Relationship Between Community Structure And Environmental Factors In Different Successional Stages Of The Karst Vegetation In Mashan In Guangxi

We investigated five different successional stages of the typical karst communities(desertified karst land, grassland, shrub, small arbor, mature forest) in Mashan in Guangxi, the research content involved species diversity of community, soil conditions, community biomass, and related environmental factors. We discussed the interactive effect between vegetation and soil in the process of karst vegetation restoration in different successional stages, explored the influence of environmental factors on undergrowth biomass, and analyzed the main influential factors to the succession and development of karst fragile ecosystem. Research conclusions are as follows:(1)With the progress of succession, species richness index, Shannon-Wiener index, evenness index were first increased and then decreased. Ecological dominance index is decreased with successional stages. In different successional stages the main drive and the key species were different, it was very clear that plants develop from lower and small to large and longevity. Heliophilous species was the dominant species in mature forest, it did not reach the climax stage, but the internal structural and stability of the community is improving.(2) With the consequent of karst vegetation succession, the surface soil bulk density (0-20cm) was decreased, porosity and moisture content were on the contrary. The soil was changed from weak alkaline or neutral to weak acid, organic matter and other nutrient content (except exchangeable calcium) was increased after a slight decline and then stayed in a higher level in the entire process of succession. Organic matter, total nitrogen, total potassium, available nitrogen, available potassium levels were significantly increased, Phosphorus (especially phosphorus) content had been maintained at a low level, there was a significant correlation among nutrient factors. Soil nutrient factors showed the similar variation in the deep-soil (20-40cm) as in the topsoil. On the extent of reduction in the surface soil and deep soil nutrients, the highest total nitrogen (average57.31%)followed by total phosphorus (57.10%), the smallest exchangeable calcium (19.57%), and the largest decline occurred in stages mature forest communities. The total element decreased more than the effective element in the same element.(3)The community biomass advanced forward from the rocky desertification stage of1.93t/hm2to104.77t/hm2in small arbor and then to224.17t/hm2in the mature forest, increased by115.15times with succession. Each layer biomass, biomass of arbor layer increased significantly with age, layer biomass of shrubs increased first and then decreased and remained at a high level, herbaceous layer biomass showed a continued downward trend, litter biomass changed irregularly.(4)The tree layer coverage, bare rock, the content of total potassium and organic matter content were the most prominent in environmental factors which affect the vegetation biomass of forest. Among them, the direct effect and indirect effect of the tree layer coverage on vegetation biomass of understory showed negative; the negative direct effect of bare rock on vegetation biomass of understory was much greater than the indirect effect of positive, total potassium was the most obvious positive effect in the indirect effect; The effect of total potassium on vegetation biomass of understory had positive direct effect and indirect effect, negative indirect effects of organic matter content and bare rock is larger; organic matter influence the undergrowth biomass mainly through indirect effects of total potassium.(5)Principal component analysis of dominant factors that affect the development of karst ecosystems showed that:soil quality conditions and years of restoration had maximum load values in the first principal component, shrub had maximum load values in the second principal component, terrain factors had maximum load values in the third principal component, herb layer had maximum load values in the fourth principal component, and the remaining factor had maximum load values in the fifth principal component. The five principal components variance contribution rates were29.93%,24.03%,14.73%,8.82%,4.94%. Soil quality and years of restoration were the most dominant factors in the restoration process of karst ecosystem, shrub played a connecting link between the preceding and the following, site conditions affected the direction and speed of succession indirectly through the redistribution of water, light and heat, herbaceous layer has certain effect on the system of material circulation and accumulation, the other factors had small effect on the development of karst ecosystem.