Climatic control on the growth and regeneration of Juniperus przewalskii at alpine treeline in the eastern Qilian Mountains, northwest China

Temperature was the dominant climatic factor that controlled Juniperus przewalskii growth before 1988. However, the limiting effect of temperature on trees gradually weakened after 1988. Treeline shifted upward since 1890, especially during the 1930–1949. The saplings of the species died due to the warming. Forest mortality triggered by drought and treeline dynamics due to the regional warming has been widely reported. In the eastern Qilian Mountains, northwestern China, sapling of Juniperus przewalskii was found dead in the high elevation at the fringe of its spatial distribution. Tree-ring widths were measured, and climatic records were related to the tree-ring width to understand the sapling’s death and the mechanism of the treeline dynamics in this region. Results showed that tree-ring widths were positively correlated with the temperatures of previous year’s September to current year’s February. However, this correlation weakened in the late twentieth century as the mean annual temperature increased. Treeline shifted upwards, and the regeneration of J. przewalskii trees peaked during 1930–1949, of which most grew at higher elevations. Sapling trees (less than 2 m tall) started to die after 1959 and the death rate peaked 2000–2004, which lagged the warming induced drought. Drought stress induced by rising temperatures explained the death of the sapling trees. This study sheds light on the impacts of climate changes for tree growth, forest dynamics, and tree mortality, and provides insights that will be useful to forest managers for managing vulnerable arid and semi-arid forests.