Which climatic factors limit radial growth of Qilian juniper at the upper treeline on the northeastern Tibetan Plateau?

There are differing views regarding the climatic factors that limit radial growth of Qilian juniper (Juniperus przewalskii Kom.) at the upper treelines on the northeastern Tibetan Plateau. In this study, trees from an upper treeline site in the Anyemaqen Mountains of the northeastern Tibetan Plateau were selected to present new evidence that low temperature limits the radial growth of Qilian juniper. Using a signal-free regional curve standardization (SF-RCS) method, a ring-width chronology for Qilian juniper was developed extending from AD 1082 to 2010. The results of correlation analysis between tree-ring index and instrumental climatic variables showed that both winter (December in the previous year and January in the current year) and summer (July and August in the current year) temperatures were significantly and positively correlated with the tree-ring index. Precipitation in June was also significantly and positively correlated with the tree-ring index, but was not as important as low temperature in controlling tree growth. To test the key limiting climatic factor for tree radial growth at different altitudes, an indicator termed the "relative distance to upper treeline" (RDUT) was developed to quantify the representativeness of collected samples for the forest's upper treeline. The RDUT showed that the upper 20% of the forest belt may be an important boundary in terms of capturing the temperature signal from tree-ring width at the upper treelines on the northeastern Tibetan Plateau. Our findings enhance the existing understanding that temperature is the limiting factor at upper treelines on the northeastern Tibetan Plateau, and will be useful in the reconstruction of past temperature in future studies