植被对多年冻土的影响研究进展

Influences of vegetation on permafrost:a review

作为冰冻圈的主体,多年冻土是岩石圈与大气圈水热交换的产物,它的存在、分布及水热过程受到多种时空尺度环境因子的控制和影响。植被是生物圈的重要组成部分,是岩石圈与大气圈热量交换的媒介,它的存在和变化影响着多年冻土的水热过程和空间分布。文章综述了近几十年来植被对多年冻土影响的研究。首先,植被参与地气之间的水热周转过程,通过反射太阳辐射、贴地植被的吸水保水作用以及截留积雪作用等,对下伏冻土产生错综复杂的影响。但是不同植被类型的反射太阳辐射能力、保水与截留能力等各不相同,产生的影响大小有别。其次,同一植被类型的不同层次(如乔木层、灌木层等)对多年冻土的影响也不同,其中贴地植被产生的影响最显著。植被截留积雪,使得地面接收的太阳辐射和地表水分重分配复杂化,从而间接地影响多年冻土环境。因此当植被发生扰动后(如森林火灾和砍伐植被),就会对其所处的多年冻土环境产生各种不利影响,引发冻土灾害,甚至导致多年冻土消融。实际上植被与冻土同为寒区自然生态系统和环境的重要组成部分,它们在长期地质和生物演化中形成生态平衡。因此,植被还常常被用于指示多年冻土及其空间分布。最后提出目前研究中存在的问题,并对未来研究方向进行了展望。

As the main body of the earth cryosphere, permafrost is formed as a result of heat-moisture exchanges between the lithosphere and atmosphere. The occurrence, distribution and thermal regime of permafrost are affected by many environmental factors of varied spatiotemporal scales. As the medium, a participant and a buffer layer of heat-moisture exchanges between the lithosphere and atmosphere, vegetation and its changes significantly affect the hydro-thermal regimes and distributive features of permafrost. In this paper, the influences of vegetation on permafrost were systematically reviewed. Vegetation has relatively high albedo and capability for water holding and retention, which vary with different types of vegetation and have great impacts on the heat-moisture regimes of permafrost. In vertical structure, there are distinct layers for a given type of vegetation. Each layer has different influences on the thermal regime of permafrost, especially the ground cover layer. Compared to the area above crown canopy, which usually reflects and absorbs probably 40% of solar radiation, the crown canopy significantly affects net radiation and ground surface temperatures in forest. Vegetation, especially the canopy structure, considerably controls the snow accumulation and redistribution through the interception component of the water budget, wind speed, and solar radiation at the snow surface, resulting in a significant impact on the heat-moisture regimes of permafrost. Therefore, deforestation, such as for farming or fuels, may alter snow accumulation, albedo, surface coverage, and capability of water holding of vegetation. This may greatly modify the permafrost environment, induce many types of permafrost hazards, or even result in the thawing and eventually decaying of permafrost patches. Permafrost and vegetation are key components of ecological system in cold regions. They are interdependent and interactive, and have reached dynamic equilibria in the geological and biological evolutionary processes. In permafrost regions, specific vegetation types or their combinations usually present because of appropriate temperatures and moisture conditions. Therefore, the occurrences of specific plant species or a plant community can largely indicate the occurrence of permafrost. Finally, some problems unresolved in the research at present are identified in this paper, and the prospect for further study is also proposed.