Climatic response of tree-ring densitometric records in a semiarid site of China

The tree-ring densitometric studies conducted in the semiarid regions are rare, among them, minimum earlywood density (MND) records the strongest climate signals than other density parameters. In contrast, maximum density of latewood (MXD) in cold and humid regions usually shows the most significant association with summer temperatures. Density parameters of Purplecone Spruce (Picea purpurea Mast.) in Mt. Shouyang, northwestern China, a typical semiarid region were obtained to test the density-climate relationships. It is showed that MXD has strong positive correlations with temperatures and a negative correlation with precipitation in the late growing season from July to September. MND is significantly positively correlated with temperature and positively correlated with precipitation during the early growing season. During early growing season, spring droughts always occur due to low precipitation. A narrow ring is built under moisture stress, since tree growth is inhibited by decreasing cell division and cell enlargement. With the intensification of monsoon, more precipitation is available, which can basically meet the needs of tree growth. During strong monsoon season with humid conditions, trees are less affected by moisture stress. In this case, high temperature could increase cell wall thickness in the latewood which strongly affects the tree-ring maximum density. It could explain why there is a significant positive correlation between MXD and summer-fall temperature.