Studies On Forest Community Structure And Plant Diversity At The Timberline Of Mt. Sejila In Tibet

Abies georgei var. smithii is the main constructive species of the forest communities at alpine timerlines of the southeast area of Tibet, and has become the dominant tree species of alpine timerlines at the shady slopes. Based on data obtained through located surveys on the environment gradients of the timerline zones at the altitudes between 3,600-5,300 m of the Mt. Sejila, the present dissertation systematically analyzed the ecological characteristics of Abies georgei, the changes of species diversity of the plant communities along with changing of the altitude gradients, and the quantitative characteristics of the forest community structures and the stabilities of the timerlines. Studies on plant diversity conservations at the timerline zone were further conducted based on the above analyses.Totally, 168 species of seed plants were recorded at the timerline zones of Mt. Sejila, and they distributed in 44 families and 117 genera. Among these plants, 20 species were endangered plants, and it is determined that Rhododendron aganniphum var. vellerum was the index species of the timerline at this area. The floristic elements of Mt. Sejila the dominant species of their floristic elements belonged to genera distributed in the north temperate zones, the elements at the highest alpine zone were dominated species, and these type of ecosystems was terribly frangible.The upper limit of closed forest of Mt. Sejila was located at the altitude of 4,320 m. There were two kinds of alpine timerlins, the alpine timerlines at the sunny slopes changed gradually, and those at the shady slopes changed abruptly. The upper limit altitude of distributions of the timerlines at the sunny slopes was 4,570 m, that at the shady slopes was 4,390 m, and the former was 180 m higher than that of the later. The vertical widths of the timerline ecotones were 250 m and 70 m, respectively. The widths of the ecotones at the sunny slopes were averagely 180 m wider than those at the shady slopes.Our investigation showed that trend of the horizontal structures of the alpine timerlines at southeast Tibet was changed in regional types from semi-arid to semi-humid then to humid along with increasing of longitudes from the west to the east. Along with the increases of the water-heat conditions, the altitudes at which timerlines occurred became higher and higher, and the widths of the timerlines were basically unchanged. The widths at shady slopes were 70-100 m, and those at sunny slopes were approximately 250 m.The spatial distribution patterns of Abies georgei, which is the constructive species of the timerlines at Mt. Sejila, were all presented in a way of aggregated distribution. Furthermore, the order of indexes of sociabilities were: seedlings > young trees > standing trees. The optimum altitudes of Abies georgei distribution were approximately 3,800 m, the diameter class structure of populations was figured in a manner typically as the letter "J" writing in a opposite way, and the species density was approximate 380 trees per hm~2 . The age structure of the populations was present as the golden tower, and they were grouped as expandable the populations.The regeneration of the Abies georgei populations occurred in a gap regeneration way. There existed a totally uptrend of the communities along with changes of environmental conditions (e.g. temperature increasing, formation of forest gaps, and so on), and this kind of changing trend was mainly happened at the shady timerlines. It is reflected from the static life table and the survival curve that Abies georgei suffered intensive environmental screening and competitive self-thinning at the early 20 years of its community formation and the period of 60-160 years in its growth and development stage, and also suffered a death fluctuation related to changing of environment conditions at the later stage. The physiological life span of Abies georgei was approximately 200 years, and its maximum life span was approximately 400 yeas. The survival curve of Abies georgei communities was generally the Deevey-III type, its mortality rate at early stage was very high, and the intensity of environmental screening toward it was high.Reproductive inputs of Abies georgei populations were relatively higher, but the communities were in a relative equilibrium, and increases of the populations were relative slower. It is shown by the Leslie matrix model that within the coming 100 years, the quantities and age structures of the Abies georgei populations will be generally stable, namely,this period will be the relative stable stage.The a diversities of species in the timerline zones were higher at the sunny slopes than those at the shady slopes. The species richness and the species diversities fluctuatedly decreased along with increases of altitudes either within the various developmental stages of arbor tresses or within the various communities of different life types of arbors, shrubs and grasses. The richness and the diversities of the communities were largely affected by herbaceous plants.The abrupt types of timerlines possessed relatively lower a diversities and higher species turnover rates than those of plant communities at the two sides of the timerline ecotones, and those of the gradual types of timerlines were happened in a opposite way. The species co-occurrences at the sunny slopes were higher than those at shady slopes. The co-occurrences between communities decreased with the increases of the differences between their altitudes. The results show that all individual communities had the composing elements in their own characteristic "family members".It is shown by result sorting using the two-way indicator species analysis (TWINSPAN) and the detrended correspondence analysis that there existed obvious different zones of the timerlines at Mt. Sejila, and the timerline zones could be classified into 7 different vegetation types. Results of the different vegetation community types show that the diversity indexes of the various types of plants at the sunny slope timerlines were ordered as follows: arbors < shrubs < grasses, and those at the shady slope timerlines: arbors < grasses < shrubs. The species turnover rates at the shady slopes were higher than those at the sunny slopes. The results of community stabilities show that the timerline communities at Mt. Sejila were the types of zonal communities, and they belonged to climaxes.The present dissertation conducted at the first time the systematical studies on the alpine timerline structures in Tibet. Both the systematization, representativeness, theories and research methods of the studies are innovative, and the results of the studies have filled up a gap concerning on the studies on the typical alpine timerlines at the southeast Tibet. The data and results obtained in these studies would lay a basic work for researches on timerlines at this typical alpine region.