How long should the fully hillside-closed forest protection be implemented on the Loess Plateau, Shaanxi, China?

BACKGROUND Restoration of degraded forest ecosystem is crucial for regional sustainable development. To protect the country's fragile and fragmented environment, the Chinese government initiated an ecological engineering project, the Natural Forest Protection Program, in seventeen provinces in China beginning in 1998. Fully hillside-closed forest protection (vegetation restoration naturally without any artificial disturbance) was one of vital measures of the Natural Forest Protection Program applied nation wide. Whether plant diversity, biomass and age structure of dominant tree species and soil nutrients in protected stands may become better with increase of protected period are still open problems. METHODS We investigated community diversity, biomass of dominant tree species, age structures, and analyzed soil chemical properties of a Pinus tabulaeformis population at protected sites representing different protected ages at Huanglongshan Forest Bureau on the Loess Plateau, Shaanxi, China. RESULTS Plant species richness of Pinus tabulaeformis community was significantly affected (p < 0.05) by forest protection and the effect attenuated with protection age. Shannon evenness index of plant species generally increased with protection age. Stands protected for 45 years had the highest tree biomass and considerable natural regeneration capacity. Contents of organic carbon, available phosphorus and available potassium in top soil increased in protected stands less than 45 years, however decreased significantly thereafter. Long-term forest protection also decreased the content of mineral nitrogen in top soil. DISCUSSION We found that the richness of shrubs and herbs was significantly affected by forest protection, and evenness indices of tree, shrub and herb increased inconsistently with protected ages. Forest protection created more complex age structures and tree densities with increasing age of protection. Content of soil mineral nitrogen at 0-20 cm soil depth showed a decreasing trend in stands of up to 30 years. Soil available phosphorus and potassium contents were higher in stands with greater proportions of big and medium trees. Long-term protection (>45 years) of Pinus tabulaeformis stands in southeast Loess Plateau, China, may be associated with decreasing plant species richness, proportion of medium to large trees, dominant biomass of Pinus tabulaeformis and soil nutrients.