Prolonged drought causes air bubbles to “choke” trees
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An x-ray micro compound tomography image showing the vessels of a conifer, which contain gas bubbles that don’t refill after drought.
Image: Brodersen et al, 2010

One of the main ways drought affects trees is by forming air bubbles in the tiny vessels that carry water inside their trunks. 

Usually after short periods of drought, these trees are able to bounce back when the rain returns. But with more severe droughts being triggered by climate change, new research is revealing that trees are now struggling to refill these air bubbles.

These air bubbles are similar to the ones that cause "the bends", or decompression sickness, in humans and kill divers who surface too quickly - and when droughts last too long, they're causing die-off in forests under frequent heat stress, such as those in Tasmania, Australia, last year.

Because we rely on forests to help store carbon and mitigate the effects of climate change, this problem could feed back into climate systems leading to more extreme weather.

A team of researchers led by Dr Brendan Choat from the Hawkesbury Institute for the Environment at the University of Western Sydney is now investigating more about what's happening inside the trunks. They're using advanced imaging techniques, similar to CAT scans, to track the progression of these air bubbles, known as emboli, during periods of drought and following rainfall.

He’s found that some flowering plants (scans shown below) are able to bounce back quickly after drought, because they can refill gas bubbles once it rains again. However, coniferous plants, such as pines and cypresses (scans shown above), are not able to dissolve gas bubbles as easily, suggesting that they may take longer to recover from droughts.

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Image shows air bubbles (emboli) in the steam of a grapevine. These air bubbles are refilled much easier following drought than the air bubbles in the vessels of coniferous plants.
Image: Brodersen et al, 2010

The next step is to model how long trees can survive drought and still bounce back. This will help forest managers determine which species are best suited to particular environments, as well as help us protect carbon sinks and important ecosystems around the planet. 

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