Divergent tree radial growth patterns of Qinghai spruce (Picea crassifolia) at the alpine timberline along a moisture gradient in the Qilian mountains, Northwest China

Tree populations at the alpine timberline are highly sensitive and vulnerable to environmental change, but how climate change alters the spatiotemporal dynamics of tree radial growth at the alpine timberline remains a critical uncertainty of forest ecosystem change. We collected 480 tree-ring cores from 10 alpine timberlines at an elevation between 3210 and 3426 m a.s.l. to study the divergent radial growth dynamics of Qinghai spruce (Picea crassifolia Kom.) across a moisture gradient. According to the 400 mm annual total precipitation threshold, the ten chronologies could be roughly classified into two categories: the eastern Qilian Mountains (EQL) and western Qilian Mountains (WQL). Daihuangshan, Junmachang, Lujiaogou, Pailugou, Sidalong and Longchanghe belong to the EQL, while Baiquanmen, Ciyaokou, Qiaozigou and Xigoukuang belong to the WQL. Additionally, the correlation coefficients between the EQL and WQL chronologies were much lower than those within the same group. There were also significant differences in the response of radial growth to climate between the EQL and WQL. The radial growth of the EQL was mainly influenced by temperature, whereas the radial growth of the WQL was primarily limited by precipitation; this was mostly related to the tremendous east-westward moisture gradient of the Qilian Mountains. These results on climate growth suggest that future warming may promote the radial growth of P. crassifolia at the alpine timberline in the EQL, while the growth trend of P. crassifolia at the alpine timberline in the WQL mainly depends on the degree of drought due to temperature and precipitation, and thus, the growth trend in the future is relatively complex.