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Most trees a sip away from death: plants' plumbing systems often on the brink of failure.

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

Seventy percent of 226 woody species in forests around the world routinely function near the point where a serious drought would stop sufficient water transport from their roots to their leaves, says plant physiologist Brendan Choat of the University of Western Sydney in Richmond, Australia. Even trees in moist, lush places operate with only a slim safety margin separating them from 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. He and his colleagues report their findings in the Nov. 29 Nature.

"I think this is a really big deal," says David Breshears of the University of Arizona in Tucson. As forest researchers 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 more worrisome data. "They all keep pointing to: 'Whoa, our forests are really vulnerable.'"

Trees don't have hearts to pump vital fluids. Instead, evaporation from tiny pores in the leaves pulls water up from the roots through masses of microscopic tubes. Called xylem tissue, these marvels of hydraulic transport can develop microscopic air bubbles when water is scarce, which then block individual tubes. Developing too many of these bubbles across the xylem kills the tree.

To judge the state of the forests, Choat and colleagues pieced together information from 81 sites spanning wet tropics to arid shrublands. The team assessed normal water transport in various tree species and the point at which each species fails.

Flowering tree species--such as maples and oaks--proved more vulnerable overall to dry conditions than conifers. But the researchers show that the majority of trees operate with only the slimmest of safety margins.

Trees have to make trade-offs to capture carbon dioxide from the air for growth and metabolism. When a tree opens its pores, it loses about 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.


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Author:Millus, Susan
Publication:Science News
Geographic Code:1USA
Date:Dec 29, 2012
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