Genetic structure and in-situ conservation of the Japanese red pine form Utsukushimatsu (Pinus densiflora Siebold et Zucc. 'Umbraculifera') as a natural monument in Hiramatsu, Shiga Prefecture, Japan

Abstract

The Japanese red pine form utsukushimatsu (Pinus densiflora Siebold et Zucc. 'Umbraculifera') is characterised by a trunk composed of numerous stems of similar diameter. It occurs only in Hiramatsu, southern Shiga Prefecture, western Japan, where it is preserved as a natural monument. However, wilt disease has caused high tree mortality in this species, decreasing the population from 450 in 1924 to 86 in 2019. To obtain population genetic data for conservation, we collected 236 samples from seedlings, saplings, and mature trees in the utsukushimatsu forest and experimental plantations. We performed multiplexed inter-simple sequence repeat genotyping by sequencing and found no clear genetic differentiation between the common Japanese red pine and its utsukushimatsu form. This suggests that the Hiramatsu red pine population has a genetic mutation that causes the utsukushimatsu form in individuals homozygous for this gene, consistent with previous reports. Inbreeding was observed in the native populations. Conservation of the utsukushimatsu habitat requires the maintenance of a population made up of individuals of all life stages (seedlings to adults); therefore, understory management through frequent undergrowth thinning is essential. Monitoring surveys should be conducted for in situ plant conservation, and seeds from various mother trees should be sown to reduce inbreeding in ex situ plant conservation.