Tree-ring experts see worse drought in future without action

Tree-ring experts see worse drought in future without action

4 November 2009

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Droughts are common throughout the history of the Southwest, but not broad-based temperature increases such as the region has been experiencing the past few decades, leading climate scientists from the University of Arizona’s Laboratory of Tree-Ring Research said last week.

Arizona’s average temperature has increased 2.5 degrees since 1976, according to state climate division records. The average annual precipitation has dropped about two inches in Arizona from 1979 to 2004, said Malcolm Hughes, a U of A professor emeritus and previous lab director.

Hughes and Lab Director Tom Swetnam said these changes are leading to earlier spring runoff, widespread death of tree stands and much larger Southwestern wildfires. They spoke last Thursday night in Prescott.

In response to an audience question about the validity of a recent “Tea Party” presentation on climate change fromYavapai College business instructor Terry Lovell that disputed the human element of climate change, Hughes urged the public to look at the qualifications of speakers and scientific evidence rather than “big sweeping statements that, when analyzed, are often based on things other than science.”

Swetnam added, “What we’re seeing now is truly extraordinary on these (multi-century) time scales.” Scientists know greenhouse gasses push up temperatures, and the gasses are producing temperature increases that scientists would predict when calculating the gasses that human activity is producing, he explained.

To understand precipitation and wildfire trends in the Southwest dating back centuries before records existed, the lab’s scientists use modern dendrochronology techniques developed by lab founder Andrew Ellicott Douglass about a century ago. He realized the width between tree rings on harvested ponderosa pine logs in the Flagstaff and Prescott areas correlated to annual precipitation. Fire scars also record years of wildfires.

The trees’ growth especially responds to winter precipitation, since the snow melts into water during their spring growing season.

Tree rings show numerous severe droughts before the 20th century. One around 1150 A.D. lasted for decades.

However, the shift to warmer temperatures the past few decades has caused more evaporation and earlier springs that exacerbate drought conditions.

“This is not a projection,” Hughes said.

And for the next 50 years, “The pattern of drought is similar to what we’ve seen in some of the toughest years,” he added. Climate experts forecast drought will be widespread across the West.

“The projected future normal resembles severe droughts of the past,” Hughes said.

The scientists also tried to offer some hope for the future.

“There is still time to adapt and change,” said Swetnam, whose talk focused on the relationship between climate and wildfires.

Analysis of tree scar data from trees across the Southwest show fires between 1600 and 1890 were highly synchronized to drought, Swetnam said. Typical ponderosa pine forests experienced fire 1-2 times per decade, thinning out the smaller trees.

“They require frequent fire,” he said of ponderosa pine stands. Other types of forests to the north, such as lodgepole pines and spruce, depend more on huge stand-replacement fires every few centuries. Piñon and juniper forests are more complicated, with some experiencing natural crown fires and some not.

The ponderosa pine scars show a huge drop-off in fires in 1890.

“It lines up really closely with livestock grazing,” Swetnam said. Huge numbers of livestock were eating up the grass that carried low-intensity fires between trees in the Southwest, he said. For example, seven million sheep were grazing across New Mexico in 1900.

About the same time, people started suppressing wildfires, and the military subdued Indian tribes that had used prescribed burns, he noted. Forests became more and more unnaturally dense during the 1900s as wildfire suppression continued.

With the increasing temperatures and spring arriving 1-3 weeks earlier the past 15-20 years, the sizes of wildfires are increasing and the fire season is lasting 1-2 months longer, Swetnam said.

“So it’s not just (too much) fuels, it’s also climate variability and climate change,” he said.

Since 1998, wildfires have burned across 4.8 million acres of Arizona, and drought and bark beetles have killed many of the trees on another 3.5 million acres, he said. Arizona has only 25 million acres of forest. In many cases, 200-year-old to 400-year-old ecosystems have been destroyed, he said.

Swetnam called for adaptive management to save the remaining forestlands. The Governor’s Forest Health Oversight Council’s statewide strategy for restoring Arizona’s forests, which he helped write, offers a good vision, he said.

“We actually need to put fire back into these forests,” he said. Fire helps recycle nutrients, and in many steep or rugged areas, mechanical treatment is impossible, he noted.

Landscape-sized forest treatment plans are a necessity to catch up, and groups such as Firescape in Southeast Arizona and the Four Forests Initiative on the Mogollon Rim are working on such scales, he said. The federal Forest Landscape Restoration Act seeks to help pay for these large projects. Firewise projects also are making a difference on private lands.

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