不同放牧压力下草地微气象的变化与草地荒漠化的发生

Micro-meteorological changes and grassland desertification due to overgrazing in Naiman, Inner Mongolia, China

过度放牧是我国半干旱地区荒漠化的主要成因之一。作者在内蒙古奈曼进行了草地放牧试验,目的在于探明荒漠化的发生机制。试验分为4个小区,即无牧区、轻牧区、中牧区和重牧区,每个小区的牧羊数分别为0,3,6和9。从1991至1994年,在试验区进行了微气象观测,并运用波文比热量平衡法和空气动力学梯度法分析了观测数据。结果表明:1)地表反射率随放牧强度的增大而增大,如过牧小区的反射率已由1992年的0.20增加到0.28,并出现明显的荒漠化。反射率是草地荒漠化发展程度的重要指标之一。2)在4个放牧试验小区,净辐射的分配格局存在明显差异,这是地表植被与反射率差异相互作用的结果。随放牧强度的增加,净辐射或净有效能减少,而且潜热交换的贡献也相对减少。牧羊的过度践踏也影响地表的热收支。3)不同放牧小区的风廓线形式不同。在过牧试验小区存在较强的风切应力作用于地表,产生风蚀,出现荒漠化。4)根据试验和植被的动态,提出该地区现有草地生长季的安全载畜量为3～4个羊单位/hm2。

Overgrazing is one of the primary causes for desertification in the semiarid zone of China. A grazing experiment in Naiman (N42°55′,E120°42′,405m asl), Inner Mongolia, China was conducted to elucidate the desertification mechanisms, especially the initiation of desertification of grassland resulted from overgrazing. The experiment field included four plots, i.e., non grazing, lightly grazing, moderately grazing and over grazing plots, in which grazing sheep number differed from 0,3,6, and 9, respectively. Micro meteorological elements were observed at these plots. The obtained data were analyzed using the Bowen ratio energy balance method and aerodynamic gradient method. The results show that:1)Ground surface reflectivity (albedo)increases with grazing intensity, e.g., albedo of the overgrazing plot has increased from 0 20 at the beginning of the experiment in 1992 to 0 28 in 1994. The overgrazing plot has been desertified. Thus, albedo becomes an important indicatior of grassland desertification. 2)Partitioning of net radiation exhibits distinct patterns among the four plots that is the results of interplay of albedo and in situ vegetation. The net radiation and contribution of latent heat flux tend to decrease due to increasingly grazing intensity. Heavy trampling by sheep also affects heat budget over the ground surfaces. 3) Wind profiles above the ground of each plot have different structures. Much stronger wind shear on the overgrazing surface is a significant driving forcing resulting in intense sand flow activities on which desertification processes are based. 4) Safe grazing capacity of the studied grassland during growing season is proposed to be 3～4 sheep or sheep equivalents per hm 2. according to the experiment and the plant community dynamics.