trees protect themselves with deeper roots, for
example, or by storing more water—but those
investments come at the expense of growing taller
to compete for light and space with other trees.
The upshot, scientists figured out in just the
past decade, is that many trees in most land-
scapes, from the hot, rainy Amazon to cold,
dry Alberta, are operating at the limits of their
hydraulic systems, even under normal condi-
tions, with little safety margin. That means a hot
drought can push them over the threshold. The
2002 drought in the Southwest did exactly that:
Tree-ring records would later show it was the
driest and worst year for growth in a millennium.
No other year even came close.
All this awakened Allen to what he now sees
as a grave global threat. “Seeing the transforma-
tion of this landscape that I’d studied my whole
adult life ... climate change wasn’t theoretical
anymore,” he told me. He started tracking the
mass mortality events elsewhere. Over the next
two decades, heat and drought would kill bil-
lions of trees directly and indirectly—in Spain,
in South Korea, throughout Australia. In cen-
tral Siberia, Russia lost two million acres of firs.
In Texas in 2011, drought killed more than 300
million trees—one out of every 16 in the state.
Increasing warmth helped deadly forest pests
spread, weakening trees and letting beetles and
moths live through the winters or reproduce more
often. Such invasions wiped out trees in Hondu-
ras, Turkey, and Algeria. In central Europe they
arrived as a shocking new plague.
On a chilly day last fall, I struggled up 227 steps
inside a former Cold War surveillance station onconflagration, allowing the woods to be reborn.
Forests, Turner realized, were resilient. It
would take time to accept how that could change.
AN EARLY WARNING CAME IN 200 2, during the
Southwest’s worst drought in five decades.
Weeks before meeting Turner, I scrambled up
a dusty embankment near New Mexico’s Ban-
delier National Monument. Beside me, Craig
Allen and Nate McDowell, an earth scientist
at the Pacific Northwest National Laboratory,
examined a picture Allen had taken in 2002.
It showed dense throngs of piñon pines, their
needles tinged orange because they were dying.
Allen swept an arm toward a nearby mesa.
He’d studied forests in this scratch of arid wood-
land near the Jemez Mountains since the 1980s.
Now the adult pines from his picture were gone.
What remained was cracked earth, hardy juni-
pers, and an occasional seedling.
A drought in the 1950s had brought even less
rain, and yet between 2002 and 2004 the impact
on trees was worse: In some areas, more than 90
percent perished, many falling victim to bark
beetles, natural predators that spread as never
before. All told, some 350 million piñons, New
Mexico’s state tree, died across the Southwest.
Unprecedented fires eviscerated hundreds of
thousands of acres of ponderosa pines.
Allen was taken aback by the severity. But
bit by bit, he, McDowell, and their colleagues
came to understand: This drought was hotter.
The slight increase in temperature attributable
to greenhouse gas emissions was already enough
to set the death of New Mexico’s trees in motion.
And what’s become ever more clear to Allen,
through his own work and that of many others,
is that trees the world over are vulnerable to
the added heat. The warmer atmosphere sucks
more moisture from plants and soil. To cut their
losses during droughts, trees close pores in their
leaves, called stomata, or shed leaves entirely.
But that limits the CO 2 they take in, leaving them
both hungry and parched all at once. When it’s
especially hot, they even leak some of the water
they’re desperate to retain.
When soil gets dry enough, trees can no lon-
ger maintain pressure in the internal conduits
that carry water up to their leaves. Air bubbles
interrupt the flow, causing fatal embolisms. Some
KEITH LADZINSKIFrom 1650 on, this
ponder osa pine survived
15 fires—but in the 20th
century most fires were
suppressed. Fuel builtup in the forest; a long,
hot drought settled
in. A monster blaze in
2011 ravaged 45 square
miles in its first night.
The result? “An extin-
guished ecosystem that
will never be seen again
here,” says scientist
Craig Allen (right).NEW MEXICO
JEMEZ MOUNTAINSA TREE’S RINGS
REVEAL A LONG HISTORY
OF SURVIVING FIRE56 NATIONAL GEOGRAPHIC