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Trends in twentieth-century tree growth at high elevations in the Sierra Nevada and White Mountains, USA

Big Sky Institute, Montana State University, Bozeman MT, USA; abunn{at}whrc.org

Lisa J. Graumlich

Big Sky Institute, Montana State University, Bozeman MT, USA

Dean L. Urban

Nicholas School of the Environment and Earth Sciences, Duke University, Durham NC, USA

We analysed a multispecies tree-ring data base to assess the degree to which twentieth-century growth trends reflect tree growth of the last millennium. We examined 1000-yr chronologies for five species of high-elevation conifers at 13 sites in western North America. Using non-parametric ordination and cluster analysis, we decomposed the variability at annual to decadal timescales into two dimensions, both of which are significantly correlated to temperature and precipitation variation. Tree-ring sites map onto the ordination axes according to species and relative position on the landscape. A spectral analysis of the ordination axes indicates a secular trend and significant quasi-periodic variation on scales of years to decades. Further, we find that the pattern of high-elevation conifer growth rates during the last half of the twentieth century are different than any time in the past 1000 years, indicating a distinct biological signature of global climate change.

Key Words: Global warming • dendrochronology • paleoclimatology • Sierra Nevada • ordination • cluster analysis • late Holocene

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