Statistical comparisons between tree-ring width sequences and climatic records provide a means of identifying climatic limitations on tree growth and allow the reconstruction of past climates. This information is especially important in the North American sub-Arctic where climate-growth relationships are poorly understood and instrumental weather records are very short, typically less than 75 yr. Dendroclimatic reconstructions before 1900 are essential for estimating a realistic range of high latitude climatic variation, because twentieth century climate is now thought to be somewhat anomalous1. While some dendroclimatic studies have been carried out in the sub-Arctic2-8, none has made full use of current multivariate statistical techniques. This study, as part of a multidisciplinary investigation of treeline fluctuations in the Brooks Range of Alaska, uses ring-width sequences of white spruce (Picea glauca [Moench] Voss) to define climatic limitations on radial growth at treeline and to reconstruct past climatic variables for Fairbanks, Alaska. Multiple regression techniques were used to analyse climate-growth relationships and the results suggest important modifications to the theory that growth at treeline is limited primarily by summer temperature2,8,9 . For example, we have found that radial growth is directly related to summer and autumn temperature and precipitation during certain months, but is inversely related to winter-spring temperature. The same ring-width sequences were used to reconstruct average May-July temperature at Fairbanks, Alaska, for the period 1829-1930. This reconstruction, which more than doubles the length of the existing climatic record, was verified by statistically comparing it to independent instrumental data. It indicates that the Fairbanks area has been warmer in the twentieth century than in the nineteenth century during these months. As the longest annual record of temperature from northern Alaska, this reconstruction provides quantitative evidence for a climatic warming similar to that occurring throughout large portions of the Northern Hemisphere during the past 100 yr (refs 10-12).