Grassland desertification by grazing and the resulting micrometeorological changes in Inner Mongolia

Overgrazing is one of the most primary causes of desertification in semi-arid zones of China. From 1992 to 1994 we conducted a grazing experiment in Naiman (lat. 42 degrees 58'N, long. 120 degrees 43'E, 345 m asl), Inner Mongolia, China to elucidate desertification mechanisms. Our experimental field, which was covered with some short grasses, legumes and forbs, included four plots where grazing sheep numbers differed (0, 3, 6, or 9 individuals). Micrometeorological measurement data were analyzed using the Bowen ratio energy balance method. Ground surface reflectivity (albedo) increased with grazing intensity. The overgrazed plot had been desertified after 3 year's grazing experiment. Albedo can be regarded as an important indicator of potential grassland desertification. When desertification occurs, then albedo will exceed a critical value. Partitioning of net radiation exhibited distinct patterns among the four plots as a result of the interplay between albedo and in situ vegetation. Ratios of the net radiation or net available radiation to solar radiation tended to decrease with increasing grazing intensity. Bur this pattern of changes was largely affected by soil moisture regime and vegetation status during the measurements. Grazing also remarkably affected wind regimes over the grazing experimental plots through altering surface roughness. Especially, sustained overgrazing decreased surface roughness length so substantially that wind could act directly on sandy grassland surface to initiate desertification. A permissible grazing capacity of the studied temperate semi-arid grassland was also proposed for future grassland management. (C) 2000 Published by Elsevier Science B.V. All rights reserved.