Environmental Factors Effect on Stem Radial Variations of Picea crassifolia in Qilian Mountains, Northwestern China

Picea crassifolia Komarov (Qinghai spruce) is an endemic tree species in China and is widespread in the Qilian Mountains, in northwestern China. High temporal resolution changes of Qinghai spruce tree stem growth remain poorly investigated and the relationships between the species growth and climate are still not completely understood. In this study, we assessed the daily and seasonal stem radial variations, and analyzed the relationships between stem radial increment of Qinghai spruce and environmental factors during the main growing period (June–August). We have found that the stem radial variations of Qinghai spruce can be divided into three phases according to the air temperature and that Qinghai spruce has two diurnal cycle patterns. The main growing period of Qinghai spruce is 30 May–31 August according to micro-core measurements, in conformity with the daily mean air temperature keeping above 5 ◦C. Precipitation and relative humidity have positive effects on the growth of Qinghai spruce, and we develop a multiple linear regression model that can explain 63% of the stem radial increment over the main growing period.