The responses of dominant tree species to climate warming at the treeline on the eastern edge of the Tibetan Plateau

Tree growth at boreal/alpine treelines is generally expected to be stimulated by climate warming. However, tree growth has been shown to have changed insignificantly or reduced in numerous studies. Tree response to climate warming also varies among species in the same eco-region. The Tibetan Plateau has experienced rapid warming in the past several decades. It remains unknown how the dominant species in this region have responded to this warming, which is expected to significantly influence treeline dynamics. We obtained 288 tree cores from fir (Abies faxoniana and Abies squamata) and spruce (Picea purpurea and Picea baifouriana) at six treeline sites on the eastern edge of the Tibetan Plateau to detect their growth trends and climate responses. The results showed that the growth of fir did not change significantly at any site, whereas spruce growth increased, especially at the Songpan and Luhuo sites. The strong negative temperature or positive moisture controls of April on fir growth indicated spring drought stress. The radial growth of spruce was positively related to June and July temperature; at the Songpan and Luhuo sites, it was also positively related to the temperature from February to April. The results indicate that both pre-growing season temperature and growing season temperature accelerate spruce growth, whereas pre-growing season temperature causes drought stress to fir. The tree species composition at the alpine treeline on the eastern edge of the Tibetan Plateau will change as warming continues.