Intra-annual stem radial increment response of Qilian juniper to temperature and precipitation along an altitudinal gradient in northwestern China

Spring temperature is a major limiting factor at the beginning of the growing season, the timing of growth initiation can increase by about 7 days/A degrees C. During the growing season, impacts of climate variables on radial growth are similar along an altitudinal gradient. Altitude is considered as an important factor affecting tree growth in mountain forest ecosystems. In this paper, the results of a 2-year field study along an altitudinal gradient in the cold and arid central Qilian Mountains, northwestern China, are reported. Twelve Qilian juniper trees (Sabina przewalskii Kom.) were monitored with high-resolution dendrometers at three altitudes ranging from 2,865 to 3,550 m. At each altitude, a local weather station was installed close to the studied trees. We identified correlations between intra-annual growth patterns derived from the Gompertz equation with local air temperature and precipitation data. The timing of growth initiation became earlier and the growing season duration increased with decreasing altitude. The onset of radial growth occurred between early May and early June, and the growing season terminated between mid-July and late August, resulting in a growing season duration that decreased from 107 to 41 days as elevation increased. June is the most important growth period at each altitude. Spring temperature, which is strongly associated with elevation, is a critical factor determining the initiation of radial growth. The timing of growth initiation was delayed by 3-4 days per 100 m elevation. When associated with the modeled altitudinal spring temperature lapse rate of -0.48 A degrees C/100 m, the onset of the growing season increased by about 7 days/A degrees C. However, during the growing season, daily stem radial increments showed a positive correlation with precipitation and a negative correlation with daily maximum air temperature at all altitudes. Our study provides new data revealing the basic growth processes of Qilian juniper trees and provides significant information to quantify the responses of tree growth to future global warming.