TUCSON -- The saguaro cactus is proving to be an upwardly mobile species.

One saguaro has been found growing under an oak tree at about 4,500 feet in Tucson's Catalina Mountains. It's flourishing in woodland terrain that is well above the species' more typical range at elevations from sea level to 3,500 feet.

While a few saguaros have been found in higher ground, scientists are now pondering: Are warming temperatures of recent decades leading to a general upward migration of saguaros and other desert plants?

"It's a good and important and difficult question," said Jeremy Weiss, a senior research specialist in the department of geosciences at the University of Arizona. "Saguaros are sensitive to freezing temperatures. If we're seeing changes as to where freezing temperatures occur and how cold they are, we're opening up a potential for saguaros to establish in different areas" including sites at higher altitude.

Weiss and other scientists say some saguaros can eke out a living at elevations at or near 5,000 feet.

One area where saguaros reach such elevations is on Tanque Verde Ridge in the Rincon Mountains east of Tucson, said Bill Peachey, a Tucson-area geologist and longtime saguaro researcher.

"But things are going to be pretty tough for them during the cold periods at those sites," Peachey told The Arizona Daily Star (http://bit.ly/JmmEra ).

Other areas where saguaros seem to push the upper limits of their range are the Hualapai Mountains near Kingman and some canyons in the so-called Front Range of the Catalina Mountains just north of Tucson.

Saguaros surviving at the highest altitude make a go of it because their seeds happened to be deposited in favorable, sheltered locations -- usually on south-facing slopes that tend to be warmer than those facing north.

A 5-foot-tall saguaro growing at about 4,500 feet near Sycamore Reservoir in the Catalina Mountains is such an example. It's on a south-facing slope and is sheltered by an oak tree serving as a so-called "nurse plant."

"The canopy of the oak tree is providing a bit of shelter in the case of freezing temperatures," Weiss said. "It will be a bit warmer under that tree. It's like when you take your patio plants and put them under a ramada" for shelter on cold nights.

How saguaro seeds might make their way to elevations above the species' more typical range could be thanks to an animal. For the saguaro growing under the oak, Peachey guessed that an animal like "a rock or tree squirrel, a bird, or a nectar-feeding bat" might have dropped a seed from saguaro fruit.

The scientists said more research would be needed before reaching a conclusion on why the cacti were moving to higher ground.