A new study of desert plants living on the slopes of Arizona's Santa Catalina Mountains has uncovered a startling fact: many trees and shrubs have moved a long way up-slope over the last 50 years. The shift could be caused by a warming trend in recent decades in the Arizona desert, or perhaps the 20-year drought.

The study was done by surveying the varied plants and plant communities lined up along the Catalina Highway, which climbs from low desert to mountain top. The data was compared to that of a 1963 survey the famous botanist Robert H. Whittaker and his colleague William Niering did along the same stretch of road.

"These old datasets are priceless things," said University of Arizona researcher Rick Brusca, who coauthored the study with Wendy Moore, and which appeared in a recent issue of the journal Ecology and Evolution. Unlike other surveys that follow transects across open country and can be difficult to precisely locate and replicate, this one along a highway makes it particularly easy. "We're actually quite surprised that no one had done it before."

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Brusca and Moore looked particularly at the 27 most common plant species found along the Catalina Highway. They found that all but a quarter of them had moved their growing elevations up-slope or at least narrowed their range by ceasing to grow at lower elevations. Eight species now begin to appear 800 feet higher than that did in 1963, 16 species live within narrower bands of elevation and some have moved higher on the mountain.

Among the most notable changes are to the alligator juniper trees which were found in 1963 to begin appearing at 3,500 feet. Now any found below 5,000 feet are dead.

The main message the researchers are getting from the plants is that their communities are reshuffling as individual species shift their ranges independently.

The study meshes intriguingly with other studies that show changes in ranges, or flowering times in the Catalinas and other parts of the Southwest United States. One such study was based on 30 years of weekly data collected along a five-mile trail that climbs more than 4,100 vertical feet in the Catalinas.

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“I've completed over 1,300 hikes to Mt. Kimball and have amassed a data set of over 215,000 records, about 115,000 of which are flowering data,” explained researcher David Bertelsen, also of the University of Arizona, but not affiliated with Brusca and Moore. “Most of the species I have been studying are not moving up in elevation (which) might also mean that species in this area are very well adapted to the considerable variability in climate over the past 1,000 years and more.”

Bertelsen expressed skepticism about the Catalina Highway plant changes and said they are very likely just responses to the long drought, which started in the 1990s.

“Of course if climate extremes become outside the 'normal' range of variability, significant ecosystem collapse could occur,” said Bertelsen. “We certainly don't know where such thresholds lie.”