Dynamics of stand density and self-thinning in Chinese fir plantations: theoretical insights and empirical validation.

Introduction: Stand density management is essential for adaptive silviculture, thinning decisions, growth modeling, and yield prediction in forestry, particularly for plantations. Despite extensive research on self-thinning rules and the maximum size-density law, significant gaps remain in the biophysical understanding and validation of the relationships among key stand variables and parameters.

Methods: This study theoretically explored and validated the relationship between maximum size-density and two key metrics: average diameter at breast height (D) and tree height (H). We used time-series data from a 30-year clear-cut, fully stocked Chinese fir plantation, a fast-growing commercial species in China, for validation.

Results: A growth balance status for fully stocked stands was proposed, wherein prior to self-thinning, the growth rate of the stand basal area (G) aligns with that of the average tree height (H), expressed as and approaching a constant slope, b. Generalized maximum size-density and stand density index (SDI) equations were developed: and with , differing from traditional equations. Additionally, a generalized self-thinning equation, or , was introduced, indicating that in fully stocked stands, tree volume or biomass depends on both tree height and tree count.

Discussion: These findings advance understanding of the maximum size-density law and self-thinning boundary, providing refined tools for managing stand density in Chinese fir plantations.