Scorched earth returns to once-favoured form

For 15 years, Craig Allen, a scientist with the United States Geological Survey, has monitored a 2.7-acre plot in northern New Mexico.

During that time, he has witnessed smaller tree species succeeding larger ones. He has seen dry years, bark-beetle infestations, large-scale tree dieback, and finally, a shift towards grassland.

To Allen, these changes tell a tale of combined human impact — 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.

Allen also suspects tree dieback in the area 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 is not all bad. Fire suppression and grazing pressure over the last century 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 quicker than comparatively long-lived piñon-juniper forest.

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.

Indiscriminate loss

The drying may have already begun. As evidence, Allen points to recent forest diebacks in disparate regions around the world: in Mediterranean Europe, the Sahel of North Africa and in Australia.

“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 is when you say, ‘It looks more like a pattern.'''

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 piñon, whose berries and nuts are carried far and wide in bird bellies, had moved in.

But this was not the simple story of species succession. A century earlier, large numbers of livestock had begun grazing in the Southwest.

The grazing favoured woody plants — livestock ate grasses and left woody plants to grow.

Land managers also began suppressing fires. Fires continually burnt back encroaching juniper and piñon trees, leaving only those trees mature and strong enough to withstand a burning understorey.

In the absence of fires, young trees moved in. During the 20th century, areas that were once grassy became piñon-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 and left many 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 from New Mexico 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 about 17C higher, part of a warming trend over western North America.

The extra heat, Allen says, 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 piñon-juniper range died, this time trees succumbed throughout their entire range.

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 17C increase has helped wipe out nearly all mature Bandelier piñons.

“How big and fast these changes can be, becomes obvious when you look at these dramatic shifts,'' says Dave Breshears, a professor of natural resources at the University of Arizona, Tucson. “We are 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.

According to the US Geological Survey, agriculture in Arizona in 2000 used four times as much water as everything else combined.

Maybe Arizona is not the best place to grow water-intensive crops such as cotton, Seager says.

Warmer temperatures may have an inordinate impact on species that inhabit the area's mountains.

A salamander endemic to the Jemez Mountains could be pushed off the mountain altogether.

The pika, a rabbit-like creature, too, could disappear locally.
But the story is not all doom, Allen says. Juniper- and piñon-dotted mesas come to mind when we imagine the Southwest.

But in many areas, their dominance is an artefact of human interference. Now, prompted by 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 piñon — will be more resilient and better able to recover from future droughts, he says.

Scientists at Bandelier are trying to speed up 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 strewing the dead tree branches across the land can mitigate, and perhaps reverse, 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 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 “Dine'' — “the people''.) 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 had lived for perhaps 1,000 years.

The drought link is less clear in settlements such as Bandelier in north-central New Mexico, which was 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, 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,'' Gauthier says. “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.''