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Science 23 January 2009:
Vol. 323. no. 5913, pp. 521 - 524
DOI: 10.1126/science.1165000
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Reports

Widespread Increase of Tree Mortality Rates in the Western United States

Phillip J. van Mantgem,1*{dagger}{ddagger} Nathan L. Stephenson,1*{dagger} John C. Byrne,2 Lori D. Daniels,3 Jerry F. Franklin,4 Peter Z. Fulé,5 Mark E. Harmon,6 Andrew J. Larson,4 Jeremy M. Smith,7 Alan H. Taylor,8 Thomas T. Veblen7

Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

1 U.S. Geological Survey, Western Ecological Research Center, Three Rivers, CA 93271, USA.
2 USDA Forest Service, Rocky Mountain Research Station, Moscow, ID 83843, USA.
3 Department of Geography, University of British Columbia, Vancouver, British Columbia V6T IZ2, Canada.
4 College of Forest Resources, University of Washington, Seattle, WA 98195, USA.
5 School of Forestry and Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011, USA.
6 Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA.
7 Department of Geography, University of Colorado, Boulder, CO 80309, USA.
8 Department of Geography, Pennsylvania State University, University Park, PA 16802, USA.

* These authors contributed equally to this work.

{ddagger} Present address: U.S. Geological Survey, Western Ecological Research Center, Arcata, CA 95521, USA.

{dagger} To whom correspondence should be addressed. E-mail: pvanmantgem{at}usgs.gov (P.J.V.); nstephenson{at}usgs.gov (N.L.S.)

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought.
H. D. Adams, M. Guardiola-Claramonte, G. A. Barron-Gafford, J. C. Villegas, D. D. Breshears, C. B. Zou, P. A. Troch, and T. E. Huxman (2009)
PNAS 106, 7063-7066
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