藏北高寒草甸生态系统碳通量变异的多重控制机制分析

The alpine meadow ecosystem in northern Tibet is quite sensitive to climate change and human activity. Therefore, it is an important indicator of potential change of the carbon budget of terrestrial ecosystems under global change context. To deeply investigate the control mechanisms of the variability of carbon balance in this alpine meadow ecosystem, the effects and interactions of climatic factors，biotic factors and grazing should be took into consideration. This project aims at revealing the effects and controlling mechanisms of these three types of controlling factors by combining statistical and observation methods. Firstly, the paralleled eddy covariance method and the statistical approach will be adopted to separate the sources of variability of carbon fluxes into climatic effects, biotic effects and grazing effects. Secondly, the effects of these factors on carbon fluxes on different time scales will be analyzed. The lag effects of climatic factors will also be considered. Additionally, the indicators and indexes which can represent the biotic and grazing characteristics of the ecosystem will be collected and observed to reveal the effects of the biotic factors and grazing. Finally, we will do a comprehensive analysis of climatic, biotic and grazing effects on carbon fluxes by using structural equation model. This project aims at indicating the effects and control mechanisms of the three types of factors on carbon fluxes and demonstrating the responses and adaptability of this alpine meadow ecosystem to climate change. These results will help the scientists predict the potential change of this ecosystem under climate change context. In addition, they would help the governments make proper polices to manage this frail ecosystem. Furthermore, this will provide a theoretical guideline for recovering the alpine meadow ecosystem in northern Tibet.

藏北高寒草甸生态系统对气候变化和人类活动的响应十分敏感，对全球变化背景下陆地生态系统碳收支的变化特征具有指示作用。要深入剖析该生态系统碳通量变异的控制机制，需要同时考虑气象因子、生物因子和放牧这三类驱动源的控制作用及其相互作用关系。本项目将围绕藏北高寒草甸生态系统碳通量变异的控制机制这一关键科学问题展开研究：首先，借助于涡度相关平行观测和统计方法，实现对气象因子、生物因子和放牧控制作用的分离；进而，借助多时间尺度分析、时滞效应分析，并结合物候观测、群落调查等手段分别对三类因子的控制作用进行深入研究；最后，借助结构方程模型等手段从整体上整合并完成气象因子、生物因子和放牧对藏北高寒草甸生态系统碳通量变异控制机制的综合评价。课题旨在阐明高寒生态系统对气候变化和人类活动的响应及适应特征，有助于在未来全球变化情景下对高寒脆弱生态系统做出准确合理的预测和管理，并为藏北草地生态系统恢复提供理论指导。

藏北高寒草甸生态系统碳收支是陆地生态系统碳循环的重要组成部分，其对全球变化响应十分敏感，对未来陆地生态系统碳收支的变化特征具有指示作用。本项目围绕藏北高寒草甸生态系统碳通量变异的控制机制这一关键科学问题，借助涡度相关平行观测技术，结合野外调查实验，应用统计拆分方法，分别对气象因子、生物因子和放牧对藏北高寒草甸碳通量的控制作用进行了深入分析。同时系统地研究了气象驱动、生物驱动和放牧驱动之间的相互作用关系，解析三大驱动源对藏北高寒草甸的共同驱动作用。研究发现：气象因子中，水分是影响藏北高寒草甸碳通量最重要的因子，具有绝对控制力。水分限制可以影响碳通量对温度的正响应，而好的水分条件可有效缓解低温胁迫。随着时间尺度的延长，气象因子控制力逐渐变弱，生物因子控制力逐渐增强。在年尺度上，叶面积指数是藏北高寒草甸碳通量的主要控制因子。然而，放牧可以直接改变叶面积指数，这将在年尺度上直接影响生态系统碳通量。同时放牧也改变了微气象环境，在短时间尺度上，同样对碳通量有重要影响。适度放牧可提高生态系统的光合能力。同时，放牧可通过草毡层的保水隔水性，提高该生态系统土壤水分含量，使该生态系统在干旱季节可维持更高的生产力。综上所述，高寒草甸碳通量受气象因子和生物因子共同驱动，且二者之间存在负反馈作用。放牧直接影响生物驱动力，间接调控气象驱动力，促进了生态系统的碳水循环。因此，适度放牧有利于保持藏北高寒草甸生态系统的稳定性，提高了其适应全球变化的能力，符合可持续发展战略要求。