The ecosystem on the Loess Plateau is seriously degraded due to soil and groundwater depletion. It is important to accelerate the recovery and restoration of degraded ecosystems in this area in order to ameliorate the regional environment and increase ecosystem productivity in Northwest China. The Ziwuling area contains zones of natural secondary forest and is important in maintaining regional climate and as a base for ecologically sustainable economic development in east Gansu and north Shaanxi provinces. The goal of this study was to quantify the successional series of plant communities in the Ziwuling area.
The area studied is located in the south of Ziwuling, and belongs to the Malan forest area in Xunyi county, Shaanxi Province, China, at 108°27′～108°52′E, 35°9′～35°33′N. It is about 40 km long from east to west, 43 km in width. There are various vegetation types in the region which have different disturbance histories. We selected areas last disturbed at different times, using the space series to replace time courses to capture a range of successional stages. We endeavored to ensure consistency of environmental conditions in the selection of sampling sites. We established 15, 10 and 21 sampling plots for investigating herb, shrub and tree communities respectively. The youngest communities are about 2～5 years post-abandonment and the oldest communities are about 130～150 years post-abandonment. The successional series was described using a combination of numerical classification and ordination. The dynamics of species composition, life form group composition, litter and humus thickness were compared across the successional series.
The plant community successional series were divided into nine stages in this area with a trend over time from herb- to shrub- to tree-dominated communities. The climatic climax was a Quercus liaotungensis community. Artificial reforestation accelerated the succession process and created alternate successional paths beginning with Robinia pseudoacacia or Pinus tabulaeformis communities. Over time, the number of families, genera and species increased.
The proportion of annuals in the early successional stages was higher than in the middle and late stages. Perennials, especially woody plants, became dominant in the middle to late stages. Perennial grasses and legumes maintained a constant ratio in the entire successional series. The species richness of every life form group increased and the compositional proportion became more consistent over time. Litter and humus thickness increased over successional time, however, the ratio of litter to humus differed among successional stages. Artificial and natural secondary forests had some differences in species composition, life form group composition, and litter and humus thickness.