Amounts of Unutilized Forest Biomass Resources Under Sustainable Forest Management, Limited by Labor–Input Constraints

In anticipation of the utilization of logging residues and small-diameter trees after the completion of the Feed-in Tariff Scheme for Renewable Energy (FIT) in Japan, this study (i) calculated the available amount of unutilized forest biomass resources over a long term by using the developed »regional forestry operations management model« and (ii) estimated the period and the ratio of the generation of small-diameter trees during cleaning and pre-commercial thinning operations to the whole biomass resources. These analyses revealed that in the first 60 years, the clearcut and reforestation of 763 ha of planted forests (average age 66 years) with the labor input of approx. 2000 man-days per year were promoted, and thus 71% of the whole area was returned to the regular harvesting cycle. During that period, the average amounts of log production and biomass generation were 7184 m3/y and 875.6 Mg/y on a dry-weight basis, respectively. During the next 60 years, when the clearcut of older, high–accumulation forests and the subsequent reforestation were stabilized, the average amounts of log production and biomass generation under sustainable forest management were 5239 m3/y and 774.8 Mg/y on a dry–weight basis, respectively, exhibiting lower amounts than those of the first 60 years. Small-diameter trees were generated by cleaning and pre-commercial thinning operations stably from 16 years after the beginning of clearcut and reforestation. Especially after 61 years, the average annual amount of small-diameter trees generation accounted for around 10% of that of the whole biomass resources and more than 50% of that of unutilized thinnings.