Elevational patterns of structures and diversities of coniferous forests in Helan Mountain.

As a typical ecozone,Mt.Helan locates in the western boundary of Pinus tabulaeformis forests and the eastern boundary of Picea crassifolia forests in China.A field survey on the coniferous forests was conducted,with selective elevational gradients and systematic sampling plots.Results show that the mean heights of trees and the total trunks’ area at breast height increase with elevation,and the trees in large diameter classes occupy a large proportion in the species composition,indicating that the coniferous forests grow better at higher elevations.However,the species richness declines with elevation,and there is a gradual change in species composition.The soil depth,organic matter and total nitrogen content increase with elevation,while the pH value decreases.The changes in soil features with elevation result in the ecological gradients influencing the coniferous forests in the study area,which indicates that the climatic gradient is associated with elevation.