Objective The effective of thinning is commonly evaluated by the growth dynamics of reserved trees. The stand diameter structure heterogeneity and growth dynamics of different tree species and diameter classes of spruce-fir mixed forest after thinning were studied in order to provide scientific guidance for operators to adjust thinning strategies.
Method Based on four permanent sample plots of natural spruce-fir mixed forest in Jingouling Forest Farm of Jilin Province, northeastern China, which included one control sample plot and three thinning sample plots with different treatments (remove 21% (light), 27% (medium), 36% (heavy) of the basal area), we applied the concept of growth dominance (GD) with the Pressler growth rate and the Gini coefficient to analyze the dynamics of diameter structural heterogeneity and the growth feedback of different species and diameter classes after thinning.
Result 5 years after thinning, the growth rates of shade-tolerant tree species (light, medium and heavy) were 2.92%, 2.73% and 3.04%, respectively. The increase of thinning intensity had no significant promoting effect on shade-tolerant species but it can promote the growth of shade-intolerant tree species significantly (2.18%, 2.91%, 3.65%). The medium and light thinning intensity could improve the growth rates of small and medium diameter trees greatly but this impact is not obvious on the dominant trees, while heavy thinning had a strong positive effect on the growth of whole stand. The non-dominant trees of shade-intolerant species responded more positively to thinning, which had stronger negative growth dominance than shade-tolerant species. Thinning of medium-diameter trees can immediately improve the heterogeneity of stand structure. The Gini coefficient decreased after thinning showed that thinning can reduce the level of stand diameter structural heterogeneity by reducing the asymmetric competition and promoting the growth of small trees.
Conclusion Heavy intensity thinning and regularly removing shade-tolerant dominant trees of weak contributors to stand growth can maximize stand productivity, while light thinning can promote the sustainable forest development, and accelerate the positive succession to the primary vegetation in northeastern China. The growth dominance can effectively reflect the growth status of trees in different diameter classes after thinning, and provide rational reference for thinning of trees.
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