Regional vegetation die-off in responseto global-change-type droughtDavid D. Breshearsa,b, Neil S. Cobbc, Paul M. Richd, Kevin P. Pricee,f, Craig D. Alleng, Randy G. Baliceh, William H. Rommei,Jude H. Kastensf,j, M. Lisa Floydk, Jayne Belnapl,m, Jesse J. Andersonc, Orrin B. Myersn, and Clifton W. MeyerdaSchool of Natural Resources, Institute for the Study of Planet Earth, and Department of Ecology and Evolutionary Biology, University of Arizona,Tucson, AZ 85721-0043;cMerriam Powell Center for Environmental Research and Department of Biological Sciences, Northern Arizona University,Flagstaff, AZ 86011;dEarth and Environmental Sciences Division, andhEnvironmental Stewardship Division, University of California–Los AlamosNational Laboratory, Los Alamos, NM 87545; Departments ofeGeography andjMathematics, University of Kansas, Lawrence, KS 66045;fKansasApplied Remote Sensing Program, 2101 Constant Avenue, Lawrence, KS 66047-3759;gFort Collins Science Center, U.S. Geological Survey,Jemez Mountains Field Station, Los Alamos, NM 87544;iForest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523;kEnvironmental Studies Program,Prescott College, 220 Grove Avenue, Prescott, AZ 86301;lSouthwest Biological Science Center, U.S. Geological Survey,Moab, UT 84532;mNatural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523-1499; andnDivision of Epidemiology andBiostatistics, University of New Mexico, Albuquerque, NM 87131Edited by Harold A. Mooney, Stanford University, Stanford, CA, and approved September 7, 2005 (received for review July 8, 2005)Future drought is projected to occur under warmer temperatureconditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggersand potential extent of drought-induced vegetation die-off remainpivotal uncertainties in assessing climate-change impacts. Of par-ticular concern is regional-scale mortality of overstory trees, whichrapidly alters ecosystem type, associated ecosystem properties,and land surface conditions for decades. Here, we quantify region-al-scale vegetation die-off across southwestern North Americanwoodlands in 2002–2003 in response to drought and associatedbark beetle infestations. At an intensively studied site within theregion, we quantified that after 15 months of depleted soil watercontent, >90% of the dominant, overstory tree species (Pinusedulis, a pin˜ on) died. The die-off was reflected in changes in aremotely sensed index of vegetation greenness (Normalized Dif-ference Vegetation Index), not only at the intensively studied sitebut also across the region, extending over 12,000 km2or more;aerial and field surveys confirmed the general extent of the die-off.Notably, the recent drought was warmer than the previous sub-continental drought of the 1950s. The limited, available observa-tions suggest that die-off from the recent drought was moreextensive than that from the previous drought, extending intowetter sites within the tree species’ distribution. Our resultsquantify a trigger leading to rapid, drought-induced die-off ofoverstory woody plants at subcontinental scale and highlight thepotential for such die-off to be more severe and extensive forfuture global-change-type drought under warmer conditions.tree mortality 兩 vegetation dynamics 兩 climate change impacts 兩woodlands 兩 Pinus edulisGlobal climate change is projected to yield increases inf requency and intensity of drought occurring under warm-ing temperatures (1–3), referred to here as global-change-t ypedrought. Protracted, subc ontinental drought in the midlatitudesis a c omplex response driven in part by anomalies associated withoscillations in sea surface temperature (2–4), which can includeoscillations over periods of decades or longer, such as thoseassociated with the Atlantic Multidecadal Oscillation and thePacific Decadal Oscillation (4), and shorter periods spanningseveral years, such as those associated with the El Nin˜o SouthernOscillation (3). Greenhouse gas forcings are expected to alterthese ocean ic ef fects on drought patterns (1–3). Indeed, the mostrecent protracted drought in southwestern North America,spann ing the beginning of the 2000 millennium, exhibited anom-alous sea surface temperature patterns consistent with projec-tions of global change response (3). Protracted drought cantrigger large-scale landscape changes through vegetation mor-t ality f rom water stress (5, 6), sometimes associated with barkbeetle infestations (5), but the potential for regional to subcon-tinent al scale vegetation die-of f under global-change-typedrought remains a pivotal uncertainty in projections of climatechange impacts (1, 7, 8). Of particular concern is regional-scalemort ality of overstory trees, which rapidly alters ecosystem type,associated ecosystem properties, and land surface conditions fordecades. The potential for this response is highlighted by a rapidshif t of a forest ecotone caused by Pinus ponderosa mortality inresponse to the 1950s drought (5). The effects of droughtac compan ied by warmer temperatures resulting from green-house forcings might be expected to produce even greater effectson vegetation change than those of periodic, protracted droughtalone (5). Yet few, if any, studies quantify rapid, regional-scaleveget ation die-of f in response to drought, key environment alc onditions triggering tree mortality, such as prior soil moisturec onditions, or how anomalously high temperatures might altersuch vegetation responses. Such relationships urgently need tobe quantified to improve climate change assessments (9).We evaluated the impacts of the recent drought on regional-scalemortality of pin˜on pine (Pinus edulis), which is sensitive to climatevariation and dominates pin˜on-juniper woodlands, one of the mostextensive vegetation types in the western North America (10–12).Specifically, we evaluated the impacts of the recent drought onregional-scale mortality in the context of the potential impacts ofglobal-change-type drought, and (i) demonstrated that the recentdrought is not as dry as the previous drought but is warmer innumerous re spects, thereby providing a case study for global-change-type drought; (ii) quantified site-specific conditions in soilwater content and local vegetation response (percentage of treemortality, and a remotely sensed vegetation index related tophotosynthetic activity and