氮限制对大兴安岭北方针叶林树木径向生长-气候关系的调控作用

Boreal forest plays an essential role in global carbon cycling and climate feedbacks. Boreal forest is sensitive to climate change and widely limited by nitrogen availability. This leads to the hypothesis that its growth response to climate warming might be progressive decreased by increasing N limitation in the long term. However, previous studies on tree radial growth response to climate change usually fail to account the effect of progressive nitrogen limitation, because it is difficult to retrieve long-term data on nitrogen availability corresponding to tree ring growth. Dendrochemistry information, such as nitrogen concentrations and δ15N isotope characteristics in tree rings, may provide a new way to assess the temporal changes in nitrogen availability. Boreal forest in Greater Khingan Mountains in Northeast China, dominated by Dahurian larch (Larix gmelinii), is located at the southern edge of Eurasia boreal forest and has been experiencing dramatic climate change for decades. Based on cross-site field investigation and approaches of dendrochemistry and dendrochronology, this project aims to assess temporal changes of nitrogen availability over time, and explore the role of nitrogen limitation in regulating the response of larch tree radial growth to climate change in boreal forest in Greater Khingan Mountains, Northeast China. The results will improve our understanding of the long-term impact of climate change on boreal forest and provide important implications for the regional forest management.

北方针叶林在全球碳循环和气候调控中有重要的作用。北方针叶林是对气候变化响应敏感的生态系统，也是受氮限制的生态系统，因而北方针叶林树木径向生长-气候关系理应受到可利用氮供应水平的调控。由于数据和方法的限制，以往关于树木径向生长-气候关系的研究常将氮限制的作用忽略。树木年轮一方面记录了过去气候变化影响下树木生长状况，另一方面树木年轮氮同位素化学特征也可反映出可利用氮水平的时序变化，这为研究氮限制对树木径向生长-气候关系的调控提供了可能的途径。本项目拟以大兴安岭兴安落叶松林为研究对象，综合树木年轮和氮同位素化学等技术手段，通过野外样带调查采样，基于树木年轮δ15N重建可利用氮供应水平时序变化，结合树木年轮和长期气候观测数据，揭示氮限制对树木径向生长-气候关系的调控作用。本项目研究结果既有助于全面认识北方针叶林对气候变化的响应机制，也将为我国大兴安岭森林生长预测和管理提供重要的理论指导。

北方针叶林分布区正经历显著的气候变化，氮限制对北方针叶林生长及其气候响应有重要调控作用，影响着未来北方针叶林的结构和功能。本项目综合样带调查和控制实验，揭示了大兴安岭北方针叶林氮限制的时空变化特征，阐明了氮限制对北方针叶林树木生长-气候响应关系的调控作用，围绕氮、磷限制大尺度时空变化格局及其驱动因素进行了拓展研究。结果表明，大兴安岭北方针叶林树木生长随土壤可利用氮水平增加而显著加速，证实该区域森林生产力受氮限制，而且氮限制强度表现出明显空间分异。气候变化对树木生长的影响在北方针叶林南部和北方针叶林-温带森林交错区存在地理分异，在北方针叶林-温带森林交错区，温度对生长的限制作用消失，而降水对生长的促进效应凸显。氮限制对北方针叶林南部及其与温带森林交错区树木生长-降水响应关系有重要调控作用，而对树木生长的温度敏感性没有显著影响。基于大尺度分析发现，氮限制在苔原、北方针叶林、温带针叶林、山地草原及灌丛较为普遍，但在全球尺度上磷限制比氮限制对自然陆地植被有更广泛的影响，而且其影响可能将进一步增强。项目研究结果增进了对北方针叶林树木生长时空分异的认识，也为全球变化背景下我国大兴安岭森林管理提供重要的科学依据。