Heat sends Southwest climate back in time
Effect of natural drought cycle and climate change is restoration of the grasslands of centuries ago.
Moises Velasquez Manoff
Bandelier National Monument, N.M.
For 15 years, Craig Allen, a scientist with the US Geological Survey, has monitored a 2.7-acre plot here in northern New Mexico. During that time, he’s witnessed smaller tree species succeeding larger ones. He’s seen dry years, bark-beetle infestations, large-scale tree dieback, and finally, a shift toward grassland. To Dr. Allen, these changes tell a tale of combined human impacts – overgrazing, fire suppression, and climate change. And they underscore how human activity can amplify the effects of natural cycles to alter a landscape dramatically.
The American Southwest may be drying, one of the predicted consequences of human-induced global climate change. Less water in an already semiarid region will affect how, and for what, people use water. Allen also suspects that tree dieback here may be part of a worldwide phenomenon. As temperature extremes have inched higher in semiarid regions globally, forests have succumbed to heat stress.
But, at least in the Southwest, the news isn’t all bad. Over the past century, fire suppression and grazing pressure have let trees reach a greater density than in times past. But now drought and higher temperatures have, in a sense, prompted the system to reset itself. Savanna will again dominate the landscape. And, given the likelihood of more intense droughts in the future, this means more resilience. Grassland can recover from disturbances more quickly than comparatively long-lived pinyon-juniper forest.
A growing desert
A 2007 paper in the journal Science forecast that the drying of the Southwest would begin sooner rather than later. Changing atmospheric circulation patterns and warmer air capable of sucking up more moisture will push the Sonoran desert of northern Mexico further north. This process will probably also occur at similar latitudes worldwide.
“We’re starting to see dieback that may be above [routine] levels, particularly in dry areas,” he says. “Increased water stress is pushing some species over the edge.”
Richard Seager, senior research scientist at Columbia University’s Lamont-Doherty Earth Observatory in New York City, and lead author on the Science paper, says, “It could be an anomaly…. But when they start lining up, that’s when you say, ‘It looks more like a pattern.’ ”
A changing landscape
When Allen first began monitoring his plot, ponderosa pine trunks littered the ground. Located 6,600 feet high in the foothills of the Jemez Mountains, the area was just below where the tall, straight pine species now grows. Allen reasons that the species had probably descended from higher elevations during the Little Ice Age, a relatively cool, wet period that ended in the mid-1800s. As temperatures rose again, optimal ponderosa conditions moved back up the mountain. The trees hung on until the drought of the 1950s.
By the 1980s, juniper and pinyon, whose berries and nuts are carried far and wide in bird bellies, had moved in. But this was not quite the simple story of species succession it seemed. A century earlier, livestock had begun grazing in large numbers in the Southwest. The grazing pressure favored woody plants over grassy ones – livestock ate grasses and left woody plants to grow.
Land managers also began suppressing fires. Until around 1910, wildfires had burned through Southwestern ecosystems about every five to 15 years locally, according to tree-ring records – apparently for millennia. Fires continually burned back encroaching juniper and pinyon trees, leaving only those trees mature and strong enough to withstand a burning understory. Absent the fires, young trees moved in unchecked. During the 20th century, areas that were once predominantly grassy – or in the case of Allen’s plot, ponderosa with a grassy understory – became scrubby, erosion-prone pinyon-juniper forest.
Then, beginning a decade ago, a series of hot and dry years ended a 17-year run of relatively moist conditions. The stress killed some trees outright and left many more severely weakened. The stage was set for the final coup: Droves of pine beetles attacked the debilitated trees. The swarms were the southernmost flank of an infestation by several species that occurred – and in some places is ongoing – from New Mexico up the Rockies to Alaska.
Here, parsing the ultimate causes gets complicated. Bark beetles are native to the area, and droughts occur periodically in the Southwest, where weather is greatly influenced by sea-surface temperatures in the Pacific Ocean. Indeed, in 1956, the tail end of the last great regional drought, conditions were actually drier in some parts than in 2002, the second-driest year on record.
But there was one crucial difference during the most recent drought: The mean temperature was 1.5 degrees F. higher, part of a warming trend over western North America. The extra heat, says Allen, overstressed the trees and pushed large portions of the system over a threshold.
Whereas in the 1950s, only those trees at the lower, drier end of the pinyon-juniper range died, this time trees succumbed throughout their entire range. Around Bandelier, 95 percent of mature pinyons have died. The dramatic shift underscores a facet of climate change that scientists are often at pains to communicate: Ecosystems are shaped by extremes of hot and cold, wet and dry. If those extremes are pushed even a little further away from the mean, the effect can be drastic, especially when relatively slow-growing trees are involved.
