New analysis suggests some trees make drought conditions worse.

The loss of trees and vegetation can have a variety of negative effects on ecological health. Often, trees and vegetation help mitigate the damage caused by extreme weather. But new research suggests the effects of vegetation on weather conditions depends on the physiology of the involved vegetation.

According to a new study -- published this week in the journal PNAS -- some tree species use precious soil water to cool themselves, worsening drought conditions.

"We show that the actual physiology of the plants matters," William Anderegg, biologist at the University of Utah, said in a news release. "How trees take up, transport and evaporate water can influence societally important extreme events, like severe droughts, that can affect people and cities."

Anderegg's previous studies have mostly focused on how the makeup of a forest and the physiology of its dominant species handle wet and dry conditions. The latest study focuses, instead, on how different tree traits impact the drought itself.

"We've known for a long time that plants can affect the atmosphere and can affect weather," Anderegg said.

Weather is largely dictated by the movement of water and heat in the lower atmosphere, and trees and forests, which pull water from the soil and exhale it into the air, can have a tremendous effect on these patterns.

The transpiration of water into the air from a dense rain forest can jump-start precipitation. When large portions of the forest are clear cut, drought conditions can proliferate downwind. The connection between forests and trees and climate and weather explains why forest health on the West Coast can influence forest health on the East Coast, and vice versa.

For the new study, Anderegg and his colleagues studied the interplay between forest characteristics and weather conditions at 40 sites around the world. Instruments recorded the movement of heat, water and carbon at each site. Scientists also studied the diversity of tree species surrounding the instruments at each site.

Scientists analyzed the data to isolate the tree traits associated with the intensification of drought conditions. Researchers found trees with the greatest rates of leaf gas exchange and water transport tended to make droughts worse.

Trees that can move gas and water more quickly are adept at keeping themselves cool, but their talent comes at the expense of soil water reserves.

"You end up getting to these conditions that are hotter and drier much faster with those plants than with other plants," Anderegg said.

Places where droughts are common tend to host an abundance of drought-tolerant species, but in some places, trees with water-intensive traits -- like oak trees in the Mediterranean -- can exacerbate drought conditions and make life more difficult for less drought-resistant species.

"Failing to account for this key physiology of plants would give us less accurate predictions for what climate change is going to mean for drought in a lot of regions," Anderegg said. "Just because we're having a good water year in the U.S. and in Utah this year doesn't get us off the hook. We need to remember that we're going to see a lot more droughts in the future."

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