How did the exposed seafloor function in postglacial northward range expansion of Kalopanax septemlobus? Evidence from ecological niche modelling

We attempted to clarify how coastal lands temporarily exposed during the last glacial maximum (LGM) contributed to the northward colonisation of Kalopanax septemlobus (Thunb. ex Murray) Koidzumi during the postglacial period in the Japanese Archipelago. Distribution records in 30-arc-s pixels were related to bioclimate variables using the maximum entropy technique to model the ecological niche of this species. Bioclimatic conditions in the exposed coastal lands during the LGM were reconstructed based on simulated palaeoclimate and fine-resolution marine topography. Potential distribution ranges were then estimated under the climatic conditions during the LGM, mid-Holocene and the present. The ecological niche of this species was influenced mainly by the temperature component of the bioclimates, leading to northward range shift after the LGM as the climate warmed. On average, 26% of the potential range of K. septemlobus during the LGM was located on the exposed seafloor. The northern edges of the species range on both sides of Honshu Island were estimated by up to several hundred kilometres farther north, when compared to the prediction assuming no sea level change. Application of ecological niche modelling provided new insight into the role of exposed seafloor as cryptic glacial refugia for this species, which has never been evidenced by fossil records. In a temperate island system characterised by long coastlines, the northern exposed seafloor would have been more important in terms of harbouring the putative source populations for northward directional colonisation during the postglacial period.