Response of height growth of regenerating trees in a Pinus tabulaeformis Carr. plantation to different thinning intensities

Chinese pine (Pinus tabulaeformis Carr.) is a widely distributed moderately shade-tolerant tree species with important economic and ecological value in northern China. The productivity of the plantations established by the government in the 1970s has apparently begun to decline. Therefore, the identification of silviculture measures that can feasibly and reasonably balance timber production and sustainable forest management is of interest to many operators and researchers. The main purpose of our study was to understand whether the height growth of trees regenerating under the canopy would positively respond to thinning and provide sufficient high-quality regenerating trees for the successful transformation to uneven-aged forests. Four different thinning intensity treatments were implemented in a 40-year-old plantation based on the number of trees that were cut down using the thinning-from-below approach: 0.0% (unmanaged), 20.0-21.0% (weak thinning), 30.0-31.0% (moderate thinning), and 40.0-41.0% (strong thinning). After five years of thinning, the differences in growth among the different thinning intensities were compared using nonparametric statistical methods. Additionally, the height growth rates of regenerating trees over the past five years were predicted using a linear mixed-effects model that included variables related to stand characteristics, soil physiochemical properties and measurements of individual regenerating trees. The results showed that thinning could significantly promote the height growth of regenerating trees in comparison to the growth in unmanaged plots. Significant differences were found among thinning intensities in terms of growth, excluding ground diameter, with the maximum and minimum values in the strong thinning and unmanaged plots, respectively. However, the models never explained more than 50% of the variation regardless of whether the plot level or the individual level was considered, although they included the majority of the evaluated variables. In conclusion, strong thinning is a potentially feasible silvicultural method for uneven-aged Chinese pine plantations in northern China. Some factors that are important in terms of the growth of regenerating trees but were not tested in our study, such as competition, light intensity and distribution, and microsite environment, are recommended to be considered in future research to improve the explanatory power of the model. Moreover, based on our results and to supplement thinning, the gap expansion harvesting system should be carried out in gaps opened by thinning to not only maintain stand stability but also facilitate the growth of regenerating trees into the canopy for canopy recruitment.