News•November 21, 2012

Drought Puts Trees the World Over 'At the Edge'

As the climate warms, scientists expect an increase in droughts around the world, causing all sorts of problems for water supplies, agriculture, and energy production. Forests will be affected too, naturally, but a new study released Wednesday in Nature shows just how widespread the effects could be.

After looking at 226 tree species at 81 locations around the world, two dozen experts from around the world have determined that fully 70 percent of trees are likely to suffer if conditions get drier — and it doesn’t matter whether those trees live in wet or dry habitats.

Aftermath of a drought-fueled fire in a Wisconsin pine forest.
Credit: Wisconsin DNR

That’s a big surprise, according to Bettina Engelbrecht of the University of Bayreuth, in Germany, who wrote a commentary on the new research in Nature. “I would have expected trees in dry areas would have a wider safety margin,” she said in an interview. “But the majority of species appear to be right on the edge. Just a little more drought will push them over.”

What actually kills a drought-stressed tree is something called “hydraulic failure.” Normally, water evaporates (or more accurately, transpires) from a tree’s leaves or needles through tiny pores called stomata. As the water exits, it creates a suction that pulls more water up from the roots through tiny channels in the trunk, in the woody tissue known as xylem. As Engelbrecht wrote in her commentary, it is “much like sucking water through a straw.”

During a drought, however, the rate of transpiration goes up, increasing the suction, while the amount of available water at the roots goes down. The extra suction can pull air bubbles in through pores in the sides of the channels, and when that happens, the channels become clogged. “How vulnerable the tissue in a particular species is to the formation of bubbles is a measure of its drought resistance,” Engelbrecht said.

It turns out that while trees in dry areas do have greater drought tolerance in absolute terms than those in wet areas, they still operate on the brink of death. The reason, Engelbrecht explained, “makes compete sense in hindsight.” The bigger the water-carrying channels that pass through the xylem, the bigger the pores in leaves or needles where water escapes. Those bigger stomata give also allow the tree to take in more carbon for photosynthesis and thus grow faster. Faster growth means that the trees with the biggest channels and stomata in both dry and wet areas tend to out-compete those with smaller openings.

Evolution dictates, therefore, that trees will tend to have the biggest possible channels, no matter what conditions they live under. The downside: bigger channels are more prone to air leakage, so even a little extra drought will push the trees into hydraulic failure relatively easily.

“We might have hoped that some forests would be relatively immune to drought, and that we didn’t have to worry so much about those,” Engelbrecht said. “Now we’re finding that we have to worry about all of them.”

Unfortunately, there doesn’t seem to be much anyone can do to prevent forest die-offs in a warming, drought-prone world. “Basically, this tells us that we should keep climate change in check as much as possible,” Engelbrecht said.