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. © 2005 Edward Arnold (Publishers) Ltd.