Megadrought
in U.S. Southwest:
A
Bad Omen for Forests Globally
Scientists
studying a prolonged and severe drought in the southwestern U.S. say
that extensive damage done to trees in that region portends what lies
in store as other forests worldwide face rising temperatures,
diminished rainfall, and devastating fires.
by
Caroline Fraser.
20
June, 2013
As
brutal fires torch tinder-dry dense forests and neighboring homes in
the American West, researchers are examining the relationships
between drought, wildfire, and a warming climate, predicting mass
forest die-offs and prolonged megadrought for the Southwest. These
forces are accelerating, they say, and already transforming the
landscape. Unchecked, they may permanently destroy forests in the
southwestern U.S. and in some other regions around the world.
Across
the West, “megafires” have become the norm. With climbing
temperatures, after a century of fire suppression, the total area
burned has tripled since the 1970s, and the average annual number of
fires over 10,000 acres is seven times what it was then. Fighting and
suppressing fires costs more than $3 billion a year, not to mention
lives lost. So understanding what, if anything, can be done to reduce
intense forest fires has assumed an urgent priority.
Currently
suffering the worst drought in the U.S., New Mexico has emerged as a
“natural experiment” in megadrought, a laboratory for
understanding drought’s deep history in the region — and what
might lay in store in an era of rapid, human-caused warming.
With
a highly variable climate, the Southwest boasts perhaps the
best-studied megadrought history in the world. It’s the home of
dendrology, the science of studying tree-rings, first developed at
the University of Arizona. The pronounced seasonality of hot summers
followed by cold winters produces well-defined rings, while
archaeological fascination with Southwestern cultures — Chaco
Canyon, Mesa Verde, and other sites where ancient peoples flourished
and disappeared — has supported the collection and study of
centuries of tree-ring data.
Temperate-zone trees lay down wider
rings in wet years, which narrow or vanish during drought. What’s
more, rings can be precisely dated, with sets matched against each
other, revealing burn scars and patterns of climate, precipitation,
drought stress, and tree mortality.
Park
Williams, a young bioclimatologist and postdoctoral fellow at Los
Alamos National Laboratory, has teamed up with other specialists at
the U.S. Geological Survey (USGS) and the University of Arizona to
wring new insight from the data set spanning the years 1000 to 2007.
Driving recently into the Jemez Mountains near his office, we pass
rust-red pines, dead or dying from drought. Later, kneeling next to a
freshly cut stump, he points to a ring near the bark. “That thick
ring right there is probably 1998,” he says, a wetter El Niño
year.
Armed
with 13,147 such site-specific cross-sectioned specimens, gathered
from more than 300 sites, Williams and his co-authors devised a new
“forest drought-stress index,” integrating tree-ring measurements
with climatalogical and historical records for a paper published
earlier this year in Nature Climate Change. Winter precipitation has
long been thought important to tree growth, but another key variable
leapt from this fresh examination of the data, related to a warmer,
dryer climate: the average vapor pressure deficit during summer and
fall, which is driven by temperature. As air grows warmer, its
capacity to hold water vapor increases exponentially, which speeds
evaporation and sucks more moisture out of trees’ leaves or
needles, as well as the soil itself.
If
the vapor pressure deficit sucks out enough moisture, it kills trees,
and there’s been a lot of that going on. Looking back in time
through the tree rings, Williams determined that the current
Southwest drought, beginning ‘The majority of forests in the
Southwest probably cannot survive in the temperatures that are
projected.’ in 2000, is the fifth most severe since AD 1000, set
against similarly devastating megadroughts that have occurred
regularly in the region. One struck during the latter 1200s (probably
driving people from the region) and another in 1572-1587, a drought
that stretched across the continent to Virginia and the Carolinas.
Few conifers abundant in the Southwest — including piñon,
ponderosa pine, and Douglas fir — survived that latter event,
despite lifespans approaching 800 years; those species have since
regrown.
The
forest drought stress index correlates strongly with these periods,
while 20th-century temperature records show a connection between
drought and tree mortality associated with huge wildfires and
bark-beetle outbreaks, such as the devastating ones of the past two
decades. Williams’ study is also supported by satellite fire data
from the past few decades, revealing an exponential relationship
between drought stress and areas killed by wildfire.
His
projections, based on climate forecasts, sparked grim headlines
throughout the region: If the climate warms as expected, forests in
the Southwest will be suffering regularly from drought stress by 2050
at levels exceeding previous megadroughts. After 2050, he calculates,
80 percent of years will exceed those levels. “The majority of
forests in the Southwest probably cannot survive in the temperatures
that are projected,” he says.
Making
matters worse in the near-term, forests hit by so-called
“stand-destroying” wildfires may not recover. During a recent
phone interview, Craig Allen, a co-author of the Nature paper and a
USGS research ecologist at the Jemez Mountain Field Station near Los
Alamos, explains that the catastrophically hot fires seen recently in
New Mexico, while a natural result of a century of fire suppression
and dense growth during wet periods, create conditions for permanent
forest loss through “type conversion.” Basically, high severity
fires that burn over a wide area subvert the ability of southwestern
conifers to reproduce, a process requiring nearby mother trees to
drop their seeds. Ponderosa pines, for example, can’t cast their
seed much more than 100 yards, virtually ensuring that large forest
gaps will be replaced by shrub and grasslands, with unfortunate
consequences for a range of forest services, particularly those
provided by delicate watersheds. “These anomalously big patches
where every tree is killed create a high risk that they won’t come
back as forests,” Allen says.
