Drought and rising temperatures weaken southwest forests

December 13, 2010

ALBANY, Calif.--Forests in the southwestern United States are changing and will face reduced growth if temperatures continue to rise and precipitation declines during this century, according to a study conducted by a team of scientists from the U.S. Forest Service; University of California, Santa Barbara; U.S. Geological Survey; and University of Arizona. Their findings were released today in the Proceedings of the National Academy of Sciences (PNAS) special issue on climate change.

Using tree-ring data and climate models, the team determined that rising temperatures and declining precipitation has led to an overall lower fitness of forests in the Southwest. This weakening of forest health has led to the trees' inability to survive wildfires and stave off bark beetle attacks. Fire and bark beetles caused high levels of mortality in 14-18 percent of forest areas in the Southwest, according to the scientists, who examined the tree rings of piñon pine, ponderosa pine and Douglas fir.

"These results have been observed previously on a case-by-case basis, but our demonstration of the pervasive effects of warming and drought should better enable water and land managers to prepare for climate adaptation in coming decades," says Connie Millar, a research climate ecologist with the U.S. Forest Service's Pacific Southwest Research Station, who co-authored the study.

Scientists analyzed annual tree-ring width data from 853 tree populations located throughout the continental United States. Of those, 235 samples represented trees located in Arizona and New Mexico. These samples were compared to each other in order to identify trends on how certain climatic conditions affect tree growth.

The projected continuing decline of these forests could mean significant ecosystem changes if the Southwestern forests continue to be impacted by wildfires and insect attacks. Drier and hotter climate conditions will continue to favor shrublands, chaparral and other invasive species.

These findings may be useful in helping forest managers make key decisions about how to adapt to climate change. The study highlighted the most vulnerable areas and suggested fuels treatment, focused fire-suppression efforts, intensive use of insect-aggregating hormones, and early detection-rapid response for invasives elimination as ways to protect high-priority areas.

The protection and preservation of forests in the Southwest is particularly important because they help maintain the area's watershed which feeds into the Colorado River. An altered hydrologic regime could cause a cascade of effects on everything--and everyone--dependent on the river's water supply.
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The study, "Forest Responses to Increasing Aridity and Warmth in the Southwestern United States" will be available at: www.pnas.org/site/misc/special.shtmlThe Pacific Southwest Research Station is headquartered in Albany, Calif. The station develops and communicates science needed to sustain forest ecosystems and other benefits to society. It has laboratories and research centers in California, Hawaii, and the United States-affiliated Pacific Islands and employs about 50 scientists. www.fs.fed.us/psw/.

USDA Forest Service - Pacific Southwest Research Station

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