Arizona without saguaros? Drought, plant invaders pose threats

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Wednesday, Dec. 1, 2010
By Grant Martin
Cronkite News

TUCSON – Saguaro National Park boasts breathtaking views, its undulating landscape studded with thousands of its namesake cactuses. It’s a sight as spectacular as it is uniquely Arizonan, as no other state can claim nearly as much habitat for the towering saguaro.

In turn, Arizona has embraced the saguaro as an unofficial state symbol – there are five on its license plates, and a saguaro features prominently on the state’s quarter design – and state law imposes fines of up to $250,000 on anyone convicted of destroying one.

Such protections, however, could prove futile in the face of sweeping ecological changes brought on by rising temperatures.

University of Arizona researchers Jeremy Weiss and Jonathan Overpeck assert that projected droughts and the decreased frequency of freezing temperatures might make the desert uninhabitable for some of its most iconic vegetation.

“You have to have a fair bit of summer precipitation, like the monsoons in the Sonoran Desert,” Weiss said in an interview. “Species like the saguaro and palo verde can’t survive in other deserts like the Mohave in southeastern California because it’s just too dry over there.”

Freezing temperatures, meanwhile, are a paradoxical boon to native vegetation. Saguaros have thrived here for centuries largely because of an ideal thermal balance: warm and moist enough for most of the year to facilitate growth, but just cold enough in the winter to inhibit the intrusion of competitive, non-native species.

“We’re seeing fall freezes come later, we’re seeing the last spring freeze come earlier, and we’re seeing a fewer number of freezes during the winter,” Weiss said. “Now the question is, how does this less limiting environment affect where we find certain types of vegetation in the state?”

Julio Betancourt, a researcher for the U.S. Geological Survey in Tucson, believes the answer is already making its presence felt in the expanses between vegetation in the Sonoran Desert.

“These bare spaces are critical,” Betancourt said. “I can go out in the desert and walk up to a saguaro with a can of gasoline, pour it over it, set it ablaze, and the fire’s not going to spread, simply because there’s so much bare ground and so little continuity in fuel.”

Without freezing temperatures to check their advance, however, invasive species of vegetation – most notably an African weed called buffelgrass – are filling these bare spaces and creating unprecedented fire risks in the Sonoran Desert.

“Now what’s happening,” Betancourt said, “is we have these non-native species, such as buffelgrass, spreading and creating heavy fuel for fires in areas that have never before burned. And because the native shrubs and cactuses are so poorly adapted to fires, they don’t really recover from fires but are instead displaced by the invasive grasses.”

Buffelgrass and other invasive species also threaten native vegetation by absorbing crucial moisture and nutrients from the ground as they spread, effectively suffocating saguaros and palo verdes.

Asked about how the landscape might be different 100 years from now, Betancourt pointed to Saguaro National Park.

“In a hundred years, I wouldn’t be surprised if it’s called Buffelgrass National Park,” he said.

Buffelgrass was first introduced to America in the 1930s on ranches in the Southwest as an inexpensive, self-sustaining food for cattle. It was deliberately introduced to the Sonoran Desert shortly afterwards as a means of combating erosion. Buffelgrass and red brome, another non-native grass, were the primary catalysts of the 2005 Cave Creek Complex fire, which charred nearly 250,000 acres of desert north of Phoenix.

At a ranger office in Saguaro National Park, a stalk of buffelgrass is taped to a piece of posterboard, its attributes and identifying features clearly labeled. It’s one of the first things new rangers at the park must study.

“We try to be as vigilant as we can with the resources we have,” said Dana Backer, a resource ecologist at the park. “It’s an enormous concern, and it’s something we take very seriously.”

During summer months, when buffelgrass spreads most quickly, teams of rangers and volunteers trudge up the park’s hillsides each morning wearing masks and gloves and carrying herbicide containers on their backs. Once a stand of buffelgrass is identified, it’s sprayed with chemicals and dyed blue to track progress.

“It’s awfully uncomfortable, especially once the sun comes out, but it’s work that needs to be done,” said Backer, who estimated that 2,000 acres of buffelgrass had already invaded the park.

“But frankly, it’s moving faster than we can keep up with it,” Backer said.