Process to extinction and genetic structure of a threatened Japanese conifer species, Picea koyamae

Since the end of the glacial age, Picea koyamae has been sparsely distributed in Japan as a relict species and is presently threatened with extinction. We investigated the population structure and genetic structure of nine populations of P. koyamae. Population size was assessed at 9-135 individuals in habitats ranging from 0.5 to 11.5 ha, and seedlings and saplings were observed in all but one particular population, which had a Sasa-type (bamboo grass) forest floor. The effective number of alleles per locus (N-e) within peripheral populations in the Yatsugatake Mountains was 1.8-2.7, much lower than that of core populations in the Akaishi Mountains (2.8-4.3) using five nuclear simple sequence repeat loci. This finding suggests that genetic variation in these populations has been reduced by isolation from other populations. The standardized genetic differentiation among populations (G'(ST)) was 0.410 and higher than that found in other Japanese conifers, suggesting that isolation and inbreeding have progressed in this species. In two isolated populations at the Yatsugatake Mountains, the fixation index (F-IS) was 0.315-0.354, much higher than the values determined for the other populations (-0.188 to 0.263). This suggests that these two populations have survived several generations while increasing the degree of inbreeding. However, the highest seedling density was in a population with low genetic variation and high F-IS. The most serious problems at present appear to be the declining number of mature trees and the deterioration of suitable environments for seedling establishment.