydric trees are those that regulate water flow in order to maintain canopy humidity within a relatively narrow range. They do this through opening and closing leaf stomata in response to changes in humidity). When the stomata close, no carbon dioxide can enter the leaf. One response of isohydric trees to drought conditions is to close the stomata in order to preserve water. Thus, during a protracted drought, the tree closes the stomata close and cannot absorb carbon dioxide and will gradually starve to death.

The second theory is that forests die due to hydraulic failure. This is a particular problem with anisohydric species (unlike isohydric trees these do not respond to drought by closing stomata, so the tree continues to absorb carbon dioxide). The problem is that if the ground water availability drops too low, there is insufficient water potential causing embolism in the xylem (the fine tubes that conduct water from the root system to the leaves) and this interrupts water flow to the leaf system.

Catastrophic failure of forests during drought conditions seems to be related to one or other of these effects. Observation suggests that droughts of long duration cause hydraulic failure, whereas drought soft high-intensity cause carbon failure followed by hydraulic failure. These observations may have substantial implications for Australia's forests. Australia has highly variable rainfall and the annual evaporation in many areas is higher than the annual rainfall. River discharges are also much lower than other than Europe, Asia, Africa or the Americas. The accepted wisdom is that temperature is the main determinant of forest mortality due to drought and usually occurs a couple of years after the drought finishes.

Professor Eamus and his co-worker, Nicolas Boulain, have developed a conceptual model that relates duration of drought conditions and their intensity to the reasons for forest failure. They question the conventional wisdom that temperature is the most influential determinant. They have developed a mechanistic model of forest behaviour that disaggregates a number of the parameters that of been incorporated into the highly sophisticated soil-plant-atmosphere (SPA) models. One important parameter is the vapour pressure deficit (VPD), a measure of canopy humidity. Modelling 15 scenarios indicated that temperature stress is not a major determinant of forest mortality; what is important is VPD. It is the combination of an unusually high temperatures and very dry conditions thereby reducing VPD that does the damage. They conclude that VPD is an important parameter that needs to be included in climate models.

Professor Derek Eamus is a plant physiologist and ecophysiologist who leads the Terrestrial Ecohydrology Research Group within the Plant Functional Biology and Climate Change Cluster at the University of Technology, Sydney.

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