Sunday, July 07, 2013

Megadrought in U.S. Southwest: A Bad Omen for Forests Globally

by Caroline Fraser

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...

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 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 piƱon, ponderosa pine, and Douglas fir — survived that latter event, despite lifespans approaching 800 years; those species have since regrown.

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 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.

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.

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