The natural restoration of woodland is important to explain the ecological process in forest ecosystem. It is significant implications to assess degraded woodland dynamics and ecological effects under artificial fostering approach in theoretically and practical for forest biodiversity conservation and sustainable management in semiarid mountains.
Study site was located in the Han Quangou basin forest distribution zone in the Qilian Mountains of northwestern China. In this area, the wood of Picea crassifolia has been depleted after logging or grazing disturbance. The wood vegetation had been degraded and changed to shrub, grassland and scattered Betula-Populus secondary forests. In 2001 year, we have established 4 fixed observation plots (10m×10m) along the elevation 50m interval range from 2468m to 2736m. In each plot, we have chose 3 grids (4m×4m) for shrubs, 6 grids (1m×1m) for grasses, 2 grids (1m×1m) for soil. In August, we have investigated vegetation community characteristics including species number, community coverage, plant density, community height, species frequency and wood diameter at breast height (DBH>1cm) in different years woodland (from 2001 to 2008). The soil physical-chemical properties of investigated plot including soil bulk density, soil water content, soil organic carbon and total nitrogen were determined in indoor laboratory. In the study, changes in the species composition and relative important value (IV) of main species, community structure, species diversity and soil properties were analyzed by methods of variance analysis.
The results showed that after human disturbance was forbidden, the habitation of woodland became well gradually within 8 years. From 2001 to 2008, the number of family, genus and species of plant community increased apparently. In the process of community succession, former species was replaced by the subsequent species frequently. The Betula platyphylla and Populus davidiana gradually become the constructive species. With shrubs and trees growing up, the vertical height increased, and community structure divided different layer from ground to community's top crest. From 2001 to 2008, biodiversity index of community increased. Patrick, Shannon-Wiener and Simpson indexes were different at different layer of community, and the order of three index among different layers (from high to low) was herb layer (HL), shrub layer (SL), and tree layer (TL). However, the order of Pielou index was tree layer (TL), shrub layer (SL) and herb layer (HL). Soil bulk density (SBD) decreased from (1.21±0.06)g/cm³ to (0.73±0.04)g/cm³. Soil water content (SWC), soil organic carbon (SOC) and total nitrogen (TN) increased with the woodland restoring.
Fencing and forest ban can restore the degraded woodland vegetation in northern slope of Qilian Mountains. Beside the vegetative form, the key species (B. platyphylla and P. davidiana) and the improving of soil moisture and nutrient, which were the main factors of succession process of degraded woodland from herbage community to trees community. When the community was at the stage of mixed vegetation, the species composition, community structure, and diversity were more complex than those in other stages. At the same time the soil quality was improved by vegetation restoration ecological. Therefore, adaptation ability of vegetation was high in the mixed vegetation stage of shrub and trees, which showed a good restoring effect.