The
West’s drastic drought-related fires may foretell what could happen
to forests globally as the climate warms. Tree-mortality has been on
the rise A study projects ‘severe and widespread drought’ by the
2060s from the Americas to Australia. across the American West,
according to a 2009 Science study, and recent analysis from the
National Center for Atmospheric Research projects “severe and
widespread drought” by the 2060s for much of the Americas, as well
as Europe, southern Africa, southeast Asia, parts of the Middle East,
and Australia.
These
conditions are already taking a toll: Earlier this year, NASA
published a study on megadrought in the Amazon, where severe drought
in 2005 caused a dieback of old-growth canopy across an area the size
of California, covering 30 percent of the Amazon. Rainfall gradually
recovered, but to the researchers’ surprise, satellite images
revealed that the damage has been lasting. Caused by the same warming
of Atlantic sea surface temperatures that empowered Hurricanes
Katrina and Rita, that once-in-a-century drought was quickly followed
by another in 2010, affecting almost half the Amazon. The forest
canopy loss appears related to prolonged drought stress.
Allen
and a host of co-authors conducted a 2010 global overview of drought
and tree mortality, a sobering list of seasonal and multi-year
droughts on every forested continent since 1970, some lasting over a
decade. While acknowledging scientific uncertainties, the overview
reiterates concerns increasingly expressed by researchers: that the
planet could see widespread and abrupt forest collapse caused by
droughts, that rainforests could begin producing more carbon than
they sequester (which the Amazon experienced in 2005 and 2010), and
that drought could push the expansion of the dry subtropics into
critically important agricultural areas, with devastating
consequences for world food production. While excluding fire-driven
mortality, the 2010 study cites well-documented recent forest
die-offs in Spain, Greece, Russia, Australia, and, of course, North
America, all places that have seen recent severe wildfires.
“I
don’t want to be a prophet of doom,” Allen says, after a week
juggling fire updates and journalists’ queries. He grants that even
extreme fires do some good, correcting unsustainable growth after wet
decades allowed forests to “fluff-up.” Recent drought is, he
says, “squeezing that excess tree biomass out of the landscape.”
He
emphasizes that forest managers know how to “treat” overgrown
forests, through thinning and controlled burns, but that prescription
is not welcomed by all. For instance, University of Wyoming
researchers have criticized Forest Service thinning plans, such as
the Four Forest Restoration Initiative in Arizona, suggesting that
high-severity fires were One expert notes that even extreme fires do
some good, correcting unsustainable growth after wet decades. once
common and continue to act as natural part of the western ecosystem.
But
that’s a minority view as foresters around the world experiment
with novel methods of maintaining or restoring woodlands, searching
for drought-adapted species and genotypes and ways to control
evaporation. Seedlings planted on a degraded, fire-damaged
Mediterranean mountainside near the coast of Spain were kept alive
with water drained from fog-collectors, which consisted of flat mesh
panels mounted above the restoration area. “We don’t have to
treat every acre,” Allen points out.
He
and other foresters have called for radically “rethinking” forest
management during drought, suggesting that we might have to consider
tree triage — pinpointing pockets of forest more likely to survive
or even irrigating beloved landscapes, such as California’s ancient
sequoias, to save them.
The
devastation wrought by extreme wildfire is on vivid display these
days in New Mexico. During my drive in the Jemez Mountains with Park
Williams, with near-record temperatures in the nineties, he directs
me onto a forest road traversing Cochiti Mesa, a spine of land at the
heart of the 2011 Las Conchas wildfire, an event that has become
legendary among students and fighters of fire.
A shoot appears beneath a tree charred by the 2011 Las Conchas fire in New Mexico. Scientists are studying megadroughts in the southwestern U.S. in the hopes of understanding how climate change could effect forests worldwide and what steps can be taken to help woodlands survive in regions facing increasingly inhospitable conditions. (Photo by Sally King/NPS)
On
June 26, 2011, an aspen tree blown onto a power line sparked fire so
hot, travelling so fast, that it consumed 44,000 acres during its
first 13 hours, nearly an acre per second. By the time it was fully
contained in August, it had burned more than 156,000 acres. Las
Conchas was the largest fire in New Mexico history, a record that
stood for a mere year, when a bigger, if less destructive fire,
burned to the south.
Even
two years later, the view from the mesa is jaw-dropping, a forest
Golgotha. In a gap 40,000 acres wide, for miles in every direction,
every tree is dead. As the wind whips grit into our eyes, Williams
tells me that trees in some areas burned so hot that the trunks
vanished, leaving ghostly holes. Others flash-burned from heat alone,
their crisp, dead needles still intact. Photographs taken afterwards
show a moonscape of ash. Few living things have returned — shrubs,
clumps of grass, a few splashes of Indian paintbrush.
But
trees, no. Ponderosa pine may not come back, without help. The
intense heat sterilized soil and destroyed virtually all biomass
across wide areas, creating huge gaps that trees will not be able to
reseed. Colorado saw this after 2002, when the Hayman fire left a
gaping, 50,000-acre hole. Except for a fraction reseeded by the U.S.
Forest Service, trees have not grown back and are not expected to for
centuries.
Gazing
across the Rio Grande valley, we can see another manifestation of
megadrought: a ballooning pyrocumulonimbus cloud from another fire,
sparked in the Pecos Wilderness the night before. Later that
afternoon, the plume reaches 20,000 feet, looming over Santa Fe,
whose own watershed has been closed to the public since 1932 because
of the threat of fire polluting two reservoirs that supply forty
percent of its water. The watershed has been thinned. But Park
Williams, with foresight granted by a thousand years of tree-ring
records, can see its future.
“It’s
not if it burns,” he says. “It’s when.”
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