Community Ecology And Biogeography Of The Mossy Dwarf Forest In Yunnan

Understanding the ecology and biogeography of the mossy dwarf forest (cloud forest)is of critical importance in protecting theses rare and endangered ecosystems. Plant diversity, physiognomy, species composition and environment of the less known mossy dwarf forest in Yunnan were studied, based on data collected from 35 sampling plots in 7 sites. The study sites are located in six national nature reserves, from east to west, which are named as Laojunshang National Nature Reserve, Fengshuilin National Nature Reserve, Huangliangshan National Nature Reserve, Wuliangshan National Nature Reserve, Ailaoshan National Nature Reserve, Daxueshan National Nature Reserve, and one well protected Sudian National Forest area. The results ofour studies are concluded as following:1) In floristic composition, the mossy dwarf forests are mainly dominated by Fgaceae, Ericaceae, Vacciniaceae, Aceraceae, Magnoliaceae, Theaceae, Aquifoliaceae, Illicieae, Lauraceae, Rosaceae etc. The dominant species vary from site to site.1) In a 2500 m~2 sampling area of each community, the total vascular species varies from 57 to 110 and the Simpson's index varies from 0.7545 to 0.9544, Shannon–Winner's diversity index from 1.8251 to 3.2905 and Pielou's evenness index from 0.6583 to 0.8982 for trees. The species richness, Simpson's index, Shannon–Winner's diversity index and Pielou's evenness of the community in Laojunshang National Nature Reserve are lower than those of other study sites. In general, the species richness and diversity of the mossy dwarf forests in Yunnan are relatively low in comparison with those forests in lower altitude in Yunnan.3) In physiognomy, the forests are dominated by phanerophyte plants, and lianas are rare. The plants with microphyllous plus nanophyllous leaves contribute to 44.32–63.46% of the total species, and the plants with an entire leaf margin account for more than 50% of the tree and shrub species. There are also less abundant tree and shrub species with drip tip leaf apex and papery leaves. In the mossy dwarf forests, evergreen species make up more than 75% of the total tree and shrub species. The forests are evergreen broad–leaved forests.4) In these forest communities, the DBH class distribution of the tree species decreases with the increase of the tree thickness, and vary obviously from one community to another community. Most trees have DBH less than 35cm.5) The cluster analysis shows that these communities have low similarity.6) At the family level, the mossy dwarf forests in Yunnan is mainly composed by Pantropic distribution, Tropical Asia & tropical America disjuncted distribution and North temperate distribution. The families of tropical distribution are more than those of temperate distributions.7) At the generic level, the flora of the forests is composed mainly by the genera of Pantropic and tropical Asian distributions. The ratio of tropical elements is almost equal to the one of temperate elements. These results indicate that the flora is a transitional one from the tropical to subtropica1.8) At the specific level, the elements of tropical Asian distribution contribute more than half of the total flora. The close relationship of the flora with tropical Asian flora is observed. The species of Chinese endemic and Yunnan endemic distributions are the second large part of the flora, which contribute to 25–42 % of the total flora.9) The soils of the mossy dwarf forest in Yunnan belong to Argosols order, brown soil group,Acid brown forest soil subgroup. Soil nutrient contents, such as total N,P,K,available N,P, K, manifested remarkable variability under different study sites. Compared with other forests in Yunnan, the soil nutrient contents are higher, and the soil pH is lower. The common pH value is about 4.0. Soil nutrient contents are not a limiting factor to tree species growth.10) The soil moisture of the summit mossy dwarf forests in Xijiaba area are higher than that of the monsoonal evergreen broad–leaved forest, the mid–montane humid evergreen broad–leaved forest, the transitional forest between the mid–montane humid evergreen broad–leaved forest and the mossy dwarf forest and Populus bonatii forest. And the soil moisture of the mossy dwarf forests manifested remarkable variability under different study years at the same times. The soil moisture significantly increases with increasing altitude.11) Same as other vegetation types in Ailao Mountain, soil moisture of the summit mossy dwarf forests show the same cycle period. In rainy season soil moisture is high and in dry season soil moisture is low.12) The soil moisture of the Populus bonatii forest shows the same variability under different layers. This result indicates the second forests of the mid–montane humid evergreen broad–leaved forest have no significant impact on the dynamic of soil moisture after a long time recover.13) The soil moisture of the 0–10 cm soil layer is higher than that of the 10–20 cm soil layers. There is commonly big discrepancy between the 0–10 cm soil layer and the 10–20 cm soil layer in rainy season but in dry season, except the summit mossy dwarf forest. The discrepancy in rainy season increases with increasing altitude. Discrepancy happens in the summit mossy dwarf forest all the year round, and the big discrepancy happens during the rainy season. The key reason is that the summit mossy dwarf forest is frequently covered in cloud or mist, so it can intercept more cloud water, and condition the 0–10 cm soil layer under wet conditions.14) Low temperature is not the necessary element for the occurrence of the cloud forest (summit mossy dwarf forest). The environments of the summit mossy dwarf forests in Yunnan are very wet and the annual mean humidity is higher than 85%. So the wet environment may be the key fact for the occurrence of the summit mossy dwarf forest.15) The summit mossy dwarf forests bear all the characters of tropical montane cloud forest and possess high percentage of tropic elements species. They should be treated as one of the world tropical montane cloud forest. The term'summit mossy dwarf forest'has the same meaning as'upper montane cloud forest'.