Trees worldwide a sip away from dehydration

Plumbing systems operate on a razor’s edge, leaving forests vulnerable

Trees in most forests, even wet ones, live perilously close to the limits of their inner plumbing systems, a global survey of forests finds.

PLUMBING TROUBLE In a colored microscopic cross section of frozen wood, blue indicates where water had been flowing properly through a tree’s plumbing. Empty water channels reveal ones that have succumbed to air bubbles, an effect of drought that can kill a tree. Yuzou Sano/Hokkaido University

Seventy percent of the 226 tree species in forests around the world routinely function near the point where a serious drought would stop water transport from their roots to their leaves, says plant physiologist Brendan Choat of the University of Western Sydney in Richmond, Australia. Trees even in moist, lush places operate with only a slim safety margin between them and a thirsty death.

“This is the first time that we’ve looked across all forest [types] and seen that there’s a convergence on risky behavior,” Choat says.

Instead of looking at the balance of water gains and losses for whole forests, he and his colleagues mined existing data to assess the dangers for individual tree species in forests from wet tropics to arid shrublands. They report their findings online November 21 in Nature.

“I think this is a really big deal,” says David Breshears of the University of Arizona in Tucson. As forest ecologists and plant physiologists confronting climate change, “we’ve been trying to be careful as a community not to be alarmist,” he says. But the new paper adds yet another perspective that’s worrisome. “They all keep pointing to: ‘Whoa, our forests are really vulnerable.’ ”

Trees don’t have hearts to pump their vital fluids. Instead, evaporation from tiny pores in the leaves pulls water up from the roots through thousands of slim tubes. Called xylem tissue, this marvel of hydraulic transport can develop microscopic air bubbles that block particular tubes. Too many of these tiny embolisms across the xylem kills the tree.

Forests cover some 30 percent of the Earth’s land, sheltering a rich share of the diversity of life. Woodlands provide timber and other products and lock up carbon that would otherwise enter the atmosphere and contribute to global warming. To judge the state of this vital system, Choat and 23 other researchers from around the world pieced together information from 81 diverse sites on normal water transport in various tree species and the point at which each species fails.

Tree species that produce flowers — such as maples and oaks — proved overall more vulnerable than conifers to drier conditions. But the researchers show that the majority of trees operate with only the slimmest of safety margins. “What surprised me is that wet forests were as vulnerable as the dry ones,” Choat says.

In plumbing systems, trees have to make tradeoffs. Capturing carbon dioxide in the air for growth and metabolism is a risky business. A tree sacrifices 400 molecules of water to evaporation to snag one molecule of carbon. The new study, Choat says, reveals that trees are maximizing their carbon capture for food even though it strains the plumbing.

For conservation in a time of climate warming, the edgy life of trees means “we cannot prioritize our efforts by forest vulnerability,” says plant ecologist Bettina Engelbrecht of the University of Bayreuth in Germany. “We have to worry about them all.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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