Alpine timberline ecotone tree growth in relation to climatic variability for Picea crassifolia forests in the middle Qilian Mountains, northwestern China.

The Picea crassifolia forests at the alpine timberline ecotone in the middle Qilain Mountains of northwestern China were examined in this study. Treering cores were sampled at four different altitudes to examine the relationship between tree growth and climatic variability. The statistical characters of the chronologies suggested that the sensitivity of tree growth to climatic variability was generally low across the alpine timberline ecotone, and did not show clear trend along the altitudinal gradient,probably due to the short vertical gradient. Tree radial growth in the timberline ecotone was strongly associated with climatic indices before growing season. Treering width was positively related to precipitation in the previous October, while negatively correlated with temperature in the previous July and August for most plots, with the later actually reflecting the effect of water availability in limiting tree growth in this semiarid region. Meanwhile, tree growth was also negatively related to the current February temperature. Our results did not show clear relationship between treering width and climatic condition in the current year growing season, suggesting that tree growth in timberline ecotone was not limited by growing season temperature in our study site. We reviewed existing evidence which showed that timberline tree growth was sometimes not sensitive to temperature changes in growing season.