Influence of variation of soil spatial heterogeneity on vegetation restoration

摘要

Numerous hypotheses and conceptional models dealing with the grassland desertification or degradation processes recognize that the invasion of shrubs in grasslands is the most striking feature of the variation of vegetation patterns in the grassland degradation or desertification processes in arid and semiarid regions. This is because the invasion of shrubs in grasslands increases the heterogeneity of the temporal and spatial distribution of primary vegetation and soil resources. As a result, the biological processes of the soil-vegetation system are increasingly concentrated in the "fertile islands" under shrub canopies, and the soil between shrubs gradually turns into bare area or moving sand under the influences of prolonged wind and water erosion. Most of relative researches support this bio-ecological interpretation for the degraded process of grassland. However, as viewed from the other aspect, the shrub vegetation distributed in patches also serves as the "trigger spots" for the grassland restoration or desertification reversion, and this has been demonstrated by the practices of combating desertification in China. Nearly 50 years of succession of artificial sand-binding vegetation in the Shapotou area and the regional restoration of eco-environment are the theoretical verification and successful example for the desertification reversion. The establishment of artificial vegetation in the region began with the installation of sand fences and planting xerophytic shrubs relying on less than 200 mm of annual precipitation under the non-irrigation condition, this made the moving sand, an originally uniformly distributed soil resource, occur the variation of spatial heterogeneity. Through the redistribution of precipitation and dustfall by the canopy of xerophytic shrubs, litter accumulation and cryptogamic crust development, soil-forming processes under shrub canopies were accelerated; in the meantime, it also created a favorable condition for the invasion and colonization of annual and perennial plant species. However, with the depletion of soil moisture in the deep layer in the sand stabilization area the coverage of shrubs in the sand-binding vegetation lowered from the highest value of 33% to 6%, the dominant position and leading effect of shrubs in the communities were weakened, furthermore they were gradually taken out from the vegetation composition. This correspondingly weakened the spatial heterogeneity of soil resource distribution. The propagation of numerous cryptogams on fixed sand surface and the colonization of annual and perennial plant species promoted the succession and restoration of the vegetation towards herb-dominated vegetation, which are similar to the primary vegetation types of the adjacent steppified desert and desert steppe. This paper, taking nearly 50 years of succession of sand-binding vegetation in the Shapotou region as an example and using the geostatistical method, puts forward and explains the conceptional model of vegetation restoration or desertification reversion of grassland in arid zones.