Height curve construction of needle and broadleaved mixed forest under main site types in Saihanba, Hebei of northern China.

Site index model is the most effective and usable method in evaluating potential productivity of forest. However, in the mixed forest with multiple tree species, multiple levels and complex stand structure, it is a scientific problem to be solved urgently that building dynamic position index model and evaluating the site productivity under different site conditions. In this paper, data from stem analysis of 348 dominant trees (174 Larix principis-rupprechtii trees and 174 Betula platyphylla trees) were used to evaluate dynamic base-age invariant site index models developed from Bertalanfy-Richards model, Hossfeld model, and Lundqvist-Korf model with the generalized algebraic difference approach (GADA) for broadleaved mixed forests in the warm temperate zone of northern China. Five dynamic site index models were created to evaluate site quality and site index conversion equations were established in different site types of mixed forest by geometric linear regression.The results showed that the optimal model for L. principis-rupprechtii was based on Bertalanfy-Richards model, and its evaluating values were R2adj 0.948, Bias 0.088, RMSE 1.089; the optimal model for B. platyphylla was based on Lundqvist-Korf model and its evaluating values were R2adj 0.966, Bias 0.066, RMSE 1.322. In different site types, the correlation coefficients between the site index of L. principis-rupprechtii and B. platyphylla were 0.87, 0.81, 0.84 and 0.91. Validation procedure showed that conversion equations were suitable for the studied mixed stands. The conversion equations can be applied in mixed species stands with inconsistent stratification or in stand conversion situations for an accurate and reliable evaluation of forest quality.