Abstract: [Objective] The diameter distribution and its changing law at storeys of typical natural broad-leaved forest in mid-subtropical zone was studied to provide the theoretical basis for revealing the structure, assisting the investigation of relevant stand description factors and directing the management planning of natural forests.[Method] Based on storey identification of maximum light receiving plane(MLRP) method, the Shapiro-Wilk(S-W test) test was used to measure the normality of the diameter distribution of storeys(including the whole stand and each storey); the Skewness(SK) and Kurtosis(KT) were used to describe the characters of the diameter distributions; the Meyer negative exponential function and the Weibull distribution function were served to fit the diameter distribution of each storey; the diameter distribution of each storey was charted by the model with better result of Chi-square test, and the changing law among storeys were analyzed.[Result] The result of S-W test showed that all the whole stand, the substorey Ⅲ and the substorey Ⅱ of the typical natural broad-leaved forest in mid-subtropical zone were not normal distribution. As to the substorey Ⅰ, the diameter distribution of sample plot 1, 2 and 3 were the normal distribution, but the substorey Ⅰ of sample plot 4 and 5 were oppo; within each sample plot, the value of S-W test increased with the increasing substorey height. In general, the absolute value of SK and KT decreased with the height of substorey increased, except the SK and KT of the substorey Ⅱ and Ⅲ in sample plot 5 and the KT of the substorey Ⅰ and Ⅱ in sample plot 2. The Meyer negative exponential function could not fit the diameter distribution of the whole stand(all the sample plots disobeyed the Chi-square test) and substorey Ⅱ(the sample plot 2 and 4 obeyed, and sample plot1, 3 and 5 disobey)well, but it could fit the substoreys Ⅰ(all sample plots obeyed) and Ⅲ(sample plot 3, 4 and 5 obeyed, and sample plot 1 and 2 disobey) well. The Weibull distribution function could fit the diameter distribution of storeys(including the whole stand storey and each substorey) better, except for the whole stand in sample plot 1, the surplus storeys in sample plot 1 and the storeys of the other sample plots all passed the Chi-square test. Used the Weibull distribution function fitted the diameter distribution of each storey in every sample plot, suggested that with the increase of storey height, the charts of storey changed from reverse "J" shape curve to right partial mountain curve and normal distribution curve, peak of curve moved to right, and the height of peak decreased.[Conclusion] The Weibull distribution function could fit the diameter distribution of each storey better than the Meyer negative exponential function at storeys(including the whole satand and each substorey) of typical natural broad-leaved forest in mid-subtropical zone. The results of S-W test, SK, KT and the status of the peak of curve shape at the chat of Weibull function indicated that the diameter distribution of each substorey in each sample plot was trend to transit gradually to normal distribution with the mean DBH and mean height of each substorey increased. The differences among the storeys(including the whole stand storey and each substorey) illustrated that the necessity of studying diameter distribution within storey identification.

Key words: mid-subtropical zone, typical natural broad-leaved forest, storey identification, diameter distribution, change law