Pieter Poot: By loosing some of its branches and many of its leaves an adult tree will actually have more water available for its remaining leaves, making it less likely that the whole tree will die. Of the four Eucalypt species that were compared, Wandoo and Powderbark Wandoo were shown to have different mechanisms to cope with drought than Jarrah and Marri. During dry and hot conditions the first two species will keep the little pores in their leaves (i.e. stomates) still somewhat open ensuring some flow of water up to their canopies. In contrast, Jarrah and Marri close their pores under these conditions, causing branches to die. As a consequence the leaves of Wandoo and Powderbark can remain active (i.e. take up carbon dioxide from the air to produce sugars) even during hot weather. However, leaving their stomates open during these conditions does dry out their leaves considerably. The research showed that both Wandoo and Powderbark can actually cope with these very dry leaves. Under high tensions (i.e. when leaves are getting dry) the water columns in the stems of these species are much less likely to break than in Jarrah and Marri. Also, when their leaves are drying the cells in their leaves will collapse (like a balloon that is loosing air) much later then those in Jarrah and Marri. This may explain why Powderbark and Wandoo have a more inland distribution.

So, if Wandoo and Powderbark have such a coping strategy, why is it that we seem to be seeing more dying Wandoos than any of the other Eucalypt species? Partly, this may be related to the particular habitat that the species occur in or have evolved in. Wandoo mainly occurs lower in the landscape than the other species and prefers clayey soils. Although these low lying soils receive a lot of run off water in winter and spring and thus seem to have plenty of water, it is extremely difficult for plant roots to obtain water from a dry heavy clay soil in summer. By being able to tolerate their leaves to dry out to a large extent, Wandoo trees are actually still able to extract some water out of this very dry soil. As these heavy clay soils are extremely difficult to penetrate some researchers suggest that Wandoo has relatively superficial roots compared to the other species and thus may rely to a larger extent on this relatively ‘young’ (i.e. last years) water in the topsoil. In contrast, the other species that often occur higher in the landscape may rely more on deeper roots that access deeper and older water. Thus, Wandoo’s allegedly relatively superficial root system, its dependency on shallow ‘recent’ water and its preference for heavy clay soils may all contribute to its vulnerability to the drought that WA is experiencing over the last 30 years. As most Wandoo trees with crown decline have been shown to be infested with insect borers and pathogenic fungi (see Ryan Hooper’s research) the drought stress may cause them to be more vulnerable to these pathogens. Future studies demonstrating the effects that drought has on a tree’s vulnerability to pathogens will have to elucidate whether these ideas are right!