Large, retained trees of Cryptomeria japonica functioned as refugia for canopy woody plants after logging 350 years ago in Yakushima, Japan

Abstract Trees retainedduring green- treeretention forestry are expected to function as biological legacies that promote biodiversity and enhance ecosystem functions in plantation forests. Investigating how historically retained treesfunction as biological legacies could help predict the long-term ecological potential of current retention practices. Here, we investigated whether large, retained treesof Cryptomeriajaponica D. Don (>1000 years old) functioned as refugia for persistence of canopy woody plantsafter logging in a 350-year-old secondary forestin Yakushima, southern Japan. We climbed five each of retainedand regenerated treesin a 1-ha research plot, measured trunk and crown structures, and tagged and measured every woody plantstem found on each tree. Compared to regenerated trees, retained treeshad twice the surface area available for canopyplant colonization. Moreover, retained treeshosted disproportionately greater abundance of canopy woody plants. Together, the five retained treeshosted 22 species of woody plantscomprising 1188 individuals, whereas regenerated treeshosted only 8 species (37 individuals). Combined with our ground-based measurement, canopywoody species contributed 16% of all woody plantspecies in the plot. Among the five retained trees, woody plantabundance increased markedly with increasing age of trunk breaks. This was because numerous epicormic branches had sprouted below trunk breaks creating large surface area upon which arboreal soil accumulated and woody plantsestablished. Canopy woody plantson retained treesshowed wide vertical distribution corresponding to complex crown structure, whereas those on regenerated treesoccurred almost exclusively on the lower trunk. Maximum stem size of canopy woody plantswas constrained by the volume of arboreal soil upon which they grew. Based on their size structures, we inferred that three species endemic to Yakushima ( Vacciniumyakushimense Makino., Viburnumurceolatum Sieb. et Zucc., and Rhododendron yakushimanum Nakai) , maintain stable populations in the retained treesby sprouting. Several other species scarcely found on the ground were also regenerating in the canopy. Our results demonstrate that retained treesof Cryptomeriafunctioned as refugia allowing canopy woody plantsto persist after logging and give support to the importance of conserving large treesfor enhancing biodiversity in forests where canopyplants contribute to species diversity.