| Purpose and SignificanceMalan Forest Region in the east of Xunyi County in Shaanxi Province locates in the south of the Loess Plateau of China. It is birthplace of the Jin River system and it is one of the forests which have the biggest forest cover rate in the loess plateau. There are few system researches on the whole and the structure of plant communities, such as different successions, different layers and so on in this region. How tree response to environmental factors? How much vegetation pattern variation can be explanted by environmental factors? How much vegetation pattern variation can be explanted by spatial factors? Whether there exits interaction between environmental and spatial factors? Whether there are the more environmental factors the better? Are there dominant factors? How much information could they take up? We always use measured factors to explain vegetation pattern variation, but what is the potential environmental factors? How important they are? Those questions weren't be answered by predecessors. Our research is due to solve those questions based on the vegetation in Malan Forest region. Solving those problems above not only enrich the quantitative research contents about the community distribution pattern theoretically, and deepen the understanding about the relationship between community and environment at different succession stages, but also has a significant meaningful to forest vegetation about protection, tending, renewal and restoration in practice.Research methodsOn the base of investigation the whole types of communities in Mala Forest Region completely, different representative community types were selected, including 60 plots which contains 45 forest plots, 10 herb plots and 5 shrub plots. Forest plots contain 5 Robinia pseudoacacia plots, 3 Populus davidiana plots, 3 Betula platyphylla plots, 10 planted Pinus tabulaeformis plots, 8 natural Pinus tabulaeformis plots and 16 Quercus liaotungensis plots. 17 variables were used as indicators of spatial position, soil characteristics and topography. Plant community types in different successional stages in this region were analyzed by TWINSPAN. Environmental factors which have significant influence (p<0.05) on the composition of plant communities in different successional stages were selected by forward selection and Monte Carlo tests. The relationship between the distribution pattern of plant communities and environmental factors was identified by DCCA,CCA and partial DCA. The influence of environmental and spatial variables and their interaction on the variation of community distribution patterns was quantitatively separated by partial CCA. How tree response to dominant soil factors was researched by GLM model.Research Results1) Forest floor, aspect, elevation and organic matter were the important measured environment gradients which impacted the distribution of forest types while other available nutrients were secondary important. Potential environmental factors could explain 57.7% information of tree composition variation. Disturbance was the important potential environment gradient which impacted the distribution of forest types.2) Salt and available nitrogen were the important environmental factors which effected the plant communities composition significantly at the early stage, and they had extracted 82% environmental information; Elevation and humus were the important environmental factors which effected the plant community structures significantly at transitional stage, and they had extracted 67% environmental information. Elevation, aspect, forest floor, humus and pH value effected plant communities composition significantly at the sub-climax stage, and they had extracted 65% environmental information. Elevation, aspect, pH value and available phosphorus were the environmental factors which effected species composition significantly at the climax stage, and they had extracted 64% environmental information. With the succession processing, environmental gradients were becoming more and more complex and climax community formed finally which were suitable to local climate, topography and soil condition (explained variation caused by environment factors alone was 66.5%). The effect of spatial position alone on plant community was the biggest at preliminary stage and the transition stage (27.4%, 41.6%) comparing to other stages. Interaction of spatial position factors and environmental factors was the most influential at preliminary stages (46.4%) while the effect was relative constant at other three succession stages (14%-22.6%).3) DCCA ordination at different succession stages suggested that distribution pattern of plant communities were in good agreement with ecological gradients in Malan forest region. The result showed that either the species composition or the habits were similar at different succession stages internal and it also corroborated the viewpoint above at the same time. With the succession processing, environmental gradients were becoming more and more complex and there were the most complex relationship between plant community and environmental factors at climax stage. As succession proceeds, the impact of environmental conditions on plant communities increases, while the total influence of spatial location, and the interaction between spatial and environmental factors decreases. The answer might be that communities were constituted by widely distributed species at early stage. Those species tended to be widespread owing to dispersal ability and other life-history traits such as long seed viability.4) Elevation, pH value and forest floor are significant factors to the structures of tree, shrub and herb layers; Humus are significant factors to the structures of tree and herb layers; Organic matter effects herb structure significantly; Total nitrogen, slop and aspect effect shrub structure significantly. From the size of explanted variation, the effect of environmental factors is big and stable (21.1-23.3%); the effect of spatial factors is small and stable (3.4-4.4%); the effect of interaction is very unstable (2.3-8.7%) in different layers. Comparing the different layers of the forest, tree and shrub layers response to environment and spatial are similarity, but herb layer is different significantly. Herb layer response to spatial factors is small (3.4%), while interaction is the biggest (8.7%). It is 3.8 time to tree layer, and it is 2 time shrub layer. That indicates the structure of herb layer is unstable. Unexplained variation take up 72.2% in tree layer, 68.3% in shrub layer and 64.5% in herb layer. The result show vegetation character is complex in Malan Region of loess plateau. They are not only affected by measured environmental factors but also affected by some potential environmental factors.5) Organic matter and pH value are the key factors to forest community structures. The response peak of pH to Q. liaotungensis is about 8, to P. davidiana is about 8.3, to B. platyphylla is more than 8.5 while P. tabulaeformis is less than 7.4 or more than 8.7. The response peak of Associated species to pH is divided into two regions: 7-7.4 and 8-8.5. The response peak of organic matter to Q. liaotungensis is more than 40 mg·kg-1, to P. tabulaeformis is about 28 mg·kg-1, to B. platyphylla is less than 20 mg·kg-1, and to P. davidiana is less than 12 mg·kg-1. The response peak of associated species to organic matter is divided into four regions: <10mg·kg-1, 20～25mg·kg-1, 30～40mg·kg-1 and >50mg·kg-1.6) From the point of succession, organic matter will became richer and richer in the succession of B. platyphylla / P. davidianaâ†'P. tabulaeformisâ†'Q. liaotungensis while pH value will became more and more acidic in the succession of B. platyphylla / P. davidianaâ†'Q. liaotungensis. The succession of P. tabulaeformisâ†'Q. liaotungensis is more special than others, because yousong exist in two different environment condition, such as pH<7.4 and pH>8.7 (related to human planted). Q. liaotungensis grow in pH=8 environments, the change action is related to the change of species composition in succession. 7) Forest floor is always thicker in planted P. tabulaeformis, and it could enrich the soil organic matter. But the soil is always more acidic, which makes forest floor decomposition more slowly. Nutrient cycling is blocked. So, the key question of accelerating succession was to find a proper way to promote forest floor decomposition, which will provide a good condition for Q. liaotungensis seeds germination and growth. |
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