Environmental factors contribute to the formation and maintenance of the contact zone observed in deciduous broad-leaved tree species in Japan.

Contact zones are defined as areas where populations from different refugia meet during a postglacial expansion and distinct DNA lineages are mixedly distributed. In Japan, contact zones of various plants and animals were reported from the Kinki-Chugoku region. These contact zones appear to be maintained without any drastic topographic barriers such as those observed in the Alps and Pyrenees Mountains. In this study, the mechanisms underlying the formation and/or maintenance of these contact zones were investigated using six deciduous broad-leaved tree species (Carpinus laxiflora, C. tschonoskii, C. japonica, Magnolia obovata, Padus grayana, and Euonymus oxyphyllus). First, the precise location of the contact zones was examined by intensive genetic analysis of the six species. Second, the relationships between the geographic location of the contact zone and various environmental factors, including climate and topography, were investigated by generalized additive models to reveal the mechanisms of the formation and maintenance of the contact zones. As a result, four of the six examined plant species clearly showed a geographically common contact zone in Hyogo Prefecture and its adjacent areas. The results of the generalized additive models indicate that the pattern of low habitat suitability estimated by ecological niche modeling was the most important factor for determining the location of the common contact zone. These results suggest that areas with low habitat suitability in Hyogo Prefecture restrict the migration and gene flow of the four species in this region, and thus, they maintain the pattern of the contact zones. This study suggests that there are major effects of habitat suitability on the formation and maintenance of the contact zones.