全球变化背景下青藏高原多年冻土退化对高寒草地的影响研究

Over the last few decades, the alpine grassland degraded on some regions of the Qinghai-Tibetan Plateau (QTP). Some studies attributed it to overgrazing; while some others the degradation of permafrost. It is believed that degradation of permafrost would cause lowering of water table, and drying of surface soil, then retrogressive succession, and finally degradation. However, effects of permaforst degrdation on water are different on different degradation stages, and different on different regions. Recently, overgrazing has received great concerns, and several measures have been made to deal with this issue. Under the context of global change, permafrost on the QTP will inevitably degrade.It is vital to understand the responses of alpine grassland under different stages of permafrost degradation and on different climate regions.In this study, we propose to investigate the effects of permafrost degradation on alpine grassland by integrating field observation and experiments, remote sensing, and modeling with a process-based biogeochemical model, which is developed by us and widely used in the high latitute regions.We aim to clarify the mechanism of permafrost degradation from the perspective of water and energy, which are two important limiting factors of growth of cold region ecosystems; and to reveal rules of the responses of permafrost and alpine grassland to climate change; and to project the changes of both under different climate change scenarios in the next 100 yrs.Our results will help government make policies and laws on the eco-environmental protection on the QTP by providing scientific basis.

过去几十年，青藏高原部分地区的高寒草地发生了退化。有研究认为，过载过牧是高寒草地退化的主要原因；也有研究认为，气候变暖、多年冻土退化会导致地下水位下降，表层土壤变干，从而使高寒草地发生逆向演替，发生退化。但多年冻土退化对地下水的影响存在阶段性和空间差异性，因而对高寒草地的影响也应存在着差异，不能一概而论。目前过载过牧问题已经得到了重视，并采取了一系列的措施；而在全球变化背景下，多年冻土将不可避免地发生退化，其对高寒草地到底有着什么样的影响，是迫切需要解决的问题。本项目拟以野外实地观测和试验为基础，结合不同尺度的卫星资料和遥感反演结果，通过改进我们最新开发的、在高纬度地区广泛应用的生物地球化学过程模型，从植被生长的水、热限制因子的角度阐明多年冻土退化对高寒草地的影响机制；揭示二者对气候变化的响应规律；预估不同气候变化情景下二者的变化，为国家制定青藏高原生态环境保护政策提供科学依据。

本项目综合利用野外观测、遥感反演以及模式模拟等手段综合开展了多年冻土退化对高寒草地生态系统影响的研究。研究结果表明：1）在流域和青藏高原尺度，遥感反演结果表明多年冻土区高寒草地植被对气候变化的响应存在着空间差异，降水较多地区，多年冻土退化后植被会变好，而干旱地区，多年冻土退化会先缓解植被生长温度限制，促进植被生长后会导致植被变差；而到一定阶段后会出现水分限制导致植被退化；2）模式模拟结果也体现出多年冻土退化对高寒草地影响的空间异质性；3）敏感性试验结果表明温度升高会同时导致蒸散发加强和多年冻土退化，而多年冻土退化导致的土壤水分流失小于蒸散发加强导致的土壤水分流失。本项目可以为地方政府制定因地制宜的气候影响减缓政策提供科学依据，但同时，多年冻土退化对生态的影响还存在很多的不确定性，比如高原土壤有着和平原不同的质地，含有大量砂砾石，在持水、导热率和养分供给等属性上有着很大差异，从而给模式预估气候变化对多年冻土以及高寒草地影响带来很大的误差，需要在未来的研究中予以改进。