| Ulmus chenmoui and Ulmus gaussenii, deciduous trees in the genus Ulmus in the family Ulmaceae, are endangered species endemic to China and mainly distributed in Langya Mountain National Forest Park, Anhui Province. Natural regeneration of populations is limited because ground under the trees solidified seriously, and growth of individuals is affected by human activities. In the past 30 years, the phenomenon of mass death of U. gaussenii old trees has appeared, while U. chenmoui retained trees become less and most of them are older plants. They are in great danger of extinction and have been listed as the national third class protected plants.We sampled at Nantianmen (32° 16.5506’N,118° 16.5685’E), the major regions of U. chenmoui population, and Zuiwengting (32°17.1317’N,118°17.0101’E), the major regions of U. gaussenii in May 2014. Based on diameter at breast height (DBH) size, two populations were divided into six size classes respectively:Ⅰ:DBH<2.5 cm, Ⅱ:2.5-7.5cm, Ⅲ:7.5-12.5cm, Ⅳ:12.5-17.5cm, Ⅴ:17.5-22.5cm, Ⅵ:>22.5 cm (in which Ⅰ size class is saplings group, Ⅱ-Ⅵ size classes are adult groups). On the basis of the analysis of age structure and dynamic, the fine-scale spatial genetic structure was studied. The main results were as follows:(1) The total number of individuals for U. chenmoui population is 116, and the number of individuals for six size classes is accounted for 58.6%,8.6%,3.4%,3.4%, 3.4% and 22.4% of the total population respectively. The results show that saplings are clumped together, and higher classes tended to be randomly distributed. Only the dynamic value (Vn) of Ⅰ and Ⅱ size classes is greater than zero, and the population dynamic index is 46.7%. U. chenmoui population is in growth pattern, but the population age structure is unreasonable. The population natural regeneration is disturbed seriously.The total number of individuals for U. gaussenii population is 422, and the number of individuals for six size classes is accounted for 61.1%,31%,3.1%、0.9%、 0.7% and 3.1% of the total population respectively. The results showed that the individuals of I and II size classes are clumped together, and higher classes tended to be randomly distributed. The dynamic value (Vn) of five size classes is greater than zero except V size class, and the population dynamic index is 59.9%. U. gaussenii population is in growth pattern but nearly in stable type. Because competition or self-thinning can result in the death of a large number of saplings and seedlings, it is difficult for individuals to grow up into canopy layer.(2)In this study, the modified CTAB method was used to extract DNA from the leaf samples, and the improved PCR method was used to develop the composite nuclear microsatellite (nSSR) primers. A total of 44 nSSR for U. chenmoui, and 82 nSSR for U. gaussenii were designed. We analyzed population genetic diversity and fine-scale spatial genetic structure using 9 pairs and 10 pairs respectively. In addition, we screened the commonality of tobacco chloroplast microsatellite (cpSSR) in U. chenmoui, and analyze population genetic diversity and haplotype analyses of U. chenmoui population.(3)The genetic diversity of U. chenmoui population is at high level of nSSR (He=0.679) and cpSSR (h=0.419). Langya Mountain Forest Park was established at 1985, so the time when population is under the disturbance of human activities is not long. For long-lived trees, the effect of habitat fragmentation, random drift and inbreeding depression on population genetic diversity may have not been revealed. The expected heterozygosity of IV size class is significantly lower than that of other size class, because the old trees maintain a high level of genetic diversity which can pass on to the offspring, but the level of genetic diversity in middle-size trees is lower due to the death of a large number of saplings and seedlings. We found seven haplotypes in U. chenmoui population, and I size class has six haplotypes and three of them are unique. There is only one haplotype in IV and V size class, suggesting that haplotype species are lost in middle-size trees. The genetic diversity of U. gaussenii population is at a low level (He=0.433), and there are no significant differences in genetic diversity between each size class. In the 90s of last century, a large number of old trees’deaths caused serious and irreparable loss to U. gaussenii population.U. chenmoui and U. gaussenii are stenochoric species, which have a limited number of parents in population, and there is no pollen and seeds move in, resulting in the high level of inbreeding in two populations. Adult individuals have lower inbreeding coefficient, because a large number of seedlings died when competition was enhanced with increasing plant density.(4) Significant fine-scale SGS is found in the U. chenmoui population within 64 m (Sp=0.0293), and saplings group and adult group also have significant fine-scale SGS within 56m and 58m. U. chenmoui fruit is samara that spread by wind. The limited seed dispersal due to high canopy density is one of the main reasons for the high population SGS intensity. Founder effect and microenvironment selection formed the SGS initially, while high level of inbreeding and limited gene flow maintained SGS in this generation. Greater overlap in seed shadows with increased population density weakened the intensity of SGS in saplings group. According to gene flow estimates, saplings group has more extensive gene flow than adult group (56m,47m), which reduced the intensity of SGS in saplings group.Significant fine-scale SGS is found in the U. gaussenii population within 46 m (Sp=0.0107), saplings group and adult group also have significant fine-scale SGS within 40m、49m. U. gaussenii fruit is samara that spread by wind. The limited seed dispersal due to high canopy density and weak wind is the main reason for the high population SGS intensity. Founder effect and microenvironment selection form the SGS initially, high level of inbreeding and limited gene flow maintained SGS in this generation. Seedlings are clumped around mother trees due to limited seed flow resulting in high intensity of SGS in saplings group. According to gene flow estimates, adult group has more extensive gene flow than saplings group (68.3m, 35m), which reduced the intensity of SGS in adult group.(5)The life of U. chenmoui and U. gaussenii populations is disturbed by human activities seriously, resulting in the lack of natural regeneration. High level of inbreeding and significant small-scale spatial genetic structure suggested that the population may be in the risk of inbreeding depression. It is necessary to carry out related in situ conservation and ex situ conservation work, including the establishment of nature reserves to reduce the interference of human activities; establish seed bank and preserve key genes type using vegetative propagation. In ex situ conservation, individuals should be sampled at apart to 64 m for U. chenmoui and 46m for U. gaussenii to reduce genetic similarity. |
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