Critical temperatures controlling the phenology and radial growth of Pinus sylvestris var. Mongolica on the southern margin of a cold temperate coniferous forest

Temperature plays a key role in phenology and growth of dominant tree species and hence in the structure and function of coniferous forests in cold temperate zones. We used a dendrochronological method to determine the critical temperatures of Pinus sylvestris var. mongolica (Mongolian pine) radial growth and the xylem growing season. Results showed that Mongolian pine radial growth is sensitive to daily mean temperatures of 4.0–5.5 °C in early spring, 14.0 °C in summer and 2.0–2.5 °C in autumn. A temperature of 4 °C might be the initial temperature required for sap flow, photosynthesis and tree germination in this area in early spring, and 5.5 °C is the most critical temperature for earlywood onset growth. The advance and extension of temperature 14.0 °C in summer can exacerbate water stress for trees and inhibit earlywood growth under the condition of insufficient precipitation. In autumn, 2.0–2.5 °C may be the minimum temperature for the xylem growth. These critical temperatures are important in predicting the changes in the phenology of cambial activity and the growth of conifers in cold temperate regions under a scenario of a warming climate. We defined the mean xylem growing season of pines in this region as 151 days from early May to early October. With the increase in temperatures during 1972–2015, the start date of the xylem growing season advanced by 0.24 days/year, the duration of the xylem growing season increased by 0.27 days/year and the basal area increment of Mongolian pine increased significantly (P