干旱与半干旱地区苔藓植物生态学研究综述

A Review on the Studies of Bryophyte Ecology in Arid and Semi-arid Areas

苔藓植物一般生活在阴湿的地方 ,但也有不少种类具有极强的耐旱性 ,在干旱和半干旱地区也有分布。在干旱和半干旱地区 ,影响苔藓植物分布的环境因子包括降水、p H值、Ca CO3 含量、植被盖度、有机质含量以及土壤质地等 ,其中起重要作用的是降水。在干旱环境下 ,苔藓植物作为环境演替过程中主要的先锋植物 ,还参与土壤结皮的形成 ,在生物防沙、固沙和生态小环境的改善中发挥着不可替代的作用。

The common habitat of bryophytes is restricted to dankness, but some bryophytes, such as the species of Pottiaceae, Grimmiaceae, Bryaceae and so on, are able to survive in arid and semi-arid areas with strong ability of drought tolerance. These bryophytes, in this sense, have to tolerate great heat and drought. Arid and semi-arid environments often support a low cover of vascular plants, which provide a niche for non-vascular plants such as mosses, lichens and liverworts. Under the pressure of extreme desiccation, bryophyte species have developed some morphological and ecological characteristics to adapt to arid environments. Generally, there are two life strategies for them to adapt to dry conditions, i.e. drought-avoidance and drought-tolerance. In order to reduce their water loss, they grow in tuft or cushion forms, which enable them to enhance ability of water retention and reduce air movement on their leaf surface. As a result, the evaporation then can be decreased. Their blades curve intensely and attach to stem when environment conditions become dry so that they can avoid great evaporation and prevent themselves from being damaged by exposure to strong sunlight. Additionally, they have white seta on top of blades, through which the plant can reflex strong sunlight. It was confirmed by experiments that certain bryophyte species possessing seta, such as Grimmia pulvinata, Tortula intermedia and so on, lost less water than other species having no seta on top of blade under similar environment conditions. Ecologically, a number of environmental factors influence the survival of bryophytes in arid and semi-arid areas. Due to their poikilohydric nature, one of the most critical of these is external water. In absence of water, photosynthesis ceases and the plants become dormant. Apart from influence of water, the distribution of bryophytes in arid and semi-arid areas are related to total annual rainfall, soil pH, calcium carbonate level, plant cover, texture, organic carbon and soil texture. Rainfall is a primary determination of plant cover, which in turn affects light, nutrient availability and therefore the distribution of bryophytes. It also can be inferred from literatures that diversity and density of bryophyte populations are much lower in deserts than that at higher elevations in nearby mountain areas. As the diversity and density of bryophyte populations increase with elevation, the number of species comprising an aggregate is also expected to increase. Meanwhile, being the pioneer plants, they involve in the process of establishment of soil crust, which has great importance in arid environment. They play an important role in desert ecosystem such as indicating of vegetation type, soil-holding, sand fixation and so on. In arid and semi-arid areas, the surface of desert may become stabilized biologically to give a softer and more permeable layer that acts as a basis for further plant colonization. The first stage is infiltration and covering with a weft of algae. Then the next stages in crust stabilization are made by bryophytes. On this type of stabilized crust, especially where there is some shade from fall sun, the bryophytes of arid regions often can reach their greatest development, with what can be a rich bryophyte flora. Ecologically, such kind of flora is of significant ecological importance in arid areas, especially in those areas where the environment problems become increasingly serious. That is why desert bryophytes, especially those species involved in crust formation, can absorb attentions of bryophyte scientists nowadays.