Tall Trees Sucked Dry by Global Warming

A well-known scientific principle describing how water moves through plants can help explain why trees may struggle to survive as the planet warms, scientists say in a new study.

Using an equation called Darcy’s law, the research also helps explain why iconic giant trees like the California redwood could be especially vulnerable to rising temperatures. The concept was outlined in a  published this month in the journal .

Plants’ vascular systems can be likened to bunches of straws, explained lead author Nathan McDowell, a researcher at Los Alamos National Laboratory’s Earth and Environmental Sciences Division, meaning water moves from the roots to the branches through tension.

The atmosphere pulls water through plants’ systems, and “the warmer and drier the air is—which is what climate change is doing—it’s increasing the evaporative demand,” said McDowell. “The warmer the air is, the more water it can hold, so it sucks harder on those straws.”

To keep from dehydrating, plants start to close their stomata—the openings in leaves through which they take in CO2.

But as stomata close during a drought, they can’t photosynthesize as effectively, McDowell said, preventing trees from taking advantage of more CO2 in the atmosphere.

“It’s like going to a buffet with duct tape over your mouth,” said McDowell.

Bigger trees, of course, have longer “straws” moving water through their systems than shorter trees, meaning they have to close their stomata even more, resulting in greater stress to the plant, McDowell explained.

For this reason, the study states, Darcy’s law shows that “shrubby, low-statured plants are most likely to survive, whereas tall, old-growth forests are particularly vulnerable to warming climate.”

Could thinning or assisted migration of trees help?, May 11).

But McDowell said it’s important to note that his paper has consequences that apply to more places than just today’s current drought hot spots.

“The big punch line of the paper is that globally, everywhere, temperatures are going up,” he said.

Because Darcy’s law applies to all plants, tall tree species around the world could be vulnerable to climate change. That includes many beyond the U.S. Southwest and other regions where scientists are already fairly certain forest ecosystems are likely to suffer.

McDowell and his co-author, Craig Allen of the U.S. Geological Survey, have a few ideas for forest managers trying to figure out how to help trees survive as temperatures rise.

One suggestion is “assisted migration,” or planting tree genotypes that are comfortable living in more southerly, low-elevation places in higher, more northerly spots, because these plants are already more resilient to hotter conditions.

Another idea outlined in the paper is the thinning of forests, which reduces the density of stands so that there is more water available for remaining trees.

“Even forests that traditionally were not exposed to frequent disturbances—and thus typically are not thinned—may benefit from reasonable stand density reductions when extreme drought conditions emerge, possibly including forests in wetter regions,” the study states.

But Allen said that because forest management is a site-specific activity, thinning trees in many ecosystems would be inappropriate, so using it as a strategy to save trees from higher temperatures would need to be evaluated on a case-by-case basis.

“This is in no way a blanket call for thinning all forests,” Allen said.

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