黄土高原植被-降水关系的临界现象及其在植被建设中的意义

Threholds in vegetation-precipitation relationship and the implications in restoration of vegetation on the Loesee Plateau,China

以退耕还林还草为主要内容的大规模的生态环境建设正在我国兴起,如何进行科学的植被配置规划,是一个急待解决的理论和实践问题。以自然植被初级生产力(NPP)表征潜在的即最大可能的植被条件,并与现在的森林覆盖率分布以及植被构成特征进行了比较。通过大量实测资料的分析,查明了降水条件对森林覆盖率和NPP的影响及其临界条件。研究表明,以NPP表征的天然植被特征值在黄土高原地区的分布具有显著的地带性。NPP与年平均降水量的关系中存在着两个临界点,分别代表着植被类型的变化,Pm=250mm可视为荒漠与草原植被之间的临界点,而Pm=480mm则为草原(森林草原)与森林(落叶阔叶林)之间的临界点。尽管次生的人工植被受人为因素的控制,由于存在着自然稀疏化过程,将使人工林地最终达到某种与水分承载能力相一致的平衡状态,自然条件如年降水仍然是决定其覆盖度的重要因素。在森林覆盖率与年平均降水量的关系中表现出一个临界点。当Pm<480mm时,森林覆盖率很低,且随年降水的增大而增大的速率很慢;当Pm>480mm时,森林覆盖率随年降水量而急剧增大。人为建造的水土保持植被,是一个自然的人工生态系统,也是一个自组织系统,它通过自我调节而趋向于达到某种平衡,建立一定的群落结构甚至于趋向于某种顶极群落,从而实现乔木、灌木、草本植物组成的最优组合。从这一原理出发,提出了一个基于实测资料进行统计分析的方法,来确定使乔木面积在乔木、灌木、草本总面积中的最优或较优的比例,得出了具体的数据。

The Loess Plateau region in China is an ecologically fragile zone, where large-scale vegetation restoration has been undertaken in recent years. How to determine the reasonable proportions of trees, shrubs and grasses is an issue important both in theory and in practice. The natural vegetation primary productivity (NPP) is adopted to reflect the maximum probable or potential vegetation conditions, and then it is compared with the present forest cover and the existing tree-shrub-grass compositions of the soil-water conservation vegetations. Based on data from more than 200 counties in the Loess Plateau region, the relationship between annual precipitation and forest cover percentage has been established, from which some thresholds have been identified. Natural vegetation indexed by NPP in the Loess Plateau region shows a clear zonal distribution. Two thresholds exist in the relationship between NPP and mean annual precipitation (P_m), corresponding to the change from one vegetation type to another. P_m=250 mm can be regarded as the threshold between desert vegetation and steppe, and P_m=480 mm the threshold between steppe and forest. Although the soil-water conservation vegetation is man made, some self-adjustment mechanism operates in this artificial vegetation system and leads the system to a balance between water supply by precipitation and water demand of vegetation. Therefore, the annual precipitation is still the major control on the cover percentages of the existing, mainly artificial forest. A threshold appears at P_m=480 mm; when P_m is lower than 480 mm, forest cover percentage (C_f) is very low and the rate at which C_f increases with P_m is also low. When P_m is larger than 480 mm, the rate at which C_f increases with P_m increases sharply. The man-made soil-water conservation vegetation can be regarded as a self-organized system. Through self-adjustment, it tends to establish some balance between water and plants, and thereby the optimal proportions of trees, shrubs and grasses in the system can be realized. Starting from this, a simple method based on empirical data has been proposed to determine the reasonable ratio of tree area to the total area of water-soil conservation vegetation including trees, shrubs and grasses. Using this method, some results have been given, which may propose some reference for the vegetation restoration planning in the Loess Plateau region.