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Drying Up: The Bleak Future for Southwest Forests

USGS researcher Craig Allen stands on the edge of Mesa Alta, amid diverse forest and woodland in the uplands of northern New Mexico; note some recently dead ponderosa pine and Douglas-fir in the field of view. Forest drought stress is strongly correlated with tree mortality from poor growth, bark beetle outbreaks, and high-severity fire.

For hundreds of years, forests of piñon pine, ponderosa pine and fir trees have covered millions of acres of mesas, plateaus and mountains in the semi-arid southwestern United States. Like forests everywhere, they provide food and shelter for countless species and help anchor vital watersheds and soils of the region.

But their days may be numbered. Based on a new study co-authored by the USGS, projected climate change impacts suggest a grim picture for current forests in the U.S. Southwest.

Research led by A. Park Williams of the Department of Energy’s Los Alamos National Laboratory came to this conclusion by comparing the tree-ring record to climate data collected in the Southwest since the late 1800s. The researchers aligned some 13,000 tree-core samples with known temperature and moisture data, further blending in known historical events such as documented megadroughts that drove the ancient Pueblo Indians out of longtime settlements such as Mesa Verde, Colorado.

Worst Forest Drought Stress in 400 Years

What they found was this: since 2000, southwestern U.S. forests have experienced more drought stress than during any other period in more than 400 years. What’s interesting about this is that precipitation totals since 2000 haven’t been exceptionally low, but temperatures have been exceptionally high. These high temperatures have caused the atmosphere’s ability to evaporate water from soil and plants also to be exceptionally high.

Think of the atmosphere as a water-thirsty sponge: the warmer the air, the thirstier the sponge.  When the summers are too hot and dry, the trees lose much of the water that they otherwise would have used for growth. If this happens too often, trees become stressed and more vulnerable to disturbances like forest fires and bark beetle infestations.

Forest Growth Rates Predictable Using Climate Records            

When Williams and the other researchers, including USGS scientist and study co-author Craig D. Allen, compared southwestern tree-ring records to climate records, they found that southwestern forest growth rates can be predicted very effectively using just two climate variables: winter precipitation and summer-fall atmospheric evaporative demand.

The researchers were able to predict future forest growth rates using these climate and tree-growth relationships, combined with projections of future climate trends. The results: all climate models project warming to cause large increases in summer-fall atmospheric evaporative demand (thirstier sponge). No models project large increases in winter precipitation. If the climate models are correct, forest growth will decrease substantially in the coming decades primarily due to increasing drought stress from warmer growing season temperatures.

Tree Rings Link Past and Current Drought Stress

Drought and beetle-killed piñon pines in Walnut Canyon National Monument near Flagstaff, Arizona, amid a few surviving trees. Forest drought stress is strongly correlated with tree mortality from poor growth, bark beetle outbreaks, and high-severity fire.

The tree-ring records in the Southwest extend back in time for more than 1,000 years. This allowed the researchers to investigate how future forest drought-stress likely will compare to periods of major drought stress in the past. The tree-ring records indicate that two “megadrought” events, during the 1200s and again in the 1500s, occurred in the past 1,000 years. Past research has shown that both of these events probably coincided with widespread forest die-off in the Southwest. So, the researchers treat forest growth rates during those extreme megadroughts as a benchmark for drought conditions strong enough to cause widespread forest die-off. If warming occurs as rapidly as projected by climate models, forest drought-stress conditions are likely to exceed those historic megadrought conditions on a regular basis by the 2050s. In fact, the study forecasted that during the second half of this century, about 80 percent of years are projected to exceed those megadrought levels.

Williams says the current drought event, which began in 2000, demonstrates how close southwestern forests already may be to reaching drought-stress levels unprecedented in at least a millennium. The team concluded that forest drought stress during 4 of the past 13 years (about 30 percent), including 2011 and 2012, matched or exceeded megadrought-type levels. The only other 13-year periods when megadrought-type conditions were reached with such frequencies in the past 1,000 years were during the megadroughts themselves.

What’s Going to Happen to Southwestern Forests in the Future?

Sunset as seen through the smoke of a prescribed burn in the Jemez Mountains, New Mexico. The burn was conducted to restore fire as an ecosystem process and reduce hazardous tree densities and fuel loads due to more than 100 years of fire suppression. Foreground trees (Douglas-fir and aspen) were killed during the Cerro Grande fire in 2000. Forest drought stress is strongly correlated with tree mortality from poor growth, bark beetle outbreaks, and high-severity fire.

As trees become more stressed from increasingly hot, dry summers, eventually they will not be able to continue to grow in their current locations, and fires and beetle infestations will take an increasing toll. We can expect to see increased numbers of trees dying, with many not being replaced. Eventually, if warming trends continue as projected by state-of-the-art climate models, the current forests will give way to ecosystems more tolerant of prolonged, severe drought. This would mean substantial changes in forest species composition to more drought-tolerant trees, or even forest-replacing shrublands and grasslands.

The article, “Temperature as a potent driver of regional forest drought stress and tree mortality,” appears in the October 2012 Nature Climate Change. Authors are A. Park Williams (LANL), Craig D. Allen (U.S. Geological Survey), Alison K. Macalady (University of Arizona), Daniel Griffin (University of Arizona), Connie A. Woodhouse (University of Arizona), David M. Meko (University of Arizona), Thomas W. Swetnam (University of Arizona), Sara A. Rauscher (LANL), Richard Seager (Columbia University), Henri D. Grissino-Mayer (University of Tennessee), Jeffrey S. Dean (University of Arizona), Edward R. Cook (Columbia University), Chandana Gangodagamage (LANL), Michael Cai (LANL), Nate G. McDowell (LANL).

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