Comparison between site index and site form for site quality evaluation of Larix olgensis plantation

ObjectiveThis study aims to examine whether site form is a valid method for site quality evaluation of Larix olgensis plantations through analyzing the relationship between site form as well as site index and site productivity.

MethodWe put forward the method of determining the reference diameter at breast height(DBH) on the basis of DBH-age logistic growth equation with 314 observation data from 168 permanent sample plots of Larix olgensis plantations, and the reference DBH was calculated as the DBH at inflection point of the growth equation. We separately developed the dominant height-age and dominant height-DBH guide curves with the Richards growth equation, calculated the site index and site form for each plot, analyzed the correlation between site index and site form, and the correlation between both of them and productivity.

ResultThe result showed that the coefficient of determination R² with the regression equation between site index and site form was 0.3627, RMSE was 1.9930m and MAE was 1.5632m. The correlation coefficients between site index, site form and productivity were 0.6764 and 0.3203, respectively. We successively classified the site index and site form into 8 grades at equal interval. The proportion from grade 1 to grade 8 by site index was 0.64%, 3.82%, 12.74%, 24.84%, 32.48%, 19.43%, 5.10%, 0.96%; and the proportion from grade 1 to grade 8 by site form was 0%, 1.59%, 9.24%, 42.36%, 44.90%, 1.91%, 0%, 0%.

ConclusionFor Larix olgensis plantations, site form could only explain 36.27% variation of site index, the correlation between site index and productivity was much larger than that of site form and productivity. The classified site quality grades by the site index and site form roughly showed the same trend in proportion at most grades, i.e. most sample plots were at the middle grade and few sample plots at both ends. But the number of sample plots in most site quality grades differs greatly. In general, we do not recommend site form as a method for site quality evaluation with Larix olgensis plantation.