Study on the experimental shape number and timber yield rate of large diameter Cunninghamia lanceolata in Huangfengqiao state-owned forest farm of Youxian County

Cultivating large-diameter timber is always the key objective of Chinese fir (Cunninghamia lanceolata) forest management. The experimental shape number and timber yield rate, as important indexes to measure the quality of the large-diameter timber fir, play a crucial role in accurately calculating its volume and evaluating its economic value.

In this study, Huangfengqiao state-owned forest in Youxian County, Zhuzhou City, Hunan Province was selected as the research area. Firstly, fifty large diameter Chinese fir trees with diameter level of 32-46 cm were selected to calculate the experimental shape number and analyze the change law with DBH and tree height. Secondly, using the data of tree height, DBH with bark and DBH without bark, the yield table of large-diameter Chinese fir was prepared. The average timber yield rate of the large-diameter Chinese fir was calculated by weighted average of the yield rate in each diameter level.

The experimental shape number of large-diameter fir tree ranged from 0.370 to 0.489, with an average value of 0.412, which decreased with the increase of DBH, and showed a trend of first increasing, then decreasing and then increasing with the increase of tree height. The average wood yield rate of the large-diameter timber Chinese fir ranged from 74.62% to 84.93%, and the average value was 82.07%, and it decreased with the increase of diameter level.

The average experimental shape number of the large-diameter Chinese fir is not much different from the experimental shape number of 0.42 usually used in practice. Thus, using 0.42 to calculate the volume of the fir would not affect the timber yield rate and economic value of Chinese fir. The average timber yield rate of the large-diameter timber of Chinese fir is higher than the average timber yield rate of 72.00% commonly used in practice. Therefore, in the stand with the majority of Chinese fir are large-diameter timber, using 72.00% to estimate large-diameter timber yield rate will underestimate the timber yield rate and economic value of Chinese fir. This study can provide a theoretical reference for the volume estimation and economic value assessment of the large diameter fir trees in Huangfengqiao Forest Farm in Youxian County and other areas with similar terrain and climate conditions.