EFFECT OF EXCLOSURE AND TOPOGRAPHY ON REHABILITATION OF OVERGRAZED SHRUB-STEPPE IN THE LOESS PLATEAU OF NORTHWEST CHINA

The purpose of this study was to clarify the effect of grazing exclosu res on the recovery and rehabilitation of overgrazed steppe vegetation on varying slope aspects in the Loess Plateau of northwest China. The annual precipitation in the area studied was 400-480 mm. Soil samples were taken on nine slopes in the five-year exclosure and on five slop es outside the exclosure after a vegetation survey; they were then ana lyzed chemically. Mean number of species recorded per 0.25 m(2) was lo wer on the south-facing slope than all other slopes. The reverse trend was observed for aerial biomass. Species diversity estimated by infor mation content was higher in the grazing zone than in a 3200-ha protec ted zone within an exclosure. From species ordination by principal com ponent analysis, species with lower coverage in the grazing zone were Poa sphondylodes, Roegneria purpurascens, Hierochloe odorata, and Pote ntilla bifurca, which are all recognized as indicator species for reha bilitation efforts. In the soil surface layer, calcium contents were l ow, and the total contents of carbon and nitrogen were high on the nor th-facing slope in the exclosure. The protection by exclosure of overg razed steppe was seen to be effective because the accumulation of soil organic matter increased and water balance improved.