Differences in canopy and understorey diversities after the eruptions of Mount Usu, northern Japan — Impacts of early forest management

Abstract

Evaluating the effects of management on successional trajectories, plant composition, and diversity has been difficult due to the scarcity of long-term studies. This study examined the composition and diversity of species in natural and artificially regenerated forests at two eruption sites of Mount Usu, northern Japan, during 2015–2019, to compare the effects of active and passive management. The two sites are Yosomi, damaged by the 1910 eruptions, and the summit, damaged by the 1977–78 eruptions. Various natural and artificial forests developed at both sites, whose species composition was analyzed by non-metric multidimensional scaling to measure similarity between the forest types, and whose diversity was compared by the true diversity index, showing the effective number of species, from order 0 (presence-absence) to order 2 (weighted species) for two layers: the canopy (woody species with DBH > 3 cm) and understorey (< 2 m high plants). Canopy diversity was measured by stem density in five 10 m × 10 m plots in each forest type, and understory diversity was measured by shoot density in four 1 m × 1 m quadrats in each plot. The canopy and understorey species compositions were distinct between the forest types, but the canopy was more affected by management than the understorey, indicating that early forest management had long-term effects on species composition. Species composition of the plantations resembled those of the natural forests when the plantations had patchy spatial structure. The naturally regenerated forests showed the highest diversities at both eruption sites, while the plantations displayed low diversities, except in one case, when the plantation showed heterogeneous forest structure. The plantations changed their species composition slowly and did not transform into natural forests. In conclusion, we suggest using a patchy plantation design with some space between patches instead of dense planting, to create resilient, diverse, and native forests after disturbances.