Drought weakens trees resistance to pests and diseases

| June 29, 2018

Forests and the trees that live in them provide for a vast array of ecosystem services and are vital for maintenance of biodiversity.

A new review paper published in Nature outlines the current knowledge on the causes of tree death from drought and dehydration using specialised non-invasive methods, a timely issue as Australia’s drought worsens.

Recent predictions indicate that surface warming may lead to longer and more intense droughts, raising concerns about the vulnerability of forests to extreme events. Droughts can kill hundreds of millions of trees within short timescales, often in combination with massive bushfires and pest-pathogen outbreaks.

Recent examples of mass dieback include extreme droughts in California and Australia. The historic Californian drought resulted in the death of over 100 million trees, while in northern Australia, over 7000 ha of Mangrove forest dropped dead.

“If we want to understand how much risk our forests are at, we have to be really confident in our measurements”, explains lead author of the study Associate Professor Brendan Choat from the Hawkesbury Institute for the Environment.

In collaboration with Australian and international colleagues, he is using advanced imaging techniques like Magnetic Resonance Imaging (MRI) and X-ray microtomography to provide a very clear picture of what happens to trees during drought, and what level of water stress will kill a given tree.

In well-watered conditions, plants are able to keep a continuous flow of water from the roots to the leaves as water is moved up the stems. However, in dry conditions it gets difficult for trees to meet the demand for water from the leaves and the water flow can break, so the stem fills with pockets of gas, much like a diver getting the bends when they resurface from the ocean too quickly.

If enough of these hydraulic connections are broken, the tree may lose most of its branches, or die completely.

“We call this hydraulic failure, and it is one of the leading causes of tree death during droughts” explains Choat. “Having access to cutting edge imaging facilities, like those provided by the Australian Synchrotron, is incredibly important for understanding this process”.

Building a database of drought tolerance of trees will allow us to better predict what impact drought will have on native forests. But is also vital for planning of urban green spaces.

As the NSW State Government announces plans to plant 5 Million new trees in Sydney, it is worth knowing which trees are likely to survive best when drought strikes.

Understanding the limits to drought of particular tree species could mean the difference between trees that see decades of life versus those that become destined for firewood.