Diameter Distribution Forest Types On The Southern Slope Of Qinling Mountains

Diameter distribution of pine-oak forest on southern slope of Qinling Mountains was studied. 127 rectangular sample plots have been established in Huoditang, Xunyangba and Xinjiashan forest farm from 2012 to 2014. Weibull distribution, the negative exponential distribution, inverted J-shaped distribution and Logistic distribution model were used to simulate the diameter distribution under the variety of dominant species, plant density and altitude conditions. χ2 test method was used to test the simulating effect of four kinds of probability density functions.This study is not only an attempt to theory discussion, but also the need of practices, for better understanding of the diameter distribution for pine-oak forest on southern slope of Qinling Mountains, and providing references for forestation, thinning design and forest planning. The main results were as follows:(1)Diameter distribution of pine oak forest on southern slope of Qinling Mountains was J-shaped distribution, which close to ideal distribution of diameter grade. Using the Weibull distribution, the negative exponential distribution, inverted J-shaped distribution and Logistic distribution of four kinds of probability density function, almost 117 plots were individually fitted. The results show that Logistic distribution function was fitting the worst, the best one was Inverted J-shaped distribution. The Weibull distribution fitting effect inferior to inverted J-shaped distribution, while a negative exponential distribution fitting results inferior to the Weibull distribution.(2)Using the parameters estimated, combining stand characteristics factor, plots characteristic factor and stand diameter distribution factor, number parameters estimated equation of inverted J-shaped distribution function which was well fitted on the south slope pine oak forest have been built. The independent plots data was used for chi-square test, 80% plots pasted, indicating that the parameters estimated equation can be used to predict diameter distribution of pine oak forest on southern slope of Qinling Mountains.(3)Stand diameter distribution of different dominant tree species showed inverted J-shaped. Using the Weibull distribution, the negative exponential distribution, inverted J-shaped distribution and Logistic distribution of four kinds of probability density function, 15 Pinus tabulaeformis plots, 30 Pinus armandii plots and 61 Quercus aliena var.acuteserrata plots were individually fitted. It was showed that inverted J-shaped distribution fits best effect, which was superior to Weibull distribution. Negative exponential distribution fitting effect was inferior to the Weibull distribution, while the worst fitting effect was Logistic distribution function.(4)The impact factors of DBH distribution: the larger stand density, the more trees of the stand, in particular the more small-diameter trees(about 5cm) of forest; The larger stand average diameter, the fewer trees of stand, indicating that the larger average diameter of stand, the large competition among trees, resulting in the fewer of trees within a stand.(5)The effect of different altitude on DBH distribution: the higher altitudes of a stand(>1700m), the more small-diameter trees of stands. Because the less human disturbance on stand environment, the better forests update, the more small-diameter trees of stands.(6) DBH distribution of different site conditions of Pinus armandii and Quercus aliena var.acuteserrata forest: The better quality of the site form, the more trees in the stand, especially the small-diameter trees(about 5cm), indicating that the well quality of stand site was good for trees update. With the site quality increased, the number of Pinus tabulaeformis was gradually increasing, except the ridge and other places of poor quality site which was more small-diameter trees of Pinus tabulaeformis.(7) DBH distribution of different species:Pinus tabulaeformis have presented normal distribution, Pinus armandii have showed waning of the left side of the "mountain-like" shape, and Quercus aliena var.acuteserrata have showed stable inverted J-shaped distribution.