Application of stand density indices for Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation management

The most important issues in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) management are the quantitative determination of stand density and the selection of appropriate density. Different stand density index models have advantages for special tree species, and this study aimed to estimate the carrying capacity of planted stands of Chinese fir and to select simple and reliable stand density indexes. Based on special experiment of different initial density, the maximum carrying capacity was estimated using Reineke’s self-thinning rule, Nilson’s sparsity theory, Beekhuis’s relative-spacing hypothesis, Zhang’s nutrient-competition rule, Curtis’s maximum stand basal area and Hui’s crowding degree based on mean tree distance. The restricted maximum likelihood method (REML) implemented with ‘nlme’ package in R software was used to refine the parameters of thinning age in Richard’s growth model. The results showed that stand density index models can describe the trends of stand density in response to tree growth: the higher the plantation initial density, the earlier age and stronger self-thinning capacity of stands. Reineke’s SDI and Zhang’s Z model are the most stable and suitable to estimate changes in the density of Chinese fir plantations, competition intensification, and the thinning age. The RD model can also be used, except at low Chinese fir densities. K, which can be affected by the mean crown width of trees, directly reflects the forest area of photosynthesis; this index is easy and simple to apply, but more research is needed to optimize the equation to evaluate whether a forest requires management and to determine the appropriate time for the first thinning and its intensity.