Characteristics Of The Vegetation Communities In The Upper Reaches Of The Minjiang River

The upper reaches of the Minjiang River, located between the Qinghai-Tibet plateau to the Sichuan Basin, is a fragile as well as an important large-scale and compound ecological transition zone , which is a typical case in terms of its complexity of natural environment, fragility of ecosystem, marginalizatioin of economical development and transitionality of social-culture in China, hence becoming one of the hot spots of world biodiversity conservation and key areas of domestic biodiversity conservation. A long history of anthropogenic influences combined with severe natural environments has resulted in its regional degraded vegetations, bringing out a series of ecological problems such as loss of biodiversity and posing a serious threat to the whole drainage basin of the Minjiang River even the ecological security and regional sustainable development in the upper reaches of Yangtze River. The readiness of regressive succession in ecological environment after being damaged and the slim possibility of restoration caused by fragility of ecological environment, makes an urgent call for a fundamental theoretical research on biodiversity conservation and restoration.Selecting the upper reaches region of Minjiang River as subject, revolving around the mutual responds and relationships between the environmental gradient and vegetation characteristics, this paper carries on the research on the vegetation classification, the connection between community distribution and environmental factors, population niche, community diversity, community complexity and community stability by investigating a large number of vegetation communities in this region and combining multivariate statistics such as TWINSPAN, DCCA, canonical correlation analysis, principal component analysis and AHP. The main conclusions this paper comes to are as follows:(1) DCCA ordination analysis shows that in the upper reaches region of the Minjiang River, the distribution of plant communities under the mixed influence of soil nutrient and water content demonstrates a high correlation with soil water content, nutrient and micro-topographigraphy, as illustrated in each axis of DCCA. Altitude, slope shape, slope degree, direction as micro topographical factors and available P, total P, soil water content, total N and soil pH as soil factors can produce a significant influence on the distribution of plant community.(2) Compared with previous studies in the gradient of community, this research on population niche in the resource gradient of soil nutrient and water content can generate a clearer demonstration of the ability of using and adaptability to environmental resources of populations. The niche breadth of main shrubs and herbs can reflect the adaptability to the environment and evenness of distribution of populations. Onosma farrerii, Caryopteris terniflora, Sorphora vrcifolia, Lespedeza floribunda, Bauhinia faberi, Scilla scilloides, Indigofera bungeana, Potentilla leuconota,Themeda triandra, Carex crebra, Spiraea alpina, Cotoneaster acuminatus, Rosa omeiensis, Potentilla fruticosa, Sibiraea angustata, Carex lanceolata, Anaphalis margaritacea, Ranunculus japonicus, Bupleurum tenue, Polygonum phaerostachyum have higher niche width and stronger adaptability, hence becoming a dominant populations in this region; while Spiraea henryi, Quercus cocciferoides, Sageretia pycnophylla, Salsola collina, Calamagrostis purpurea, Artemisia igniaria, Tragus berteronianus,Salix cheilophilla, Rubus mesogaeus, Hippophae rhamnoides, Artemisia roxlourghiana, Polygonum viviparum,Fragaria gracilis have lower niche. Not all populations with lower niche are dominant populations. This peculiarity depends on the range and evenness of distribution of the certain population. Populations with higher niche breadth always have higher niche overlap, but not all populations with smaller niche breadth have lower niche overlap.(3) Among the plant communities in arid valley, with an increase of altitude, both the a diversity and β diversity of herbs layer and shrubs layer display a "high-low-high" trend. The α diversity and β diversity of herbs layer are higher than those of shrubs layer. Both the herbs layer and shrubs layer have higher a diversity and β diversity at an altitude range of 1400～1600m and 2000～2200m, which is a potent proof that a low biodiversity occurs in regions of a medium altitude, while a high one exists in regions of high and low altitudes.The diversity varies in different formations: Form. Pertya sinensis, Jasminum humile .Form. Berberis wilsonae, Lonicera japonica, Form. Spiraea Spp.have higher a diversity and β diversity. Form. Leptodermis purdomi, Form. Caryopteris spp. and Form. Bauhinia faberi , Sorphora vrcifolia have lower α diversity and β diversity; both a diversity and β diversity in different slope directions and shapes: shaded >slope half-shaded> slope sunny slope, concave slope> straight> convex slope; in different positions: lower slope> upper slope> middle slope . The a diversity of shrubs layer and herbs layer alike in different slope positions: upper slope> lower slope> middle slope, while the β diversity, lower slope> upper slope> middle slope.Among the plant communities in origin region of the Minjiang River, with an increase of altitude, diversity of both the herbs layer and shrubs layer display symmetrical changing trends that they are almost the same at altitude ranges of 2700-3300m and 3300-3900m. The altitude ranges of 2900-3100m and 3500-3700m see a higher diversity, while those of 2700-2900m, 3100-3400, 3700-3900m see a lower one. The maximum diversity of shrubs layer and herbs layer occurs at the altitudes of 3000m and 3600m respectively. The diversity in different vegetation types follows the trend: forests formations >shrubs formations >meadow formations. The diversity in herb layer of Form. Salix spp., Form. Sibiraea angustata, Form. Spiraea spp.and Form. Caragana spp.are at high level, while Form. Hippophae rhamnoides and Form. Berberis amurensis are at lower level.In meadow formations, the diversity of Form. Imperata cylindrica and Form. Carex spp.are higher than that