Optimal equilibrium curve of natural secondary forest of Pinus massoniana in Hunan Province of Central China

  Objective  In this study, with the support of existing growth models, the potential equilibrium curves for natural secondary forest of Pinus massoniana in Hunan Province of Central China were compared and the optimal equilibrium curves were finally identified as the quantatitative management target.

  Method  Based on the comprehensive analysis of the structure of Masson pine natural secondary forest and the results of previous studies, the values of basal area (B) in this study were to be selected as 35, 40 and 45 m²/ha, respectively. Ratio of tree number of two consecutive diameter classes (q) was selected as 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, respectively. The values of maximum retained diameter classes (D_max) were 40, 45 and 50 cm, respectively, and 54 potential equilibrium curves were constructed. With the support of variable-parameter transition matrix models, these potental equilibrium curves were simulated and the optimal equilibrium curve was determined.

  Result  The 80-year simulation results produced by the transition matrix model showed that the equilibrium curve 12 (B was 35 m²/ha, D_max was 45 cm, and q was 1.7) exhibited obvious better performance in terms of volume increament, stand volume, average DBH of harvested stems, and stable species compostion. When the equilibrium structure was reached, it had the largest annual growth of stock volume (iv) , iv was 1.18 m³/(year·ha). At the same time, other evaluation indicators of the equilibrium curve were also excellent. The variance of the stem number around the target (var) was 0.12 < 0.5, the stock volume V was 268.13 m³/ha, and the quadratic mean diameter of the removed trees (dg) was 17.99 cm. In addition, in the last 20 years of the simulation period (2077−2097), the composition of tree species basically remained stable, and no obvious fluctuation was observed. Therefore, equilibrium curve 12 was determined as the optimal equilibrium curve.

  Conclusion  The simulation results of the current forest stand show that if the forest structure is adjusted in time according to the equilibrium curve, the forest structure will gradually approach the equilibrium curve and finally achieve the equilibrium state. The composition of forest tree species will also change accordingly. The hard broadleaf trees of high economic value and oak will become the main part of the forest, while the proportion of Masson pine will gradually decrease. At later stages, the Masson pine will gradually deteriorate and be replaced by other broadleaved trees. Moreover, the number of large diameter trees will increase, and the value of the forest will gradually increase.