New Mexico: Desertification takes over researcher’s forest research

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.

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 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. 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’s 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 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.

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