南亚热带常绿阔叶林优势树种固碳对降雨变化和氮添加交互作用的响应

The variation of input quantity and pattern of precipitation and nitrogen will have an impact on forest net primary productivity (NPP) and carbon sequestration. Will the increased nitrogen deposition and seasonal changes in precipitation under future climate change scenarios exert a synergistic effect on carbon assimilation or an antagonistic effect instead, and how will be the effect intensity and mechanism of the two combined environmental factors on forest carbon sequestration, are key scientific issues for elucidating the interaction mechanism between forest carbon cycle and global changes. This research will be conducted at a designed seasonal precipitation redistribution and nitrogen addition manipulated experimental platform in a subtropical secondary evergreen broad-leaved forest. We will estimate carbon assimilation rate of dominant tree species using the multi-layer canopy conductance-constrained carbon assimilation model (4C-A model) based on sap-flow based canopy stomatal conductance and photosynthetic parameters, investigate NPP by combining diameter at breast height (DBH) increment and allometric equations, and analyze the individual and combined effects of seasonal rainfall redistribution and nitrogen addition on carbon assimilation of dominant tree species. This method distinguishes the sunlit and shaded leaves at different canopy layers, avoiding the deficiency of neglecting light intensity heterogeneity inside the canopy with previous model methods. Additionally, this 4C-A model uses the sap-flow based canopy stomatal conductance to constrain the modelled results so as to improve accuracy. The research will be of practical significance and theoretical value in understanding the responses of forest carbon sequestration to environmental changes and predicting regional carbon storage under future climate change scenarios.

降雨和氮的输入量和输入模式的变化会改变森林的净生产力和固碳能力，全球变化背景下氮沉降增加和降水季节变化对森林树木碳同化产生“协同”还是“拮抗”效应、以及两因子交互作用影响森林净固碳量的程度和机制，则是阐明森林碳循环与全球变化因子相互作用的关键科学问题。本项目依托南亚热带次生常绿阔叶林模拟降水季节变化和氮添加野外控制实验平台，以碳同化多层模型(4C-A模型)整合基于树干液流测定值的冠层气孔导度和光合参数，估测优势树种的碳同化速率，结合树木生长量和异速生长方程估算净初级生产力，探讨模拟降水季节变化和施氮的单独和联合作用对优势树种固碳的影响规律。该方法区分冠层不同层段的阳叶与阴叶，克服以往模型研究未考虑冠层内部光照非均匀性的不足，以基于树干液流测定值的冠层气孔导度约束模拟结果，提高模拟精度。研究结果对认识森林固碳对环境变化的响应和预测全球变化情景下的区域碳储量，有明显的学术价值和现实意义。

降雨和氮的输入量和输入模式的变化会改变森林的碳同化能力，研究全球变化背景下氮沉降增加和降水季节变化对森林树木碳同化能力的影响机制，是阐明森林碳循环与全球变化因子相互作用的关键问题。本项目依托南亚热带次生常绿阔叶林模拟降水季节变化和氮添加野外控制实验平台，探讨模拟降水季节变化和施氮的单独和联合作用对优势树种碳吸收能力的影响规律。实验共包括4个处理：对照处理(C)，100 kg ha−1yr−1 N添加(N)，干季更干、湿季更湿(P)，干季更干、湿季更湿和氮添加(PN)。研究结果表明降水变化和氮添加处理均未对荷木(Schima superba)和火力楠(Michelia macclurei)叶片最大碳同化速率(Amax)和气孔导度(Gc)产生影响。然而，PN显著降低了荷木叶片的最大羧化速率Vcmax，降水变化和氮添加处理均未对火力楠Vcmax和最大电子传递速率Jmax产生影响。由于PN处理对荷木Vcmax的负面效应，PN处理亦对DBH相对生长率产生影响。在6月和12月，PN和P处理下荷木DBH相对生长率显著低于N处理，表明在大气氮沉降增加背景下，降水季节变化将降低6月和12月荷木的生长。降水变化和氮添加处理均未对荷木和火力楠的内在水分利用效率(WUEi)产生影响。综上，在降水季节变化和氮沉降增加的背景下，荷木的碳同化速率和固碳量在特定时段将受到负面影响，然而火力楠在降水季节变化和氮沉降增加情形下受到影响较小。此外，本项目改进了估算碳同化速率的碳同化多层模型(4C-A)，改进模型在初始模型的基础上，增加水汽压亏缺(VPD)对气孔导度的效应，同时考虑胞间CO2浓度(Ci)随着气孔行为的变化，改进模型显著提高了碳同化速率估算的准确性。项目执行过程中，发表SCI论文3篇。