Conservation Ecology and Genetics of Two Dioecious Plant Species in South Korea

Abstract

Dioecious plants may pose special conservation problems because the population dynamics can be affected by sex-specific trade-offs between growth and reproduction, and differences of adaptability for environmental conditions. Moreover, if the population size is the same, dioecious plants only about half the individuals produce seeds compared with cosexual plants. Therefore, for conservation of the endangered dioecious species needs to be well-integrated knowledge derived from ecological, demographic and genetic attributes related with the sexual system. This study was conducted to propose a long term conservation plan through analysis of the ecological traits - sex ratio, size distribution and spatial pattern - and genetic aspects - genetic diversity and spatial genetic structure - in two dioecious plant species of South Korea. Ilex cornuta 호랑가시나무 (Aquifoliaceae) is a dioecious evergreen shrub, and an endemic species of East Asia. Inland populations of I. cornuta in South Korea are known to almost have been reduced or destroyed due to excessive exploitation and habitat destruction. However, in Jeju Island, the ecological information such as distribution and structure of I. cornuta is almost unknown. Thus, I analyzed sex ratios, sex-related ecological traits and spatial distribution patterns for I. cornuta populations of Jeju Island. In Yongsu-ri population (YS population hereafter), an overall sex ratio of reproductive trees was male-biased (0.66), while Gotjawal Provincial Park population (GP population hereafter) did not differ significantly from 1:1. Females were slightly larger and taller than males in two populations, but these were no statistically significant intersexual differences. I. cornuta genets usually formed a clump with several basal sprouting stems (ramets), but the differences between sexes were not significant. Vitality was significantly associated with sex in YS population, and males had more defense against infection than females. The significant spatial segregation was not observed between males and females in YS and GP populations. Unlike GP population designated as a protected area, YS population is surrounded by the farmland and exposed to risk factors such as biased sex ratio, high infection probability and anthropogenic damage. For YS population, suitable protective devices are needed to prevent I. cornuta trees being disappeared due to habitat disturbance by anthropogenic activities. GP population has a problem of small population size. Therefore, the conservation program for assisted natural regeneration of I. cornuta trees is needed. Torreya nucifera 비자나무 (Taxaceae) is a dioecious gymnosperm currently distributed in southern parts of South Korea and Japan. Torreya population in Jeju Island is known as the largest (N = 2,861) and oldest (mostly 200-400 years old, max. ~900 years old) in the world. In this study, I examined sex- and size-related spatial distributions in three quadrats of old-growth T. nucifera forest in extremely heterogeneous Gotjawal terrain of Jeju Island. Univariate spatial analysis showed that spatial patterns of T. nucifera trees differed among the three quadrats. In quadrat A, individual trees showed random distribution at all scales regardless of sex and size groups. When assessing univariate patterns for sex by size groups in quadrat B, small males and small females were distributed randomly at all scales whereas large males and large females were clumped. All groups in quadrat C were clustered at short distances, but the pattern changed as distance was increased. Bivariate spatial analyses testing the association between sex and size groups showed that spatial segregation occurred only in quadrat C. Males and females were spatially independent at all scales. However, after controlling for size, small males and small females were spatially separated. Diverse spatial patterns of T. nucifera trees across the three sites within Torreya population is considered to reflect various factors such as the ecological characteristics of individuals as well as the environmental heterogeneity of Gotjawal terrain and artificial interference. This study was performed to examine genetic diversity and spatial genetic structure using nine microsatellite markers in an old-growth Torreya population of Jeju Island. Overall genetic diversity (HE = 0.513) of Torreya population was lower than other tree species with similar ecological characters. Also, Torreya population showed a significant bottleneck signature (P < 0.001), and a relatively small effective population size (Ne/N = 0.56). Spatial autocorrelation analysis using Moran’s I and the kinship coefficient (Fij) revealed individuals being genetically more similar at a distance of up to approximately 180 m and 150 m, respectively. The intensity of spatial genetic structure (Sp = 0.0130) was relatively strong to than the other species that share similar life history traits with T. nucifera. We found a genetic sub-structure with the north-south directions according to results of the genetical bandwidth mapping and the location of genetic barriers in Torreya population. GENELAND analysis based on a Bayesian model revealed that Torreya population was composed to four clusters (K = 4). The distribution of four clusters also showed the north-south directions, especially the southern area included all four clusters of Torreya population. According to the results of the spatial genetic analysis, this population was divided into three genetic groups (Group I, Group II and Group III), and 4% of total genetic variation was attributed to differences among groups (P = 0.010). The present study, for the first time, displayed low genetic diversity, significantly genetic sub-structure with three genetically distinct groups and strong spatial genetic structure in Torreya population. This results are emphasize that the urgency of conservation activities to maintain or improve the level of genetic variations. And the spatial genetic sub-structure within Torreya population should be considered for future conservation and management practices. Specially, we propose Group III need to be conserved with top priority as the major source because it shows both large heterogeneity and all four genetic clusters. For ex situ conservation of T. nucifera the sampling strategy is efficient at least above 150 m between individual trees to reduce genetic relatedness. In South Korea forests, dioecious woody species is known to be about 24%, which is relatively higher compared to 8% in the world. In South Korea, however, few ecological studies with dioecious sexual system have been conducted. In this study, I have provide conservation strategies for the dioecious I. counuta and T. nucifera based on informations such as sex-related ecological traits, spatial structure and genetic assesment. These results will help to determine the future direction of the research for the conservation of the endangered dioecious plants in South Korea.

Keyword: Conservation of dioecious plants, Genetic diversity and spatial genetic structure, Ilex cornuta, Sex ratio and sex-related distribution pattern, Torreya nucifera