Do larger snags stand longer?—snag longevity in mixed conifer–hardwood forests in Hokkaido, Japan

Abstract

More than half of the dead trees fell down within decade. Besides, DBH effect on snag longevity varied by tree species and was not detected for some species. Management of long-lived snags in temperate mixed forests should be reconsidered. Snags (standing dead trees) provide essential habitats for many species. Those with large diameter at breast height (DBH) are prioritized for retention; one of the reasons is that snag longevity (recruitment to falling down) increases with increasing DBH in conifer-dominated forests. Despite its importance, the DBH effect on hardwood snags is inadequately understood. The aim of this study is to investigate whether and how the DBH effect varies by tree species in mixed conifer–hardwood forests. We checked the current status (standing or falling) of dead trees that had been recorded in 37 permanent plots and applied a generalized linear mixed model. We used tree species, time since death, DBH, and interaction terms as variables. Half of the dead trees with a DBH of 25 cm were estimated to have fallen down within 1–6 years for most of the species. While snag longevity increased with increasing DBH in Abies sachalinensis and Phellodendron amurense, we detected no such effect in Picea and three hardwood species. For highly decayed or long-lived snags, retaining large snags is not always effective. Managers of species-rich mixed forests should consider the variability of the DBH effects and management of dead wood before and after tree mortality.