Abstract: This paper studied the mixed spruce-fir coniferous and broadleaved forest in Jingouling Forest Farm and analyzed survey data of 41 fixed sample plots from 1987 to 2005,calculated the q value which was the ratio of number of stems of adjacent diameter,described the diameter structure of the mixed coniferous and broadleaved stand.The research results show that:1) the q value of the small diameter trees is declining year by year.From 1987 to 2005,the q values of I-1 and I-3 district decreased from 1.39 and 1.52 to 1.14 and 1.06 respectively,which showed that the small diameter tree structure had become irrational in diameter structure.2)Saplings whose diameter below 6cm regenerated less.3)The mean q values can not display diameter structure of stand actually,while the q values calculated by the small diameter wood,the medium diameter trees and large diameter trees can display stand diameter structure more truly.

Key words: Jingouling Forest Farm, q value theory, diameter structure, mixed coniferous and broadleaved forest