Diameter structure and understory diversity in Chinese pine plantations in Beijing low mountain areas.

In order to examine the basic site conditions needed for the normal growth of Chinese pine plantations and maintenance of high biodiversity, 97 plots of Chinese pine plantations and 134 shrubherb subplots were established in the Beijing low mountain areas. The diameter of pine trees and species diversity were measured under four different site conditions. The results showed that diameter structure of Chinese pine plantations showed a normal distribution at the north and northeast slopes with deep soils, while the reverse Jshaped diameter distributions appeared in the other three site conditions, i.e. north and northeast slopes with shallow soils, south and southwest slopes with deep or shallow soils. Plant species diversity was high under the canopy of Chinese pine plantations; there were 63 species, including 33 shrub species and 30 herbaceous species. Overall, species richness at the north and northeast slopes were significantly higher than those at the south and southwest slopes. The dominant species in the shrub layer were Grewia biloba var. parviflora and Vitex negundo var. heterophylla, while those in the herb layer were Rubia cordifolia, Oplismenus undulatifolius, Leonuru japonicus and Arthraxon hispidus. The Patrick index, Simpson index, ShannonWiener index and Alatalo index of the understory vegetation were higher in the Chinese pine plantations at north and northeast slopes with deep soils than in the other sites investigated.