Community Structure, Population Dynamics And Persistence Mechanism Of The Rare And Endangered Tree Zelkova Schneideriana In The Mountainous Area Of Southern Jiangsu

Zelkova schneideriana,an endangered and rare tree species,has been listed as a second-class state-protected plant in China. It is mainly distributed in the area to south of Huaihe River,the middle and lower reaches of the Yangtze River and its southern region. Jiangsu is one of the main distribution areas of the Z. schneideriana. Due to human activities, some habitat of this species is destroyed, thus resulting in a serious threat to the survival of Z. schneideriana populations. According to the second key protected wild plant resources survey in Jiangsu in recent years, the south mountain region of Jiangsu province is the main distribution area of wild Z. schneideriana trees.Based on the thorough investigation, we chose the Longtan Forest Farm, Huijiacun and Shijiashan Forest Farm where the Z. schneideriana is distributed concentratedly as sampling sites, and carried out their community structure, population dynamics and structure diversity researches. This study aimed to understand the survival situation,development trend and maintaining mechanism of the Z. schneideriana in Jiangsu area. Meanwhile, the research also provided the theory evidence for reasonable protection and scientific management. The main results were as follows:Community structure: In Longtan Forest Farm, Huijiacun and Shijiashan Forest Farm, seven plots (20 m×20 m) were established respectively. Each plot was divided into four tree layer plots (10 m×10 m), and in each quadrat, a 5 m×5 m shrub layer plots and 1 m x 1 m herb layer plots were set. Using a stratified sampling method to investigate the plants of tree layer, shrub layer and herb layer, we analyzed the community species composition and species diversity of Z. schneideriana. (1) The Z.schneideriana communities were mainly distributed in the valleys or slopes with an elevation of 300 m below. In Longtan, Z. schneideriana was mainly distributed in the hills and valleys. In Huijiaucn and Shijiashan, it was mainly distributed in the hills.(2)There were two community types. One was deciduous broad-leaved forest and it was the main community type; the other was bamboo forest in which some Z.schneideriana trees occurred. (3) The communities within the three sites had abundant species which contained 170 vasular plant species, belonging to 138 genera and 75 families. Longtan forest farm had a total of 79 species of vascular plants, belonging to 47 families and 71 genera. The Huijiacun had a total of 86 species of vascular plants,belonging to 53 families and 77 genera. Shijiashan Forest Farm had a total of 92 species of vascular plants, belonging to 51 families and 78 genera. Overall, most of them were angiosperms and fewer were gymnosperm across the sites. (4) Most of the genus geographical components of the flora in southern Jiangsu were temperate and the second was tropical genera. The vegetation had the nature of the transition from tropical to temperate zones. (5) All of the 21 Z. schneideriana plots were divided into Z. schneideriana+Celtis sinensis community and Z. schneideriana community. Their species diversities showed a similar trend between the two communities. On the whole, the species richness index, Shannon-Wiener index, community evenness index and ecological dominance index on the tree layer were lower than those on the shrub and herb layers. (6) The vertical structure of the Z. schneideriana community was obvious and simple. It could be divided into tree layer, shrub layer and herb layer.Tree layers were mainly composed of big and tall Z. schneideriana trees. The shrub layer had some juveniles of Z. schneideriana and the number of Z. schneideriana in herb layer was the fewest. (7) The Z. schneideriana community was unstable. The ratios of cumulative inverse of species number to cumulated relative frequency were 31/69, 30/70, 28/72 in Longtan Forest Farm, Huijiacun and Shijiashan Forest Farm respectively. The results were considerably different from 20/80, suggesting that the tree layer of Z. schneideriana community was not stable in the three areas.Population dynamics: In order to understand the structure and dynamic of Z.schneideriana population in the mountainous areas of southern Jiangsu, a total of seven (20 m×20 m) plots in Longtan, Huijiacun and Shijiashan were established respectively. We used the DBH class to represent the age class structure and adopted the method of