高寒草地净初级生产力对降水梯度的响应：新生物量分配假说的验证和机理解释

Tibetan Plateau possesses distinct vegetation dynamics and biomass allocation due to its unique geographical features, such as high elevation, barren soil and arid and cold climate. Precipitation, as one of the major limiting factors to alpine vegetation dynamics, is now experiencing dramatic changes in distribution patterns. As a result, the vegetation net primary productivity and allocation process become more complicated in response to the huge variations in precipitation. Based on the precipitation simulation in different stations and belt transect survey along the rainfall gradient, this project presents a comprehensive analysis on both aboveground and belowground net primary productivity (ANPP and BNPP) by using various methods, including carbon isotopic tracer method, in-growth core approach and soil coring method, in monthly scale. Multiple environmental factors that affecting net primary productivity (such as climate variables, soil properties, community compositions, plant conditions, and root characteristics) have been monitored, sampled, investigated and analyzed at the meantime. By acquiring high precision datasets of ANPP and BNPP, we verified the new biomass allocation hypothesis, and quantified the weights that assigned to ANPP and BNPP. Also, community economic spectrum that affecting changes in ANPP and BNPP is established to better understanding the biomass allocation mechanism of alpine grassland under varying precipitation patterns. In this project, we aim to compare the different responding processes and driving mechanisms of alpine grassland ANPP and BNPP to precipitation gradient, and the variations between different types of grassland.

青藏高原高海拔、干旱、寒冷和土壤贫瘠等自然地理特征，使该区域植被动态以及生物量分配过程非常特殊。作为高寒植被动态主要限制因素之一的降水，其分配格局正发生剧烈的变化，势必导致植被净初级生产力及其分配过程对降水变化的响应更为复杂。本项目基于站点尺度模拟降水实验结合自然降水梯度样带调查，开展多频次（每月）和多方法（碳同位素示踪法、内生长法和根钻法）地上和地下净初级生产力（ANPP和BNPP）研究。同时对多个环境因子（如气候要素、土壤性质、群落组成、植物状况和根系特征等）进行监测、调查、取样和分析。获取高精度ANPP和BNPP数据集，对新生物量分配假说进行验证，同时量化ANPP和BNPP分配的权衡值，进一步构建影响ANPP和BNPP变化的群落经济谱，解读降雨格局变化下高寒草地生物量分配机制。旨在比较研究高寒草地ANPP与BNPP对降水梯度的响应过程和驱动机制，以及不同草地类型之间的差异。

本项目以青藏高原高寒草地为研究对象，通过研究不同草地类型生物量与净初级生产力的差异，分析降水梯度下生物量（AGB和BGB）与净初级生产力（ANPP和BNPP）地上-地下权衡模式，量化不同草地类型地上-地下权衡变化，阐明对地上-地下生物量及净初级生产力权衡的驱动机制，为准确预测高寒草地植被动态变化，优化草地管理提供依据。主要成果如下：（1）AGB与ANPP存在显著差异，高寒草甸（AM）地上-地下净初级生产力均高于高寒草原（AS），且BGB对降水更敏感；（2）在降水梯度下土壤全氮、有机碳及含水量与群落加权平均值（CWM）呈现非线性响应，其中物种多样性（0.69）和叶干物质含量（0.42）调控AS生物量分配，叶干物质含量（0.48）和叶片碳含量（﹣0.45）调控AM生物量分配，且AM和AS生物量均向地上权衡，权衡值分别为0.203和0.088；（3）AM和AS净初级生产力均向地下权衡，权衡值分别为0.017和0.035。结构方程模型建模发现，年均降水量（0.50）、土壤容重（0.66）和速效氮（0.47）是影响AM生产力权衡的主导因子，而年均温（-0.32）和年均降水量（0.43）是影响AS生产力权衡的主导因子；（4）结合群落ANPP、植物碳含量数据与青藏高原的物候指标，发现低、中、高（0.15、0.31、0.84 g/m2/day）三种植物群落碳积累速率（CAR）模式。表明水热有效性通过调节土壤养分调控CARs，进而调节植物的生物量分配。同时，ANPP群落稳定性受到植物物种多样性和地下生态系统多功能性（BEMF）的显著影响。