Imagine that you're lost in a desert, or some other inhospitable environment. You've got two choices. One is to stay in place and conserve supplies and water, in order to make them last. The other is to push on tenaciously and hope that you find a way out of your predicament.
As it turns out, trees are like that too, when it comes to coping with the hotter, drier environment created by climate change. In a new article in the journal Global Change Biology, University of Washington researchers studied two common tree species in southwestern Colorado, and found that each had developed a different survival strategy.
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"We really wanted to identify the entire suite of strategies that a plant can use to grow in drier environments, as well as which of these strategies each tree would employ," UW biology professor Janneke Hille Ris Lambers, who co-authored the article with graduate student Leander Anderegg, said in a press release.
In the summer of 2014, the two researchers studied the slopes of the La Plata Mountains in Colorado's San Juan National Forest, where a drought-ravaged ecosystem that's become 1 degree Fahrenheit hotter over the past 30 years is putting stress on trees.
One of the species under pressure is the ponderosa pine, whose habitat at the lower levels overlaps with that of the trembling aspens, a tree that predominates further up the mountainside.
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Anderegg and Hille Ris Lambers led a team of undergraduates who collected leaf, branch and tree ring samples of both trees. They studied that material to figure out how the trees adapted to drought conditions, and measured qualities such as the trees' growth rate and the water tension inside their woody tissue.
The researchers found that the ponderosa pine used a strategy of "drought avoidance" by conserving water, especially by shutting the tiny openings on its leaves to prevent water loss and slowing growth.
The trembling aspen, in contrast, simply tried to keep growing - at least for a while - during drought, with no change to water conservation strategies. "On the dry end of their range, the trembling aspens are relatively short with these really fat leaves," Anderegg explained. "Internally, they also grow really strong xylem vessels, which move water inside of the tree. As a consequence, they are much denser and they also grow slower."
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Both of the strategies have downsides. The trembling aspen's push to grow might make it more vulnerable to severe or prolonged drought, especially at its dry lower range. The ponderosa pine's strategy is likely to gradually reduce its range.
According to the U.S. Environmental Protection Agency, climate change is likely to shift the geographic range of many tree species, with species unable to tolerate heat moving northward or to higher elevations.