Response of stem radial growth of Qinghai spruce ( Picea crassifolia ) to environmental factors in the Qilian Mountains of China

Exploring the response of intra-annual tree growth to environmental factors is critical for the understanding of the effects of climate changes on tree seasonal growth patterns. We monitored stem radial changes of Qinghai spruce (Picea crassifolia) over two growing seasons (2014 and 2015) in the Qilian Mountains of China using high-resolution point dendrometers. Our results showed that the onset of the main growth period occurred around 11 May. Soil temperature was a key environmental factor influencing the onset of tree growth. Stem radial growth commenced when soil temperature at 40 cm depth was >0 °C. The timing of radial growth cessation exhibited greater variability than onset, ranging from late July to early August, and was probably unrelated to climatic conditions. We found close relationships between maximum daily shrinkage (MDS) and air temperature, precipitation, soil water content, and soil temperature. Maximum and mean air temperature, relative air humidity, vapor pressure deficit, and precipitation had significant effects on daily stem radial increment (SRI). Precipitation was one of the most important factors influencing stem radial growth. Moreover, the number of precipitation days was more significant in promoting cumulative seasonal growth of Qinghai spruce than total precipitation. With climate warming, the onset of radial growth may occur earlier than before, and seasonal growth patterns of trees may change.