Are disjunct populations of Betula costata in the Japanese Archipelago glacial relict? An attempt at verification by species distribution modeling

A glacial relict, a remnant of a cold-adapted species or population from a postglacial retreat, is essential for understanding the formation processes of present-day vegetation. Although past distributions have been reconstructed for several woody species in Japan, few studies have focused on the cold-adapted woody species that survive in the Japanese Archipelago. Betula costata Trautv., a species considered a glacial relict, is a common canopy tree in the cool-cold temperate forests in Northeast Asia. However, B. costata is rare and has a disjunct distribution in the Japanese Archipelago, and the reasons for its rarity and disjunction in Japan remain poorly understood. Therefore, we investigated the distribution of B. costata in Northeast Asia since the last glacial maximum (LGM; between ca. 27,000 and 19,000 years ago) using species distribution models (SDMs). We tested the working hypothesis that B. costata was once abundant in the Japanese Archipelago during the LGM. However, postglacial climate warming caused it to become a disjunct and relict. Two types of SDMs, Maxnet and Random Forest, predicted suitable habitats for the species with three time periods: The LGM, mid-Holocene, and present. The resulting maps of suitable habitats during the LGM showed extensive distribution across continental Asia and the Japanese Archipelago. By the mid-Holocene and the present, suitable habitats expanded northward in continental Asia but retreated in the Japanese Archipelago due to increases in the minimum coldest quarter temperature and precipitation. These simulation results support the hypothesis that the Japanese population of B. costata is a glacial relict.