Heat sends Southwest climate back in time
Effect of natural drought cycle and climate change is restoration of the grasslands of centuries ago.
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.Skip to next paragraph
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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.