Application of R-distribution to model cumulative distribution of basal area of stands in Chinese fir plantations

Stand basal area distributions were obtained in order to investigate the R-distribution,a new function derived from the Richards function.After analyzing advantages and shortcomings of 3-parameter Weibull function,we applied the R-distribution to model basal area distributions in un-thinned stands of Chinese fir plantations at different ages,densities and site indices.We found that the R-distribution had high precision in modelling basal area distributions and its parameters appeared theoretically meaningful.The relative scale parameter p was highly correlated with stand age and quadratic mean diameter at breast height(DBH);the location parameter q was highly correlated with quadratic mean DBH and the shape parameter r indicated the inflection point of the distribution.Ordinates of the inflection points of the cumulative distribution curve of stand basal areas mainly ranged between 0.4-0.6;most were found to be above 0.5.Both abscissa and ordinate of the inflection points were differently affected by age,quadratic mean DBH,density and site index.Based on parameter prediction method and parameter recovery method,the three parameters of the R distribution could be predicted well.