of Form. Sub-alpine and Form. Kobresia spp. As for shrub layer , Form. Caragana spp. has the highest diversity, Form. Hippophae rhamnoides has the lowest diversity. Diversity in herb layer is higher than that of shrub layer.Under a great influence of altitude gradient, species compositions in plant communities in the origin area of Minjiang River show significant differences and the β diversity is more easily affected by the change of herbs layer. Along the altitude gradient, there are higher species turnover rates at the altitude ranges of 2900-3100m and 3500-3700m, and the community has a higher heterogeneity. While at the altitude ranges of 2700-2900m, 3200-3300m, 3800-3900m,, the community has a lower heterogeneity and species turnover rate. The change of β diversity in shrubs layer is comparatively stable along the altitude gradient, with its climax occurring at an altitude of 2900m and the minimum at 3300m.A significant quadratic curve relationship between the β diversity and soil moisture and nutrient shows that the soil nutrient and water content of the plant community with the highest species turnover rate are at a medium level, instead of a high level of nutrient and water content. (4) The very significant positive relationship exist between the aboveground biomass of shrub communities and the altitude, available K, soil moisture, organic matter, total P, hydrolyazble N, total N, available N in the upper reaches of the Minjiang River, while the very significant negative relationship exist between slope gradient and pH value, among which altitude, available K and soil moisture play a leading role in affecting the aboveground shrub biomass. The aboveground biomass of shrub communities increases with the raise of species diversity and species richness and has a very positive linear relationship with species diversity and species richness.(5) Huffman coding method of communication theory is used to study the plant complexity and the relationship with environmental factors and reveal that between plant community complexity and diversity , evenness. Both total and structural community complexity vary with degree, shape and aspect of slope, with an order of upper slope>middle slope>lower slope, shaded slope> half-shaded slope>sunny slope, and concave slope>straight slope>convex slope. Form. Pertyasinensis,Jasminumhumile have the highest total complexity, and Form Leptodermispurdomi and Form Ceratoidesarborescens communities have the lowest total complexity, while Form Quercnscocciferoides and Form Onosma farrerii communities have the highest structural complexity.The plant formations in origin area have a high total complexity at the altitude ranges of 2900-3100m and 3500-3700m. The total complexity is as the following: Forest plant formations>shrub formations>meadow forations. Form. Picea asperata is ranked as the highest in total complexity, Form. Kobresia spp have a lowest one . In shrub formations, Form. Caragana spp.have the highest total complexity, Form. Hippophae rhamnoides have the lowest one. In meadow formations, the total complexity of Form. Carex spp. and Form. Imperata cylindrica are at high level, Form. Kobresia spp.and Form. Sub-alpine are at lower level. Similar to the correlation of β diversity with soil nutrient, The total community complexity had significant quadratic correlations with soil water content and nutrient. However, the soil factors that influence total community complexity in origin area and arid valley are different, therefore, it does not grow in a simple linear way with the increase of soil nutrient and water content.Total community complexity is positively correlated with community diversity, evenness and species richness, while structural complexity is negatively correlated with community evenness. As two components of total community complexity, structural complexity is more sensitive than diversity to the change of species in the community, which is not only related to community evenness, but also to community richness. In the communities with a high evenness, the difference between total complexity and diversity is not quite obvious, and structural complexity is lower. However, the exceptional case exists in which a low evenness cannot push the low structural complexity of the communities with a low species richness to its climax. The relative contribution of structural complexity and diversity to total complexity would be different for different study area or ecosystems.(6) The stability of typical plant formations in the upper reaches of Minjiang River is analysed by using AHP methods, combining quantitative character indices reflecting vegetation stability, such as vegetation coverage, diversity, complexity and degree of succession with external environmental factors such as climate, topography and soil character, hence supplementing the insufficiency that previous researches on vegetation stability assessment take the only vegetation character into account. The result indicates: the plant formations of origin region, Form. Picea asperata, Form. Salix spp., Form. Sibiraea angustata, Form. Abies faxoniana, Form. Spiraea spp. Have higher stability, while Hippophae rhamnoides and Form. Berberis amurensis have lower one. Form. Imperata cylindrical and Form. Imperata cylindrica have a higher stability, Form. Kobresia spp.and Form. Sub-alpine have lower one. In the plant formations of the arid valley region, Form. Spiraea spp,Form. Daphne spp., Form. Onosma farrerii, Form. Bauhinia faberi, Sorphora vrcifolia, Form. Caryopteris spp., Form. Quercus cocciferoides have a higher stability.Under a constant external disturbance, regressive succession of the vegetation occurs in the upper reaches of the Minjiang River. Regressive succession model of the vegetation in the arid valley is: Form. Quercus cocciferoides, Form. Spiraea Spp.→Form. Pertya sinensis, Jasminum humile, Form. Onosma farrerii→Form. Ceratoides arborescens, Form. Caryopteris spp. Form. Bauhinia faberi , Sorphora vrcifolia; the regressive succession model of the origin region is Form. Abies faxoniana, Form. Picea asperata→Form. Sibiraea angustata→Form. Caragana spp., Form. Berberis amurensis→Form. Imperata cylindrical, Form. Kobresia spp.