Maximizing the canopy openness for replanted Korean pine seedlings under secondary forests in northeast China

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Accelerating the recovery and successional processes of secondary forests is an important task for sustainable forest management around the world. However, previous studies have mainly focused on the establishment and survival of seedlings, whereas the differences of light requirements in different growth stages of understory seedlings have been overlooked for a long time. Therefore, the present study developed a tree-level harvest decision tool that can account for adjustments in the light environment of the understory and optimize the stand structure of the overstory concurrently. A set of spatial (canopy openness, species mingling, and uniform angle) and nonspatial (the number of species and diameter classes, as well as the q-value of diameter distribution) parameters were employed to formulate the joint objective function. Then, three thinning scenarios were generated by adjusting the priority of the replanted Pinus koraiensis and their companion species. The developed tool and assumed thinning scenarios were then tested using four 100 m × 100 m mapped secondary forests in northeast China, where approximately 500 seedlings per hectare of P. koraiensis were replanted under the canopy in the 1980s. The results indicated that the tree-level harvest decision tool improved the joint objective function values by approximately 3.6–6.9 times for different thinning scenarios on average. Hereinto, the light conditions of P. koraiensis were improved by about 32 %, meanwhile the stand structures of the overstory were also increased by about 11 % of species mingling and about 1 % of the horizontal distribution. Thus, the developed tree-level harvest decision tool was effective and can be applied to manage secondary forests.