Understory thinning reduces wood-production efficiency and tree species diversity in subtropical forest in southern Japan

Based on the simulation of a spatially explicit individual-based model, this study examined the long-term impacts of understory thinning on stand structure, tree growth and number of tree species in a subtropical forest on the Ryukyu Islands, Japan. The treatment options for understory thinning were defined by combinations of thinning cycle and critical size of thinning trees. The results of the simulation demonstrated that understory thinning had no major influence on the growth of canopy trees, and conversely reduced the stand biomass over time. Moreover, tree species diversity drastically decreased with thinning of understory trees. Frequent thinning of smaller trees prevented the regeneration of understory species. Our predictions suggested that the prescription of understory thinning has a negative impact on the economic efficiency of timber production and the resilience of tree species diversity in a subtropical forest in southern Japan. This is contrary to the expectation that understory thinning, regarded as being analogous to the traditional prescription for coppice woodland, fosters a productive forest with species diversity.