Abstract: [Objective] Taper equation is a key tool to describe stem form variations. By far, different taper equations have been proposed in the world. Among them, the segmented taper equation is one of the most commonly used equation.The method of ordinary least squares regression (OLS) is commonly used to estimate the parameters in the model. However, in practical terms, the application of the segmented taper equation was restricted because the estimated parameters of inflection points a₁ and a₂ (the relative tree height with numerical region of 0 to 1) obtained by OLS regression do not ensure to fall into the region of 0 to 1. Based on the above issue, the aim of this research is to find out the optimal fitting method by using TS(the two-factor automatic selection algorithm)and OLS, which provides technical supports for the construction of tree curve model and fine material.[Method]The three rare species such as Erythrophleum fordii, Castanopsis hystrix and Tectona grandis were developed using 120 individual data (40 for each species), the segmented taper equation was constructed for each species using OLS and TS, respectively. They were evaluated and compared based on the indexes of the coefficient of determination, the residual sum of squares, the mean prediction error, the variance of prediction errors and the root mean square error.[Result]The results show that both the OLS regression and TS fitting accuracy are preferred more than 95% and their coefficient of determination, residual sum of squares are the same. For the variance of prediction errors and the root mean square error TS is the best for E.fordii, C.hystrix and T.grandis; for the mean prediction error OLS method is the best for E.fordii,but TS is the best for C.hystrix and T.grandis, however, the difference between the two methods corresponding the mean prediction error is small; stem profiles prediction results are also very similar; TS can ensure that the inflection point of the estimated parameters is in the (0,1),and it is simple, fitting result is stable.[Conclusion] TS could explained theoretically the optimal value of segmented taper equation parameters a₁ and a₂. Therefore, it was recommended to fit the segmented taper equations with TS.

Key words: rare tree species, stem profiles, segmented taper equation, two-factor automatic selection algorithm