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It was love at first sight of stark, towering, reddish-brown cliffs that led David Bertelsen to start scaling the Finger Rock Trail in the Catalina Mountains' front range.

It was sheer curiosity that led Bertelsen to start recording every plant he saw up the five-mile trail to the top of Mount Kimball and to make the journey as often as once a week for more than 20 years.

But it is the reality of climate change that draws scientists to Bertelsen's data. A retired probation officer, he has no science degree. But several scientists say his discoveries on the Finger Rock Trail point to signs of ecological change due to global warming, drought, or both.

As the 5-11, 205-pound Bertelsen walks up the mountain on a recent sunny spring morning, he points to a dead saguaro, shorn of its green skin, left with a hulking shell. He comes across stalks of tall yellow buffelgrass, a scourge to the native desert, climbing a hillside.

As he heads up mile two, he points to a withering, bare cottonwood tree. Two cottonwoods out of eight in Finger Rock Canyon have died since the Southwest's drought worsened in 2002, he says.

He's made 111,012 separate observations of floral blooms over 20 years. Since 1984, Bertelsen has recorded the name of every plant he has seen on a 30-foot-wide swath on either side of the trail. He has made the hike nearly 1,170 times.

The changes he has seen make Mount Kimball a symbol and a microcosm of how global warming and drought are affecting the Sonoran Desert and to some extent the entire West, scientists say.

● On the last mile, Bertelsen has seen about 60 plant species, about one-tenth of the trail's total, "move up the mountain" by 400 to 1,000 feet, he said.

● Since 2002, he's seen 71 dead mature saguaros — more than in the previous 18 years.

● More than 50 Arizona white oaks, 42 mature ponderosa pines and 15 alligator junipers died in that period, he says — a scene similar to die-offs attributed to drought throughout the West.

● He's noticed major population declines in bird, mammal, reptile and amphibian species.

● In 1990, he started seeing 19 non-native grass species including buffelgrass invade the trail in patches; some scientists say global warming has made it easier for buffelgrass to thrive here. Since 2002, he has watched buffelgrass shoot four feet high and move uphill from 3,300 feet to 4,500 to 5,000 feet up.

● Perhaps most significantly, in recent years Bertelsen has seen more than a dozen wildflower varieties bloom for the first time on the trail's last mile. High-elevation blooming — and blooms earlier in the year, both here and elsewhere — are believed by many scientists to be a symptom of human-caused global warming.

Bertelsen grew up near the wilds, on a northwest Illinois farm. His mother told him that "flower" was the first word he ever spoke. At age 10, he fell in love with the Sonoran Desert while reading his grandmother's Arizona Highways magazines, and he knew then that he would someday move here. He did, in 1979.

On his first trip to the top of Mount Kimball in 1981, he was captivated and intrigued by the dramatic changes in plant and animal life as he went uphill. In January 1984, nearly 30 climbs later, he decided to start recording what he saw.

He would paste or tape flowers and leaves he gathered into a loose-leaf notebook, and write what he saw on a checklist that mushroomed in size.

"It is difficult to explain, but I really feel I belong in this canyon, that I am truly a part of it," he says.

In the early 1990s, he saw the first real sign of change, red scarlet morning glories blooming in July and August on the trail's last mile. They had previously petered out at about 6,000 feet. He also saw at high elevations the yellow, daisy-like New Mexico ground sel that formerly grew farther down the hill.

It never occurred to him to think about climate change until he showed his research to Theresa Crimmins, a scientist in UA's Arid Lands Department, in 2006. The higher-elevation blooms are "statistically significant, for sure," Crimmins says.

She points to the fact that average summertime temperatures rose about one degree Fahrenheit in 13 weather stations in Southern Arizona between the periods of 1984-1993 and 1994-2003.

While these changes seem benign, they have several potential ill effects, Crimmins says:

● If some species move uphill and others don't, that can affect the stability of plant communities and increase the risk of high-elevation wildfires.

● If plants bloom earlier, it could mean hummingbirds and other migratory birds will arrive too late to drink the nectar, hurting their survival chances.

● Earlier-blooming plants will spread seeds earlier, giving foragers more time to eat them before rains arrive to nourish the seedlings into new plants.

● Plants that used to live at high elevations may someday cease to exist there because they can't coexist with the heat.

"If we see more species move up the slope, does that mean things are more vulnerable to invasion, or to fire?" Crimmins asks. "There's a lot of things we don't know yet."

Bertelsen's speed and robustness are diminishing now that he's just turned 65.

For him, a round-trip hike up the canyon has jumped from four to 15 hours. He now starts hiking at midnight. He lost more than a year of research in 2004-05, after breaking a leg and an arm in accidents and undergoing triple-bypass heart surgery.

But he has no intention of stopping. "I will continue to collect data as long as I'm able," he promises. dean knuth / Arizona Daily Star