Michelle Nijhuis is a contributing editor of High Country News. Her work has also appeared in Smithsonian, National Geographic, The New York Times, and the anthologies Best American Science Writing and Best American Science and Nature Writing. She and her family live off the grid in Paonia, Colorado.

Excerpt:

By 2006, close to 150,000 acres of Colorado aspen were dead or damaged,
according to aerial surveys. By the following year, the grim phenomenon
had a name—sudden aspen decline, or SAD—and by 2008, the damaged
areas had exceeded half a million acres, with 17 percent of the state’s
aspen showing declines. In many places, patches of bare and dying treetops
are as noticeable as missing teeth, and some sickly areas stretch for miles.
Aspen declines are also underway in Wyoming, Utah, and elsewhere in the
Rockies. Surveys of two national forests in Arizona showed that from 2000
to 2007, lower-elevation areas lost 90 percent of their aspen.

Aspen grow in clones, or groups of genetically identical trunks. Some
clones are thousands of years old, although individual trees live 150 years
at most. One especially large stand in Utah, known as Pando, after the Latin
for “I spread,” was recently confirmed by geneticists to cover 108 acres. It is
variously said to be the world’s heaviest, largest, or oldest organism. Disturbances such as wildfires or disease usually prompt clones to send up a slew of fresh sprouts, but new growth is rare in SAD-affected stands.

….The most extensive SAD is in the hottest and driest areas—low-lying,
south-facing slopes. The pattern suggests that the region’s extreme drought
and high temperatures—both possible symptoms of global warming—have
weakened the trees, allowing more disease and insect attacks.
It seems that new stems aren’t growing back after trees die because
drought and heat have stressed the trees. During drought, aspen close off
microscopic openings in their leaves, a survival measure that slows water
loss but also slows the uptake of carbon dioxide, required for photosynthesis.
As a result, the trees can’t convert as much sunlight into sugar. Worrall
speculates that the trees absorb stored energy from their own roots, eventually
killing the roots and preventing the rise of new aspen sprouts. “They
basically starve to death,” he says.

The drought here has lasted nearly a decade, and climate scientists predict
that severe droughts will strike even more often in parts of the West as
greenhouse gas levels continue to rise and contribute to global warming. “If
we have more hot, dry periods as predicted, SAD will continue,” says Worrall.
Aspen at lower elevations will likely disappear, he says, and those at
higher elevations will be weaker and sparser.