Abstract: In forest,competition intensity can be expressed with the total basal area of breast-height or the number of trees per hectare as the degree of crowding neighboring trees.Competition plays a very important role in forest composition,structure and function,it influences the individual survival,population dynamics and physiological process of forest directly or indirectly.In this study,the goal was to determine a reasonable range of competition among individuals for the forest management and the research objective.The Pinus sylvestris var.mongolica plantations planted on sandy land in the Zhanggutai area,Liaoning Province were focused on,where the average annual precipitation was 518.24 mm during the period of 2005-2015.Four sample plots with different stand density were separately selected from the young P.sylvestris var.mongolica plantations (10 a) and the mature ones (40 a),the densities in A1,A2,A3 and A4 were 975,1 175,1 425 and 1 625 plants per hectare in the young forest,and those in B1,B2,B3 and B4 were 350,750,1 200 and 2 975 plants per hectare in the mature forest,respectively.The effects of competition on tree height growth of P.sylvestris var.mongolica plantations on sandy land were assessed with the methods of normality test and variance analysis.The results showed that the tree height of individuals met the normal distribution (α≤0.05) in young P.sylvestris var.mongolica plantations (10 a) and mature ones (40 a) on sandy land,meanwhile,the tree height of individuals met the normal distribution (α≤0.05) in eight different stand density sample plots.The distribution curves of individual tree height of all 10 a stand age sample plots were left skewed,the coefficient of skewness (g₁) was in an order of A1<A4<A3<A2<0,and the kurtosis coefficient (g₂) was in an order of A4<A2<A3<A1<0.Both the individual tree height and average tree height reached the maximums in competition of 1 425 plants per hectare.Moreover,there were significant differences between competition 1 175 plants per hectare and competition 975 plants per hectare but not in competition 1 625 plants per hectare (P≤0.05).The value of the skewness (g₁) kept an order of B1<B2<B3<0<B4,and the kurtosis coefficient (g₂) was in an order of B3<B4<B2<0<B1 in stand density.

Key words: competition, Pinus sylvestris var.mongolica, sand-fixation plantation forest, tree height growth