|其他摘要||Yongde Snow Mountains, which located between 24°00′--24°12′ N and 99°32′--99°43′ E, are administratively belong to the Yongde County, Southwest Yunnan, China. They are located in the south of the continued parts of southwestern Hengduan Mountains (i.e. Nushan Mountain), and mainly made up of Mt. Daxue (alt. 3504.2 m), Mt. Xiaoxue (alt. 3363 m), Mt. Boluo (alt. 2991 m), Mt. Mannaixueshan (alt. 3364 m), Mt. Niufengdui (alt. 2974 m) etc. geographically. The study area, Yongde Snow Mountains region, which is composed of seven towns (including Daxueshan, Wumulong, Yalian n, Yongkang, Banka, Dashan and Chonggang), covers from Mahuangqing in the east to Yongkang River in the west and from Wenhua village in the north to Nanting River in the south,. The total area of the Yongde Snow Mountains region is about 1700 km2.
Based on the previous research result on the Yongde Snow Mountains region, we have carried out further study on the vegetation and the floristic characters of the region, and also on the pattern of species richness along altitudinal gradients in the region. The result of this study is offered in the dissertation.
The characteristics of vegetation in the Yongde Snow Mountains region
The vegetation of the Yongde Snow Mountains region distributes in no definite rule latitudinally or longitudinally, and the base-belt vegetation is mixed with each other along different slopes of the mountains. Due to the climate difference caused by altitudinal gradients, the vertical vegetation belts can easily be distinguished from each other. From the valley of Nanting River and Yongkang River to the peak of Daxue Mountain, a sequence of vertical vegetation types display obviously, containing the vegetation series from tropical to temperate. Besides the distribution characteristics, the vegetation of the Yongde Snow Mountains region has also some other features as below:
1. The evergreen broad-leaved forests are the best vegetation in the region, which includes several Formations, such as Form. Lithocarpus truncatus, Form. Castanopsis delavayi, Form. Lithocarpus xylocarpus, Form. Lithocarpus variolosus, and so on.
2. Tsuga and broad-leaved trees mixed forests distribute in a zone in the area, but no pure Tsuga forest occurs here. Tsuga forests are widely distributed in western Sichuan, southeastern Xizang and northwestern Yunnan. The Yongde Snow Mountains region is different from those previously mentioned in that the Tsuga plants in the area don’t form pure forest.
3. Alpine shrub vegetation is fully developed but alpine meadows are somewhat not so developed.
4. Owing to the heterogeneity of landforms, soil substrates and other factors in different areas, there are some non-zonal vegetation types that are restricted to certain areas. The Juniperus formosana community which can only be found on the range of Bodao Mountain is such a type of non-zonal vegetation whose coming into being has something to do with the landforms and the limestone soil.
The floristic characteristics of the Yongde Snow Mountains region
Based on the floristic statistics and analysis at the level of family, genus, and species, 8 points about the floristic characteristics and the pattern of species richness along altitudinal gradients have been concluded:
1. The Yongde Snow Mountains region is rich in seed-plant species. So far, 190 families, 859 genera, 2025 species (including 53 varieties/subspecies) from the region have been recorded. Among these taxa, 6 families, 10 genera, 15 species are gymnosperms; the other 184 families, 849 genera, 2010 species (including 53 varieties/subspecies) are angiosperms. Through comparing with the adjacent regions in numbers at the level of family, genus and species, it is obvious that the Yongde Snow Mountains region is abundant in plant diversity.
2. Among the190 families in this area, 98 families (excluding the cosmopolitan ones) are tropical, and 45 families are temperate. The percentage in total of tropical families is higher than that of temperate families, which proves that the flora of this region has a close relationship with tropical flora historically and geographically. However, some typical and tropic-endemic families, such as Crypteroniaceae, Dipterocarpaceae, Lecythidaceae, Myristicaceae, etc., don’t occur in this area. Of the 859 genera, 493 (excluding the cosmopolitan genera) are tropical, and 313 are temperate. Among the 2025 species (including 53 varieties/subspecies), the number of tropical ones accounts 837 (excluding the cosmopolitan species), and 1233 species is temperate. Considering the flora characteristics of the area at the level of family, genus and species, it is obvious that the amount of tropical and temperate elements in this region are almost equal, which implies that the Yongde Snow Mountains region is located in an area with a flora transitioned from tropical Asian to East Asian. In another word, the region is a key one that connects the two floras, and its flora has prominent subtropical feature but is essentially more tropical.
