图书馆知识库

Most Tibetan people make their living through pastoralism, with livestock grazing being widespread throughout the Tibetan Plateau. The Tibetan Plateau is also home to the northern hemisphere's highest treelines but the effects of human activities, in particular pastoralism, on these treelines and their influence on the treeline responses to climate change remain unclear. To disentangle the effects of pastoral disturbances and climate on treeline dynamics, we surveyed three plots at grazing-disturbed Balfour spruce (Picea likiangensis var. balfouriana) treelines in the Yushu region (eastern Tibetan Plateau) and compared this data with published results of five undisturbed spruce treeline sites. Spatiotemporal changes in the position and structure of treelines during the past 400 years were reconstructed using dendrochronology. Tree recruitment was significantly linked to long-term summer and winter temperature. Under low local population densities and grazing pressure but consistent climate warming, disturbed treelines shifted upwards by 23.9-82.6 m during the 1611-1910 period. However, under higher local population densities, increased grazing pressure and climate warming over the last century, disturbed treelines showed no significant upward shift. In contrast, undisturbed treelines shifted upward by 25-68.5 m over the last century, thus implying that pastoralism was suppressing treeline expansion in disturbed sites. Together, these results underscore the importance of anthropogenic disturbances in influencing the structure and position of alpine treelines on the Tibetan Plateau. The role played by human disturbance should be carefully considered when evaluating the potential for treeline advance under future climate warming.