Stand characteristics and aboveground biomass of young coastal Pinus thunbergii Parl. stands with different tree densities

To consider density control practices for the extra-high density Japanese black pine (Pinus thunbergii Parl.) forests, we investigated stand characteristics, aboveground biomass, and biomass production in young stands of various density during 2019–2021 (14–16 years old). We used the density-adjusted plots established in an even-aged natural forest in 2010; i.e. 1,000, 2,500, 5,000, 10,000 trees ha−1, and the control plot (ca. 50,000 trees ha−1) without thinning. Mean tree height did not differ significantly among the plots, whereas mean clear bole length was longer and mean trunk diameter was smaller with increasing tree density. Spindly tree form was observed especially in the control plot, indicating higher vulnerability to mechanical disturbances. Aboveground biomass (YTotal; Mg ha−1) in the control and 10,000 trees ha−1 plots was greater than that of the published data. These two plots were considered to reach the full density condition which is higher than the full density condition in the published data. It implies the difference in site quality among sites. Positive correlation was observed between leaf biomass (Mg ha−1), and the increment of YTotal (ΔYTotal), if the control plot was excluded. Together with the spindly tree form, it was too late for thinning in the control plot. In contrast, no thinning would be required for 1,000 and 2,500 trees ha−1 plots, since the reduction of ΔYTotal was not observed in these plots. Thinning would be required for 5,000 and 10,000 trees ha−1 plots. Further quantitative analysis will be necessary for future density control.