Science - Global Study Projects Drought-Driven Loss Of 40-50% Of All Planet's Forests w/i 40 Years
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Brodribb is the lead author of a new study in the journal Science that analyzed 10 years of research on how forests react to droughts, which are becoming more common and severe due to climate change, even though the damage may be less evident to the eye than the Australian bushfires. “There’s no smoke,” he says, “but there’s a big problem.” In fact, another paper in the same issue of Science found that climate change is driving a decades-long “megadrought” in the U.S. Southwest. Across the world, forests cover almost a third of all land — and in some places, the ratio is even higher. But if climate change continues unabated, many forests will suffer. Researchers have used models to understand potential changes to the forest canopy, and “you can easily see 40 to 50% of forests being destroyed in 40 years,” says Brodribb.
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In laboratory scenarios trees grew faster and larger when there was more CO2 in the atmosphere. That’s led some researchers to wonder if climate change could be good for forests. Faster growth might mean that species can more easily adapt and more quickly spread their young into new areas. More robust trees might help forests survive the other challenges that climate change will bring: more drought, fires, pests and pathogens. Outside the lab, though, it’s more complicated. For trees, carbon emissions have one big upside, but a lot of downsides.
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For one thing, high levels of CO2 might spur more leaf growth, but “if you make 20% more leaves, each plant will actually need more water in a high CO2 world,” Danielle Way, an associate professor of biology at the University of Western Ontario who was not associated with the study, wrote in an email. In a world with more droughts, that’s a problem for trees. Water molecules are sticky. They cling to one another and pull each other up the inside of a tree’s trunk through the xylem. As the water evaporates out of the leaves, it draws on the water behind it. Most of the time this process, known as transpiration, provides leaves with the constant supply of water they need for photosynthesis, but it also puts water and xylem under extreme tension.
When there is not enough water, the tension can collapse the xylem, a process known as hydraulic failure. Higher temperatures make this failure even more likely because trees “kind of sweat,” says Brodribb. When there is little water, cells called stomata usually close up to keep a tree’s water from evaporating, but under higher temperatures trees keep the stomata open. Water escapes and the tree draws even more moisture out of the soil. This is what the authors of the Science study call a “universal vulnerability.” Some species may be better adapted to drought and less susceptible to hydraulic failure, but with a perfect storm of warming temperatures and drier conditions, every tree in every forest in the world could be vulnerable.
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https://therevelator.org/forests-drought-climate/