spatial sequence as a surrogate for temporal change. The population structure and population dynamic were analyzed. (1) There are 782 individuals,among them, 247 individuals in Longtan, 243 individuals in Huijiacun, 292 individuals in Shijiashan. (2) The Z. schneideriana trees were divided into ten age classes in Longtan and Shijiashan, nine age classes in Huijiacun. The population structure was fluctuant across the three sites. The seedling and sapling trees was the most, the medium trees were the second, the adult trees were the fewest. The population in each site was a typical increasing population. (3) The expected longevity was the highest at the class I in the three areas. On the whole, it generally decreased with the increases of age classes. (4) The mortality rate was the highest in the medium tree (class ? in Longtan and Shijiashan, class V in Huijiacun) in the three areas. The vanishing rate reached the highest at the largest classes (class X in Longtan and Shijiashan, class IX in Huijiacun). (5) The survival rate gradually reduced with the increase of age classes in Longtan, Huijiacun and Shijiashan.Population survival accorded with Deevey II type.Population structure diversity: the relationship between the reference tree and neighbouring tree was studies by using the structural diversity index. We established the plots of 30 m× 50 m, 30 m×50 m and 40 m×50 m in Longtan, Huijiacun and Shijiashan respectively. (1) In general, the Z. schneideriana reference tree was surrounded by the two and more than two other trees. The Z. schneideriana was highly interspersed and the interspecific competition was more intense. (2) There was a short distance between the reference tree and neighbouring tree in the three areas.The average tree-tree interval distance was 3.33 ± 0.18 m in Longtan,3.35± 0.20 m in Huijiacun, 2.42 ± 0.17 m in Shijiashan respectively. Hence the stand density was the biggest and the intensity of competition was the strongest in Shijiashan. (3) Averagely,the DBH of Z. schneideriana was larger than those of the three neighbouring trees,showing that the Z. schneideriana occupied a certain advantage in horizontal plane. In Shijiashan, this advantage was more obvious. (4) The reference tree of Z.schneideriana was much higher than its three nearest neighbouring trees, showing that the Z. schneideriana occupied a certain advantage in vertical plane. (5) All the recruitment was mainly seedlings but fewer saplings. The Z. schneideriana depended on seeds to reproduce. The density of recruitment was 1.84 n /m2 in Longtan, 1.92 n/m2 in Huijiacun, 3.04 n/m2 in Shijiashan respectively.Persistence mechanism: first, the results of structure diversity showed that most adult Z. schneideriana were surrounded by heterospecific neighbouring trees with short distances between them, suggesting that the Z. schneideriana was facing an intense interspecific competition. However, the Z. schneideriana had great advantage over their neighbouring trees both in horizontal and vertical planes based on the results of the diameter and height differentiation indices. Hence Z. schneideriana could obtain more environmental resources on account of its higher competitive capacity. Second, there were many recruitment in the sampling plots, and they could occur on forest edge, slope and beside creek valley, showing that this species had strong regeneration ability. In addition, Z. schneideriana mainly depend on seeds for reproduction rather than sprouting, thus enhancing its genetic diversity. Probably because of these reasons, Z. schneideriana population had complete age structures and was a typical increasing population. Meanwhile the Z. schneideriana can be widely distributed in various deciduous broad-leaved forests in which it can occupy the dominant position in the tree layer. Aditionally, the Z. schneideriana can also grow in bamboo forest.Therefore, it is suggested to take effective measures to improve conservation management of Z. schneideriana, especially for its community habitats. For example,it may be practicable to cut some stalks of Phyllostachys edulis when it has invaded into the deciduous forests of Zelkova. Another possible practice is to make the thick ground layer sparse in some forest stands to facilitate the seedling establishment of this tree species. At the same time, it is also important to improve the ecological protection consciousness of villagers and tourists concerning the rare and endangered species by propagating the importance and necessity of ecological construction.