Influence of desertification on vegetation pattern variations in the cold semi-arid grasslands of Qinghai-Tibet Plateau, North-west China

In arid and semi-arid grassland, many models hypothesize that desertification leads to the replacement of grassland by shrubland vegetation; however, the theoretical interpretations are open to debate. Therefore, a field study was conducted in the Guinan desertified grassland of the Qinghai-Tibet plateau, North-west China, to test the hypotheses on a regional scale. We used four field sites to represent the four stages of desert development: slight, moderate, severe and very severe. A total of 40 quadrates were investigated in each site. Plant coverage, above-ground biomass, species richness and life-form were recorded; species diversity was calculated using the Simpson index and soil parameters were also measured. Our results indicate that the proportion of silt decreases from 12% in the slight stage to 1% in the very severe stage, clay from 71% to 42% and sand from 17% to 93%. Soil water-holding capacity clearly decreases from the slight to the very severe stage. Soil organic matter (OM) is also reduced with desert development, which leads to destruction of the stability of soil physical structure and nutrient content, such as progressive N, P and K loss in surface and subsoil layers. In response to changes in soil properties, vegetation altered as regards species composition, species diversity, coverage, structure and life-form. Consequently, with desert development, herbaceous species, especially grasses, were lost from the community composition and replaced by xerophytic shrubs or semi-shrubs. Finally, psammophytic annual plants-dominated vegetation composition, while shrub maintained a low coverage. Although our results partially support previous hypotheses at the regional scale, it is considered that, apart from soil texture, soil OM and nutrients are the main factors mediating the dominance balance between shrub and herbaceous species.