Impact of a tree improvement program on the genetic diversity of sugi (Cryptomeria japonica D Don) plantations

Abstract The main purpose of this study was to investigate how a shift in seed sources owing to the implementation of a tree improvement program affected the genetic diversity of plantation forests. We investigated the genetic diversity and pairwise relatedness between individuals in sugi ( Cryptomeria japonica D Don) plantations established before (“Pre-B”) and after (“Post-B”) the start of the Japanese tree improvement program. Based on SSR and SNP markers, most genetic diversity indices and the degree of relatedness within plantations were comparable between “Pre-B” and “Post-B”. However, there were large variations in the proportion of closely related pairs within plantations in both groups. Additionally, in “Post-B” plantations, parental contributions greatly diverged among plus-tree clones. The results of this study indicated that the shift in the seed sources, caused by the implementation of a tree improvement program, did not have a significant effect on the genetic diversity of plantations. However, the results also indicated that the degree of relatedness within plantations varied significantly according to how seeds are collected regardless of the type of seed sources. Furthermore, it was also revealed that genetic diversity in “Post-B” plantations could have decreased because of a deviation from equal parental contributions in seed orchards. Our results indicated that effective management of seed orchards is important to secure the genetic diversity of plantation forests.