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Hotter, Drier Conditions Forcing Desert Plants To Migrate

By Katie Valentine  

"Hotter, Drier Conditions Forcing Desert Plants To Migrate"

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Desert plants living in Arizona’s Santa Catalina Mountains have moved surprisingly far upslope in the past 50 years, a trend that’s likely due to warmer, drier temperatures in the region, new research has found.

The study, led by researchers from the University of Arizona, surveyed the locations of desert trees and shrubs that grow along the Catalina Highway, which stretches from desert lowlands to the top of Mount Lemmon in the Santa Catalina Mountains. Researchers compared the present-day data to a 1963 survey of plant life along the road. The results were striking: of the 27 most common plant species found along the road, 15 had shifted their lower boundaries upslope, and eight of those shifted more than 800 feet upward from their 1963 lowermost boundaries. The researchers think that a warmer, drier climate is causing some plants at lower elevations to undergo water stress and die, thus moving the plant population’s range upslope.

But only four of the plants extended their upper regions in order to compensate for their reduced lower boundaries, while eight lowered their upper boundaries and 15 remained unchanged — a finding that suggests shifts in climate are not just moving some plants to higher elevations, but may also also be restricting their overall habitat size. Researchers said the findings could spell trouble for the future of some plant species, especially those that experienced the most dramatic shifts in elevation: the alligator juniper, for instance, began growing at 3,500 feet in elevation in 1963, and today is found no lower than 5,000 feet up the mountainside.

“If climate continues to warm, as the climate models predict, the subalpine mixed conifer forests on the tops of the mountains — and the animals dependent upon them — could be pushed right off the top and disappear, ” said Richard C. Brusca, a lead researcher on the study.

The study isn’t the first to attribute changes in vegetation patterns to climate change — a report released this month documenting climate change’s effects in California noted that forests in the Sierra Nevada have also been moving upslope over the past 60 years and are shifting from conifer-dominated ecosystems to broad-leafed deciduous forests, a trend that will likely impact the birds and mammals that depend on certain tree species for survival. The report called the retreat of conifers upslope a “clear biological signal that conditions are changing.” The report also noted that all plant species in the Deep Canyon Transect of Southern California’s Santa Rosa Mountains had moved upward compared to their 1977 ranges.

And it’s not just in California — a report from 2011 that analyzed more than 100 previous studies on shifting plant and animal regions found that as a whole, species around the world are migrating uphill at a rate of 36 feet per decade and away from the equator at a rate of 10 miles per decade.

“The more warming there’s been in an area, the more you would expect a species to move, and the more they have moved,” Chris D. Thomas, lead researcher on the study, told the Washington Post. “This more or less puts to bed the issue of whether these shifts are related to climate change. There isn’t any obvious alternative explanation for why species should be moving poleward in studies around the world.”

That report raised concerns that many species, especially those that already dwell in high elevations, won’t be able to move quickly enough to survive changing temperatures. Species that can move fast enough could disrupt the wildlife already found in higher elevations, which has the potential to lead to increased competition among species and possible extinction of local populations.

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