Case in point: A mere 1.5 degrees F. increase has helped to wipe out nearly all mature Bandelier pinyons.
“How big and how fast these changes can be becomes much more obvious when you look at these dramatic shifts,” says Dave Breshears, a professor of natural resources at the University of Arizona, Tucson. “We’re really going to be living in a different landscape in the future.”
The new climate regime will probably affect how people use water in the area, says Columbia’s Dr. Seager. Thanks to rivers like the Colorado, the Southwest has a healthy water supply, especially compared with northern Mexico, he says. But it’s hardly unlimited. Scarcity will probably force people to rethink how water is divvied up.
According to the United States Geological Survey, for example, in Arizona in 2000, agriculture used four times as much water as everything else combined. But farming contributed less than 1 percent of the state’s GDP. Maybe Arizona is not the best place to grow water-intensive crops like cotton, he says. (The state ranks ninth in the US as a cotton producer.)
Warmer temperatures may have an inordinate impact on species that inhabit the area’s mountains – essentially cool, wet islands surrounded by hot dry deserts. A moist dirt-loving salamander endemic only to the Jemez Mountains, which are only 11,500 feet at their highest, could be pushed off the mountain altogether.
The pika, a high-elevation rabbitlike creature, could also disappear locally, although they’ll probably persist in the northern Rockies.
But the story is not all doom and gloom, says Allen. Juniper- and pinyon-dotted mesas come to mind when we imagine the Southwest. But in many areas, their dominance is an artifact of human interference. Now, prompted by more intense extremes, the ecosystem has come full circle, returning to something approximating its original state. The new landscape – grasslands with a much lower density of juniper and pinyon – will be more resilient, and better able to recover from future droughts, he says.
“For a while we went through this ‘woodification’ trend,” says Allen. “Now that excess is being squeezed back by dieback and by fire. In a structural sense, it’s actually somewhat restorative.”
Scientists at Bandelier are trying to speed along the restoration. Much of the area’s nutrient-rich topsoil, itself a remnant of cooler, wetter conditions that prevailed here during the last Ice Age, which ended 10,000 years ago, has eroded away. But scientists have found that simply strewing the now-dead tree branches across the land can mitigate, and perhaps reverse, this desertification.
The pine needles and woody matter slow down running water so that it seeps into the ground rather than flowing off.
“You’re sort of redistributing organic material,” says Brian Jacobs, a botanist with the National Park Service in Bandelier. They also shade the earth from the sun, lessening evaporation and helping grasses to establish themselves.
“We’re trying to hold on to the soils,” he says. “And we’re using the least invasive methods for doing that.”
When the Spanish arrived in what is now the US Southwest in the late 1500s, they found a settled, agricultural people throughout the river valleys of the area. They dubbed these native Americans “Pueblo” (“village”) Indians. But ruins of an earlier people – the Anasazi – dotted the mesas and hilltops of the region. To the arriving Spanish, the settlements already stood abandoned. What had happened?
Archaeologists are quick to correct several widespread misconceptions in this story. First, the Anasazi didn’t vanish at all. The Tewa-speaking Pueblo Indians are, in fact, their descendents. They didn’t disappear; they simply moved into the river valleys.
Second, don’t call them “Anasazi,” Navajo for “ancestor of our enemies.” They’re “ancestral Puebloans.” ("Navajo," for that matter, derives from a Tewa word meaning "fields adjoining an arroyo," a reference to the tribe's agricultural practices. The Navajo call themselves "Diné" – "the people.") [Editor's note: The original story misstated the origin of the word "Navajo."]
Why did the ancestral Puebloans move? One theory: climate change. A megadrought coincided with the abandonment of the Four Corners region of the US in the late 1200s, where they’d lived for perhaps 1,000 years. The drought link is less clear in settlements like Bandelier in north-central New Mexico, abandoned by 1550. But soil exhaustion from centuries of farming and deforestation may have also played a role, says Rory Gauthier, archaeologist at Bandelier National Monument in New Mexico.
As today’s inhabitants contemplate a changing climate in the Southwest, they could probably learn a few things from the successes and failures of the ancestral Puebloans, Dr. Gauthier says. Foremost among them: the art of conserving water: The Pueblo’s ancestors developed intricate irrigation systems and terraced hillsides to better conserve water. They designed their gardens rather like waffles: a series of raised ridges created sunken squares that trapped and held water for crop use.
“I’m hopeful,” says Gauthier. “If we take the Pueblo people as a model, they’ve survived for a long time. They’ve made social adjustments. And they’ve been able to meet that challenge.”