升温突变对川西马尔康树木生长的影响

以采自川西马尔康林区的岷江冷杉(Abies faxoniana)和岷江柏(Cupressus chengiana)树轮为研究对象,对树轮指数与气候要素进行相关分析,研究1995年升温突变前后该区主要针叶树种的树轮响应变化。结果显示:1955—1994年时段,高、中海拔岷江冷杉径向生长对温度正响应,年轮指数呈缓慢的上升趋势,低海拔岷江柏受温度影响不大,年轮指数上升较快;1995—2012年时段,随着温度升高,3样点树木年轮指数并未上升,甚至呈下降趋势,表现出"响应分异现象"。对月气候要素的响应,树种间存在差异:高海拔岷江冷杉在前一时段主要表现为与冬季温度的正相关,后一阶段则转变为与4月温度的显著负相关及与4月份降水的显著正相关,受到显著的4月干旱胁迫影响;中海拔岷江冷杉后一时段温度敏感性消失,5月干旱胁迫加剧;低海拔岷江柏主要受降水影响,后一时段4月份干旱胁迫加剧。升温突变后,川西马尔康林区岷江冷杉和岷江柏均表现出"响应分异现象",在今后的气候重建及碳循环模拟中应加以考虑。

Effects of abrupt warming on main conifer tree rings in Markang, Sichuan, China

Temperature is considered to have the greatest influence on annual tree growth in cold regions, resulting in a pronounced positive relationship. However, an anomalous reduction or even loss of temperature sensitivity and/or a divergence in tree growth has been detected at previously temperature-limited northern sites in recent decades. This phenomenon, known as the "divergence problem," has potentially significant implications for large-scale patterns of forest growth, the development of regional climate reconstruction, and carbon cycle models based on tree-ring records. The main objective of this study was to explore the responses of the main conifer species (Abies faxoniana and Cupressus chengiana) to abrupt warming around 1995 in Markang County of western Sichuan, and determined whether the "divergence problem" exists in this typical high-elevation cold region. We analyzed the climate-growth relationships of the conifer species in two distinct periods: 1955-1994 and 1995-2012. The results showed that the growth of A. faxoniana at middle- and high-altitude sites had a positive temperature response in 1955-1994 and the tree-ring index was relatively high in this period, whereas C. chengiana at the low-altitude site was marginally influenced by temperature in 1955-1994 and the tree-ring index was apparently high at this period. In contrast, the value of the three tree ring chronologies along the elevational gradient remained steady or decreased in 1995-2012. This "growth divergence" found in this study is expressed as an offset between warm instrumental temperatures and their underestimation based on tree rings. The ring width index of A. faxoniana at high-altitude site was positively correlated with winter temperature prior to 1995, while it was significantly negatively correlated with April temperature and significantly positively correlated with April precipitation after 1995. This indicated the potential existence of spring drought stress at the high-elevation site. The temperature sensitivity of A. faxoniana at the middle-altitude site disappeared in 1995-2012, accompanied by the intensification of the effect of spring drought stress on radial growth of trees. The growth of C. chengiana at the low-altitude site was mainly controlled by precipitation, and the spring drought stress on forest growth became a noticeable feature after 1995. Conifer forests are the most productive and widespread forest types in the west Sichuan region. Thus, any coherent, climate-related change in conifer forest growth is likely to be an important factor in CO₂ uptake. Since the conifer species in Markang County of west Sichuan exhibited "growth divergence" with the abrupt warming after 1995, the current assumption of tree-growth and climate models of a uniform positive relationship of forest growth to high-elevation warming will lead to potential uncertainty in estimating the regional climate change and carbon cycle of west Sichuan.