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.

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 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.

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