Grassland desertification by grazing and the resulting micrometeorologicalchanges in Inner Mongolia

Overgrazing is one of the most primary causes of desertification in semi-ar id zones of China. From 1992 to 1994 we conducted a grazing experiment in N aiman (lat. 42 degrees 58'N, long. 120 degrees 43'E, 345 m asl), Inner Mong olia, China to elucidate desertification mechanisms. Our experimental field , which was covered with some short grasses, legumes and forbs, included fo ur plots where grazing sheep numbers differed (0, 3, 6, or 9 individuals). Micrometeorological measurement data were analyzed using the Bowen ratio en ergy balance method. Ground surface reflectivity (albedo) increased with gr azing intensity. The overgrazed plot had been desertified after 3 year's gr azing experiment. Albedo can be regarded as an important indicator of poten tial grassland desertification. When desertification occurs, then albedo wi ll exceed a critical value. Partitioning of net radiation exhibited distinc t 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 radia tion to solar radiation tended to decrease with increasing grazing intensit y. Bur this pattern of changes was largely affected by soil moisture regime and vegetation status during the measurements. Grazing also remarkably aff ected wind regimes over the grazing experimental plots through altering sur face roughness. Especially, sustained overgrazing decreased surface roughne ss 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 grasslan d management. (C) 2000 Published by Elsevier Science B.V. All rights reserv ed.