Structure Characteristics of Pinus massoniana Natural Secondary Forest at Different Altitude of Enshi

The Pinus massoniana natural secondary forests of different altitudes in the state of Enshi from different altitudes were taken as the research objects, and the upper limit exclusion method was chosen to set up 4 elevation gradients:the behind the mountain of Hubei institute of nationalities (T₁), Lotus pond village in Enshi City (T₂), Jixin village in Enshi City (T₃), Mao Jia yard in Lichuan City (T₄), 4 stands of 20 m×20 m were bulit at each elevation gradient using total station instrument. The stand structure characteristics were investigated in each plot and the collected data were analyzed using spatial structure and non-spatial structure indicators such as uniform angle index, mingling index, neighborhood comparison, species composition and diameter distribution. Results show that there are many small diameter classes trees in the P. massoniana natural secondary forest from different altitudes, and exhibit good natural regeneration. The width of the diameter distribution is highly significant positively correlated with the elevation height. The average uniform angle indices of different forests are tending to be distributed randomly as the altitude rises, which is in a middle state. From T₁ to T₃, there is a highly significant positive correlation between the elevation and the uniform angle indices of the different forest stands. The average mingling of each forest at different altitude is low in general, and is between weak mixed and moderate mixed degree. The relationship between the elevation and mingling is highly significant positively correlated of the P. massoniana natural secondary forest stands from the altitude T₂ to T₄. The overall average neighborhood comparison of each forest is about 0.5, which is in a middle state. From T₂ to T₄, there is a highly significant positive correlation between the elevation and the neighborhood comparison of the different forest stands. As the altitude rises, the coniferous trees of natural secondary forests such as P. massoniana, Cunninghamia lanceolata and Cupressus funebris become the dominant species.