Distribution characteristics of tree size of Pinus sylvestris var. mongolica nature forest on sandy soil

This study aims to analyze the univariate distribution and bivariate distribution characteristics of DBH and height of Pinus sylvestris var. mongolica nature forests on sandy land at Honghuaerji of Inner Mongolia of northern China in order to provide scientific basis for their sustainable forest management. Two 1 ha Pinus sylvestris var. mongolica nature forest plots with different densities(940 and 1 149 plant/ha) were established and all trees with DBH>5 cm were investigated. Density distribution characteristics of DBH and height and their bivariate distribution were analyzed on whole stand level and on different categorized tree height size classes, and the statistical calculation was implemented in R language software. The results showed that:1) on the whole stand level, the distributions of DBH and height of tree in two plots generally showed double-peak curve. The average DBH in plot 1 and plot 2 was basically the same while average height in plot 2 was greater than plot 1, but number of large size class trees both in terms of DBH and height in plot 1 was greater than plot 2. Analysis based on different categorized height layers showed that distributions of DBH and height of overstory trees were unimodal distribution in both plots and distribution of DBH in plot 1 was normal distribution. Average diameter of overstory trees in both plots was basically the same but average height of overstory trees in plot 2 was greater than plot 1, and number of overstory trees in plot 1 was more than plot 2. Additionally, distributions of DBH and height of understory trees in two plots showed unimodal distribution and the distribution of DBH in plot 2 was normal distribution. Average diameters of understory trees in plot 1 and plot 2 were basically the same but average height of understory trees in plot 2 was more than plot 1, and large trees of overstory trees in plot 2 were also more than plot 1. 2) The bivariate distributions of DBH and height in two plots were multi-peak distribution. Overall, the ratio of large tree size class was more than small tree size class in plot 1, but this situation was opposite in plot 2. Meanwhile, the ratio of height to DBH of large tree size class was less than small tree size class in plot 1, but the ratio was almost the same in plot 2. From different height layer analysis, the ratio of large tree size class was more than small tree size class among overstory trees in the two plots, and the ratio of height to DBH of overstory trees was not significantly different between large and small tree size classes. The ratio of large tree size class was greater than small tree size class among understory trees in plot 1, but this situation was opposite in plot 2, and the ratio of height to DBH of large tree size class was less than small tree size class of underwood in the two plots. Stand density had no obvious effect on average diameter in Pinus sylvestris var. mongolica nature forests. High density can elevate the growth of tree height of whole stand, but number of dominant trees of whole stand decreased in high density stand, especially for overstory trees. In the meanwhile, stand density had effect on the distribution of energy in the radial and longitudinal growth of trees, and the biomass was always distributed to tree height growth in the high density stand. Therefore, high density forest should be properly thinned in order to decrease intertree competition, which can allocate tree biomass reasonably and improve the proportion of dominant trees in whole stand. For low density stands, the forest should be taken properly above story thinning in order to improve the growth of understory trees.