Genetic characteristics reflecting the population size and disturbance regime of Siebold’s beech (Fagus crenata Blume) populations at the northernmost distribution

To understand the population dynamics of tree populations at the range limit of a species’ range, it is important to determine which population size and disturbance regime are critical to genetic diversity. Siebold's beech (Fagus crenata Blume) is a major canopy tree species of cool-temperate forests in Japan, with the northernmost distribution reaching the Kuromatsunai Depression in southwestern Hokkaido, Japan. We examined the genetic variation and dynamics of three beech forests, with different disturbance history and population attributions in the northernmost population. The Tsubamenosawa (TSU) and Sannosuke (SAN), both natural forest, have historically experienced little artificial disturbance, whereas the Soibetsu (SOI), a secondary forest, was intensively logged, and thus strongly disturbed in the past. In total, 35 alleles were detected among 12 loci, encoding 9 enzyme systems. At TSU, He and RS were 0.141 and 2.280, respectively. At SAN, He and RS were 0.142 and 2.604, respectively. At SOI were 0.182 and 2.628, respectively. Parameters of genetic diversity changed with population size, small isolated population indicated low values. Gene flow distance for low density mature trees in the natural forests was greater than that for high-density secondary forest. However, effective population sizes (Ne) were 34.7, 64.3 and 60.3 in TSU, SAN and SOI, respectively, reflecting differences in the density of mature individuals. The results suggested that the population with the low density of mature trees kept genetic diversity through long distance gene flow. The mature tree density affected the effective population size in the northernmost beech populations.