Global Warming Articles:
Reading the Tree-Rings
Massive Pinyon Pine Die-off
Polar Melting
Hard Numbers on Global Warming
African Lake Demonstrates Effects of Global Warming
Global Climate Change Lecture Series
Fall 2006 Alumnus Global Warming update:
Forest Fires
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The University of Arizona Alumnus — Summer 2006 / Global Warming Underlying Cause of Massive Pinyon Pine Die-off Revealed
The high heat that accompanied the recent drought was the underlying cause of death for millions of pinyon pines throughout the Southwest in 2005, according to UA researchers. “We documented a massive forest die-off — and it’s a concern because it’s the type of thing we can expect more of with global warming,” says research team leader David D. Breshears, a professor of natural resources in The University of Arizona’s School of Natural Resources in Tucson and a member of the UA’s Institute for the Study of Planet Earth. The resulting landscape change will affect the ecosystem for decades. Hotter temperatures coupled with drought are the type of event predicted by global climate change models, and the new findings suggest that big, fast changes in ecosystems may result from global climate change. At study sites in Arizona, Colorado, New Mexico, and Utah, the team found that from 40 to 80 percent of the pinyon trees (Pinus edulis) died between 2002 and 2003. The researchers confirmed the massive regional dieback through both aerial surveys and analysis of satellite images of pinyon-juniper woodlands. “Scientists are concerned about how fast vegetation will respond to climate change, but we don’t have many examples to test our ideas,” Breshears says. “Here we’ve clearly documented a case that shows how big and fast the die-off can be.” The drought, coupled with particularly high temperatures, set the trees up to be susceptible to insect infestations. Bark beetles delivered the knock-out punch. “It was the drought — beetles don’t get trees unless the trees are really water-stressed,” Breshears says. Breshears, Neil S. Cobb, director of the Merriam-Powell Center for Environmental Research at Northern Arizona University in Flagstaff, Paul M. Rich, research scientist at Los Alamos National Laboratory in New Mexico, and their colleagues reported their findings in October 2005 in the online Early Edition of the Proceedings of the National Academy of Sciences. Los Alamos National Laboratory (LANL), the National Science Foundation, the U.S. Forest Service, the U.S. Geological Survey, NASA, and the National Institutes of Health funded the research. Ecologists want to learn more about long-term changes in ecosystems that occur in response to climatic and other environmental variation. So in 1987, researchers established a study site in the pinyon-juniper woodland zone at LANL. Roughly every two weeks, Breshears and his colleagues measured soil moisture on the 100-by-150-meter plot (about three times the size of a football field). Temperature and precipitation data were recorded at a nearby site. In 1992, the team began tracking tree mortality. “I could see the plot from my office window,” remembers Breshears, who used to work at LANL. When the recent drought hit, the scientists were well-positioned to compare how the vegetation fared before and during the drought. Initially, the trees managed, but in 2002 the pinyon pines began to die. By the end of 2003, more than 90 percent of the pinyons on the plot were dead. “I would see the trees go from vibrant green to pale, gasping green to pale brown to dropping all their needles,” Breshears says. Pinyon pines all over the Southwest were doing the same thing. The U.S. Forest Service’s aerial surveys of the region’s pinyon-juniper woodlands in 2002 and 2003 revealed significant tree die-off covering more than 4,600 square miles (12,000 square km). The effect was so dramatic it could be detected by satellite. The region’s 60,000 square miles (about 155,000 square km) of pinyon-juniper woodlands became a lot less green starting in 2002, the team found. Moreover, the NDVI (Normalized Difference Vegetation Index) measurements for the site at LANL showed that the plot’s greenness dropped at the same time and in a similar way. During a previous multi-year drought in the 1950s, not as many trees died. To see how the two droughts differed, the researchers compared the four driest consecutive years of the earlier drought, 1953-1956, with those of the recent drought, 2000-2003. “By every measure we looked at, the recent drought was hotter,” Breshears says, adding that, if anything, the 1950s drought was drier. The high heat combined with the extreme dryness put the trees under so much water stress that the attacks from bark beetles finished them off. Under such conditions, the trees cannot make enough pine sap to defend themselves against the insects. “These trees are slow-growing trees, so we aren’t going to have woodlands of this type back in this area for decades,” Breshears explains. He adds that the lack of pinyon nuts will have negative effects on wildlife and on people who harvest the nuts for food and for sale. Paul Rich says, “The fate of the pinyon-juniper forest depends on what happens next, especially in terms of weather. If it’s wetter, the trees may come back. If not, we’ll probably see shifts to species from drier ecosystems.” Having such a wealth of data and a range of expertise was crucial for figuring out what happened, says Neil Cobb, leader of the Drought Impacts on Regional Ecosystems Network (DIREnet). “The NSF DIREnet project allowed us to bring all these researchers together.” Breshears said the team’s next step is developing ways to predict how bad a drought must be to cause such large-scale die-offs. Breshears, Cobb, and Rich’s co-authors include professors at the University of Kansas in Lawrence; Colorado State University in Fort Collins; Prescott College in Arizona; Northern Arizona University; the University of New Mexico in Albuquerque; and researchers at the U. S. Geological Survey in Los Alamos, N.M.; the Los Alamos National Laboratory; and the U.S. Geological Survey in Moab, Utah; The team’s paper, “Regional Vegetation Die-off in Response to Global-change Type Drought,” will be published in an upcoming edition of the Proceedings of the National Academy of Sciences.
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