From The Week (Ryan Cooper):
A team of scientists, led by Dr. William Anderegg of Princeton, have been working on the aspen question, and their results were published today in Nature Geoscience. The answer is something called “xylem cavitation.” And unless we do something big about climate change soon, it will kill most of the aspens in the Southwest.
Here’s what that means. Trees transport water through their xylem tissue (one example of which is regular old wood), basically composed of millions of tiny tubes, or “conduits.” Xylem doesn’t work like a mechanical pump — instead, water flows up the tree trunk through capillary action. That flow is maintained through evaporation at the leaf surface, removing water at the top so more can replace it, and supplied by the roots drawing water from the soil.
In hot, dry conditions, like the early 2000s drought, water evaporates more quickly from the leaf surface — and there is less water in the soil to maintain supply. Anderegg and his team quantified both of these with a factor they called “climatic water deficit.” When the deficit is high, the water pressure inside the xylem decreases due to tension between the top and bottom of the tree.
If the pressure gets low enough, gas bubbles will spontaneously form in the water column — which is called cavitation. A bubble instantly blocks that particular xylem conduit and prevents the water from flowing. Block enough conduits, and the tree desiccates and dies.
It’s “like a tree heart attack,” says Anderegg. He and his team constructed a model of this cavitation mechanism, calculated a threshold at which aspens should die, and compared it with historical data on the early 2000s drought. They found pretty clear agreement, explaining about 75 percent of the tree mortality during that time (a good result, given how complex forests are).