大兴安岭北方针叶林-温带针阔混交林交错区树木径向生长特征及其对气候变化的响应

The impacts of climate change on the boreal forest have received significant attention because of the essential role of boreal forest in global carbon cycling and climate feedbacks. Boreal-temperate forest ecotone shows high sensitivity to climate change and may provide an opportunity to improve our understanding of how southern limit of boreal forest responds to climate change. However, little effort has focused on the transitions between boreal forest and temperate forest. Although temperate broadleaved trees are expected to shift with climate warming into the boreal forest, field studies are lacking to detect the predicted range shifts due to the decadal to centurial time lags and slow shifting rates. Tree growth responds promptly to climate change and specific responses of tree species may provide insights of how species distribution, community composition and primary productivity will change with rising temperature and changing precipitation regimes. A reduction in forest growth indices and temperature sensitivity, which is known as the ‘growth divergence’, has been detected in the high latitude boreal forest since the middle of the 20th century. The divergence problem has potentially significant implications for large-scale patterns of forest growth and the global carbon cycle in response to climate change. Trees may show diverse growth responses to climate change because of differences in species-specific traits. In addition, responses of tree growth to climate warming may show great asymmetry. The Greater Khingan Mountains in Northeast China, containing large area of boreal forest with Dahurian larch (Larix gmelinii) as the dominant tree species, is located in the southern edge of Eurasia boreal forest. On the east slope of Greater Khingan Mountains, boreal forest gradually changes to a boreal-temperate forest ecotone with the Mongolian oak (Quercus mongolica)-Dahurian larch forest as one typical forest type. This program combines field investigation and climate data to assess how tree radial growth responds to climate change, analyze the species-specific responses to climate change, and test whether growth divergence has occurred in the boreal-temperate forest ecotone in Northeast China.

北方针叶林在全球碳循环和气候调控中起到极为重要的作用，气候变化对北方针叶林的影响是当前全球变化研究关注的热点问题。生态交错区对气候变化尤为敏感，而针对北方针叶林-温带针阔混交林交错区的研究仍十分薄弱。研究北方针叶林-温带针阔混交林交错区树木生长特征及其对气候变化的响应将有助于全面认识气候变化对北方针叶林的影响过程和机制。研究发现，20世纪中期以来北半球高纬度地区北方针叶林树木生长对温度的敏感性减弱，出现“生长分异”现象。由于不同物种自身属性的不同，其对气候变化的响应可能存在差异。此外，树木生长对气候变化的响应可能存在不对称性，其机制尚有待研究。本项目将针对上述问题系统研究大兴安岭北方针叶林-温带针阔混交林交错区兴安落叶松和蒙古栎生长特征及其气候变化的响应规律，验证生长分异现象是否发生并对其机制进行探讨。

北方针叶林在全球碳循环和气候调控中起到极为重要的作用，气候变化对北方针叶林的影响是当前全球变化研究关注的热点问题。通常认为，生态交错区对气候变化尤为敏感，而当前针对北方针叶林-温带针阔混交林交错区的研究仍十分薄弱。中国大兴安岭北方针叶林是由东西伯利亚南泰加林向南延伸形成的以兴安落叶松为单优势种的明亮针叶林，在大兴安岭东坡蒙古栎-兴安落叶松林是北方针叶林向温带针阔混交林过渡的地带性植被类型。树木年轮径向生长对气候变化的响应有较强的敏感性和时效性，因而是研究森林生态系统对气候变化响应过程和机制的有效切入点。本项目通过典型样点树木年轮采样分析，建立了大兴安岭北方针叶林南缘及北方针叶林-温带针阔混交林交错区兴安落叶松树木年轮径向生长及环境因子综合数据库，基于该数据库研究了兴安落叶松径向生长特征的时空格局及其对1980-2015年间气候变化的响应规律。主要研究结果如下：（1）从大兴安岭北方针叶林到北方针叶林-温带针阔混交林交错区，兴安落叶松径向生长速率有显著的梯度变化，该梯度随着树轮生理年龄增长而发生变化，生长竞争和气候以外的其他限制因子（如土壤养分）可能是影响该梯度的重要因素。（2）兴安落叶松径向生长对温度和降水的响应存在明显的物候期不对称性，兴安落叶松径向生长对生长季起始月温度和降水有较高的敏感性，而对非生长季及生长季结束月均温和降水的变化无显著的响应；从大兴安岭北方针叶林到北方针叶林-温带针阔混交林交错区，兴安落叶松径向生长对生长季温度和降水的响应随背景气候条件而发生变化。（3）通过对1980-1995和2000-2015两个时期的比较发现，兴安落叶松径向生长对温度和降水的响应均发生了明显的时间分异，生长分异现象随着背景气候条件而表现为多样化。本项目研究结果既有助于深入认识北方针叶林南缘森林生长对气候变化的响应过程和机制，也可为气候变化背景下大兴安岭林区制定可持续的森林经营管理方案提供理论指导。