3. After the general analyzing of the flora in the Yongde Snow Mountains region and the floristic comparison with the four adjacent regions, it can be concluded that the flora of Yongde Snow Mountains is the closest to that of Mt. Wuliang. Similar to that of Mt. Wuliang, with many East Asian families and genera, the flora of Yongde Snow Mountains still lacks of typical and tropic-endemic families though having a higher percentage of the tropical elements in the region. Therefore, the floristic position of Yongde Snow Mountains is: East Asiatic Kingdom (Ⅲ)---Sino-Himalayan forest subkingdom (ⅢE.)---Yunnan plateau region (ⅢE13)---SW. Yunnan mountain and hill subregion (ⅢE13c).
4. The modern seed-plant flora of Yongde Snow Mountains mainly comes from tropical Asian, China-endemic, and Sino-Himalayan elements, and during the long geological history, the process of fusion of these three elements is a key to the origin of modern flora in the Yongde Snow Mountains area. In addition, because the region is located at the border of East Asiatic Kingdom and in the transitional area between Paleotropical Kingdom and East Asiatic Kingdom, some elements from Indo-Malaysia, Sino-Himalaya and North Temperate developed into variously endemic taxa in the region. Moreover, the region became the northern or southern distribution border of many taxa. All these factors formed the framework of the modern flora in Yongde Snow Mountains region during geological periods.
5. The phenomenon of horizontal and vertical replacement in the region is obvious. On the one hand, some constructive species and dominant species of the communities show a geographical replacement, on the other hand, some close species belonging to the same genus also show a vertical replacement along altitude.
6. The endemism of the flora in the Yongde Snow Mountains region is rather remarkable. In the region, there are 7 families that are endemic to Eastern Asia, which suggests that its flora has a close relationship with that of East Asia during its flora originating and developing. Among the 18 China-endemic genera, some are primitive and old elements, and much more others are new and young elements which probably came into being with the rising of Mt. Himalaya and the upheaving of the Qinghai-Tibet Plateau. All these indicate that the Yongde Snow Mountains region not only has preserved some primitive and old floristic elements, but also developed many young and advanced ones. A large number of China-endemic species show this region’s contact with other natural and geographical regions of China, and the 36 species endemic to Yongde Snow Mountains can make its floristic characteristics more clear and notable.
7. After a statistic analysis on the species richness of each altitudinal gradient (with an altitude range of 100 m) in the Yongde Snow Mountains region, it can be concluded that the species richness pattern along altitudinal gradients is: in the low altitude area, the species richness rises with the increasing of altitude in general and reaches the maximum at the altitude of 2300--2600 m, then it drops with altitude rising. In conclusion, in the Yongde Snow Mountains region the relationship between the species richness and altitude gradient is a unimodal curve, which means that the maximum species richness appears in the middle altitude region. We ascribe the distribution pattern to the probable best combination of water and heat at the middle altitude and the existence of the “hard boundary” of species’ distribution. Furthermore, the other factors such as the areage, heterogeneity of habitats and interference may also affect the distribution pattern of species along altitudinal gradients.
8. Based on the analysis of the disposition in vertical structure of 219 dominant and frequent species in the mid-montane humid evergreen broad-leaved forest, it can be shown that only Tropical Asian, Sino-Himalayan and China-endemic elements can occur in all the synusiae including the A、B、C tree layers, shrub layer, herb layer and so on. Among 219 dominant and frequent species, the Sino-Himalayan and China-endemic elements take up a large portion, and the Tropical Asian elements play an important role as well, which indicates that the mid-montane humid evergreen broad-leaved forest in Yongde Snow Mountains is floristically intervenient between subtropical and temperate. On the other hand, the further analysis of China-endemic species shows that the region is floristically transitional. All these evidences indicate that Yongde Snow Mountains are on the border between Sino-Himalayan flora and Tropical Asian flora, and its transitional floristic characteristics are rather obvious. Through the comparison of dominant and main companion species between Yongde Snow Mountains region and its neighbouring regions, it shows that the mid-montane humid evergreen broad-leaved forest in the region is more “humid and warm”.|