The high heat that accompanied the recent drought was theunderlying cause of death for millions of pinyon pines throughout theSouthwest, according to new research.
The resulting landscapechange will affect the ecosystem for decades. Hotter temperaturescoupled with drought are the type of event predicted by global climatechange models. The new finding suggests big, fast changes in ecosystemsmay result from global climate change.
"We documented a massiveforest die-off – and it's a concern because it's the type of thing wecan expect more of with global warming," said research team leaderDavid D. Breshears, a professor of natural resources in The Universityof Arizona's School of Natural Resources in Tucson and a member of UA'sInstitute for the Study of Planet Earth.
At study sites inArizona, Colorado, New Mexico and Utah, the team found that from 40 to80 percent of the pinyon trees died between 2002 and 2003. Theresearchers confirmed the massive regional dieback of vegetationthrough both aerial surveys and analysis of satellite images of thosestates' pinyon-juniper woodlands.
"Scientists are concerned abouthow fast vegetation will respond to climate change, but we don't havemany examples to test our ideas," Breshears said. "Here we've clearlydocumented a case that shows how big and fast the die-off can be."
Thedrought coupled with particularly high temperatures set the trees up tobe susceptible to insect infestations. Bark beetles delivered theknock-out punch.
"It was the drought – beetles don't get trees unless the trees are really water-stressed," Breshears said.
Breshears,Neil S. Cobb, director of the Merriam-Powell Center for EnvironmentalResearch at Northern Arizona University in Flagstaff, Paul M. Rich,research scientist at Los Alamos National Laboratory in New Mexico, andtheir colleagues will report their findings the week of Oct. 10 in theonline Early Edition of the Proceedings of the National Academy ofSciences. A complete list of authors and their affiliations is at theend of this release. Los Alamos National Laboratory (LANL), theNational Science Foundation, the U.S. Forest Service, the U.S.Geological Survey, NASA and the National Institutes of Health fundedthe research.
Ecologists want to learn more about long-termchanges in ecosystems that occur in response to climatic and otherenvironmental variation.
So in 1987, researchers established astudy site in the pinyon-juniper woodland zone at LANL. Roughly everytwo weeks, Breshears and his colleagues measured soil moisture on the100-by-150 meter plot (about three times the size of a football field).Temperature and precipitation data were recorded at a nearby site. In1992, the team began tracking tree mortality.
"I could see theplot from my office window," said Breshears, who used to work at LANL.When the recent drought hit, the scientists were well-positioned tocompare how the vegetation fared before and during the drought.
Initiallythe trees managed, but in 2002 the pinyon pines began to die. By theend of 2003, more than 90 percent of the pinyons on the plot were dead.Breshears said, "I would see the trees go from vibrant green to pale,gasping green to pale brown to dropping all their needles."
Pinyon pines all over the Southwest were doing the same thing.
U.S.Forest Service's aerial surveys of the region's pinyon-juniperwoodlands in 2002 and 2003 revealed significant tree die-off coveringmore than 4,600 square miles (12,000 square km).
The effect was so dramatic it could be detected by satellite.
Richand his colleagues at LANL and the University of Kansas analyzedsatellite images of the region's pinyon-juniper woodlands for the years1989 through 2003 using a measure of vegetation greenness known asNormalized Difference Vegetation Index (NDVI). The weekly compositeimages came from data collected by AVHRR (Advanced Very High ResolutionRadiometer) sensors on National Oceanic and Atmospheric Administrationsatellites.
The region's 60,000 square miles (about 155,000square km) of pinyon-juniper woodlands became a lot less green startingin 2002, the team found. Moreover, the NDVI measurements for the siteat LANL showed that the plot's greenness dropped at the same time andin a similar way.
During a previous multi-year drought in the1950s, not as many trees died. To see how the two droughts differed,the researchers compared the four driest consecutive years of theearlier drought, 1953-1956, with those of the recent drought, 2000-2003.
"Byevery measure we looked at, the recent drought was hotter," Breshearssaid, adding that, if anything, the 1950s drought was drier.
Thehigh heat combined with the extreme dryness put the trees under so muchwater stress that the attacks from bark beetles finished them off.Under such conditions, the trees cannot make enough pine sap to defendthemselves against the insects.
"These trees are slow-growingtrees, so we aren't going to have woodlands of this type back in thisarea for decades," Breshears said. He added that the lack of pinyonnuts will have negative effects on wildlife and on people who harvestthe nuts for food and for sale.
Rich said, "The fate of thepinyon-juniper forest depends on what happens next, especially in termsof weather. If it's wetter, the trees may come back. If not we'llprobably see shifts to species from drier ecosystems."
Havingsuch a wealth of data and a range of expertise was crucial for figuringout what happened, said Cobb, leader of the Drought Impacts on RegionalEcosystems Network (DIREnet). "The NSF DIREnet project allowed us tobring 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 coauthors include Kevin P. Price and Jude H. Kastens ofthe University of Kansas in Lawrence; Craig D. Allen of the U. S.Geological Survey in Los Alamos, N.M.; Randy G. Balice and Clifton W.Meyer of Los Alamos National Laboratory; William H. Romme of ColoradoState University in Fort Collins; M. Lisa Floyd of Prescott College inArizona; Jayne Belnap of the U.S. Geological Survey in Moab, Utah;Jesse J. Anderson of Northern Arizona University; and Orrin B. Meyersof the University of New Mexico in Albuquerque. The team's paper,"Regional vegetation die-off in response to global-change typedrought," will be published in an upcoming edition of the Proceedingsof the National Academy of Sciences.
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