Dying aspen trees sound alarm for world's forests

They say what doesn't kill you makes you stronger, but that's not true for the US's iconic aspen trees.

They appeared to survive a severe drought between 2000 and 2003, but it is now clear that it fundamentally weakened them. If the same is true for other tree species, climate change may be pushing many forests perilously close to a tipping point.

Ever since the drought in western North America, aspen trees have been dying at an alarming rate, a phenomenon now known as sudden aspen decline. In some places, nearly a fifth of the trees ? one of the main species in western US forests ? have died.

Earlier this year, William Anderegg of Stanford University in California and his colleagues showed that the trees were dying mainly from water stress (PNAS, doi.org/fgfbsk). But it was unclear why they were still dying years after the drought had ended.

Air bubble intrusion

To find out, the team looked at the xylem, or water-conducting pipes, of declining aspen trees in Colorado. As trees lose water through their leaves, they suck up more through their roots. The drier the tree, the harder it must suck, but if it sucks too hard, air bubbles can creep into the xylem, interrupting water flow.

When Anderegg experimentally induced such cavitation in aspen twigs, he found that it developed more readily in those that had been through the drought, whether apparently healthy or dying. "There seems to have been some damage that would leave them more vulnerable to future water stress," says Anderegg.

Some evidence suggests that the stress of previous cavitation during the drought may have cracked the seams where xylem tubes connect, making it easier for air to creep in ? and this so-called "cavitation fatigue" is still there nearly a decade later.

Tipping point

This means prior drought stress may make trees more likely to die from a later drought, says Anderegg. Since much of the world will experience more severe droughts as climate changes, researchers will need to take this into account when trying to predict the fate of the world's forests. "Accounting for history is going to be pretty key," says Anderegg.

This could be very bad news for forests. A recent study showed 70 per cent of tree species in 81 forest sites worldwide, from moist forests to arid ones, are already close to their threshold for drought-induced cavitation.

If every severe drought lowers this threshold, many of these species might be pushed past the tipping point to mortality. "Our forests are potentially going to change a lot," says that study's co-author John Sperry at the University of Utah in Salt Lake City.

Journal reference: Global Change Biology, DOI: 10.1111/gcb.12100

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