Elevational Differentiation in Earlywood and Latewood Density Responses of Pinus sylvestris var. mongolica to Climate in the Northern Greater Khingan Range

Annual density chronologies of Pinus sylvestris var. mongolica L. at high and low elevations in the northern Greater Khingan Range were analyzed, and the responses of earlywood and latewood densities to climatic factors were examined. Significant elevational differentiation was observed in the growth response. At low elevation (630 m), earlywood density was positively correlated with temperatures in c4, c6, and c7 (c, current year) and negatively correlated with precipitation in p11 and c6 (p, previous year). Latewood density was negatively correlated with temperatures in c1–c5 and positively correlated with temperatures in c7–c8. At high elevation (1000 m), earlywood density was significantly negatively correlated with temperatures in p10, p11, c3, and c6, and with precipitation in p11, c2, c3, c5–c7, and c9; latewood density was significantly negatively correlated only with temperature in p11. Following an abrupt temperature shift in 1987, the low-elevation earlywood density chronology shifted from a decreasing to a strongly increasing trend, the low-elevation latewood chronology shifted from a strongly decreasing to a strongly increasing trend, and the high-elevation latewood chronology shifted from a strongly decreasing to an increasing trend. July temperature in the year of the shift drove the trend changes in the low-elevation earlywood and high-elevation latewood chronologies, and May temperature drove the trend change in the latewood density chronology at low-elevation, thereby explaining the shift in the latewood trend. Sliding-window correlation analyses further showed that low-elevation trees are more sensitive to climate fluctuations and exhibit lower growth stability, whereas high-elevation trees are less sensitive to climate fluctuations and show higher growth stability. Thus, the growth of low-elevation P. sylvestris var. mongolica is affected by combined water and heat stress, while the growth of high-elevation trees is primarily limited by temperature. Under ongoing warming, growth potential is likely to increase near the treeline but decline at low-elevation sites. These results provide a robust scientific basis for elucidating the response mechanisms of mountain forests in the Greater Khingan Range to climate change.