Study On Soil Anti-scourability Of Secondary Successional Plant Communities In Jinyun Mountain

Large numbers of relatively integrated plant communities at different secondary successional stages have formed in Jinyun Mountain due to the effects of tourism, trampling, felling, fires and other human activities. The secondary succession of plant communities in Jinyun Mountain is the epitome of the vegetation restoration and reconstruction in the Three-Gorge reservoir area. Therefore, researching the soil erosion mechanism of plant communities in Jinyun Mountain and discussing the relationship between the plant succession and the soil anti-scourability have some theoretical and practical value to the protection of natural forests and the eco-environmental construction of vegetation in the Three-Gorge reservoir area. Soil as a target being eroded, the soil anti-scourability (the ability of soil to resist the physical action of overland flow scouring and transporting) has an important impact on the soil erosion, and it's one of the quantitative indexes to measure the soil anti-erodibility. In this paper, taking ten plant communities at different secondary successional stages in Jinyun Mountain as the research object, the soil anti- scourability is studied by means of the scouring experiment with undisturbed soils. The variation rule of the soil anti-erodibility in the succession of plant communities is revealed. The main conclusions are as follows:1. Under the condition of discharge flows of 1L/min, 1.5L/min, 2L/min, the differences in Reynolds numbers and Froude numbers of slope sheet flows of ten plant communities are small, fluctuating around a certain value. However, under the condition of different discharge flows, there's a larger difference in every plant community's own Reynolds numbers and Froude numbers. With the increase of the discharge flows, both Reynolds numbers and Froude numbers of slope sheet flows of ten plant communities have a tendency to increase, but they don't have qualitative change, that is, the overland flows are in the laminar state, and are regarded as turbulent flows.2. According to the relation between the erosion and sediment yield by flow and the soil critical shear stress: M=A (t - t₀) t·s, it's calculated that the soil critical shear stresses of ten plant communities vary in the range of 0.397～0.889 (N/m²). Based on the magnitude of critical shear stress, the anti-scourabilities of ten plant communities are approximately divided into four grades from weak to strong, that is, that of Pinus massoniana and Pinus massoniana & Cinnamomun camphora is poor; that of Phyllostachys pubescens, Cunninghamia Lanceolata & Cinnamomun camphora and Neosinocalamus affinis is ordinary; that of Machilus pingii, Cinnamomun camphora, Gordonia acuminata and Castanopsis fargesii is better; that of Imperata cylindrica is best.3. The soil anti-shear strength of ten plant communities is determined using 14.10 pocket vane tester. Through the regression analysis, it's found that the relationship between the soil anti-shear strength and the critical shear stress is close, and the linear regression model is established that t=0.7165k+0.2095, R²=0.9638. Therefore, the soil anti-scourability can be judged quickly and relatively exactly in the field by determining the soil anti-shear strength.4. From the point of view of energy, the soil anti-scourability is expressed by the amount of soils scoured by the unit flow energy consumption per unit time and area. Its formula is K=M_æ²™/(E_耗·T·A). Based on this formula, the K values of Cinnamomun camphora plant community under the conditions of three kinds of discharge flows of 1L/min, 1.5L/min, 2L/min and three kinds of gradients of 5°, 10°, 15°are calculated. The analysis of variance is used for the K values, and the reuslt is that under the condition of the same discharge flow, the change of K value is not significant with the change of gradient, while under the condition of the same gradient, the change of K value is not significant with the change of discharge flow either. This indicates that it's feasible to measure the magnitude of the soil anti-scourability by the flow energy consumption. The magnitude of the soil anti-scourability is only related to the properties of soil itself, not related to the outside factors. The different discharge flows and gradients only make the scouring power of slope flow different, and them don't lead to any change of the magnitude of the soil anti-scourability.5. The Bivariate correlation analysis is performed to seven types of indexes(22 in total) which are critical shear stress and compaction (soil specific gravity, soil bulk density), mechanical composition (coarse sand, fine sand, coarse silt, fine silt, coarse clay, colloidal particle), water-stable aggregates (>5mm, 5～3mm, 3～2mm, 2～lmm, 1～0.5mm, 0.5～0.25mm, >0.25mm), soil infiltration (initial infiltration coefficient, steady infiltration coefficient), porosity (total porosity, capillary porosity, non-capillary porosity), organic matter and pH value. The result is that the sequence of correlation coefficients from big to small is >0.25mm (0.877**), >5mm (0.706**), coarse clay (0.693**), capillary porosity (0.664**), fine silt (0.632**), colloidal particle (0.574**), coarse sand (-0.536**), fine sand (-0.501**). The stepwise regression is made between the critical stress and eight indexes which correlation analysis achieves a prominent level. The regression equation is as follows:Y=-0.584+0.19X1+0.006X2-0.004X3 (X1: >0.25mm water-stable aggregates, X2: coarse sand, X3: fine sand). The regression equation indicates that the leading factors influncing the soil anti-scourability are >0.25mm water-stable aggregates, coarse sand and fine sand.6. According to the soil anti-scourability grades of ten plant communities and their own secondary successional stages, it can be found that except Imperata cylindrica, the soil anti-scourability of other plant communities has a tendency to increase along with the chronosequence of the secondary succession of plant communities.Imperata cylindrical plant community is at the primary stage of secondary succession, but its soil anti-scourability is best. This is because that the soil of Imperata cylindrical plant community belongs to the category of cooling sand yellow mud, while soils of other plant communities belong to the category of cooling sand yellow soil. The adhesion between the particles of soil of cooling sand yellow mud is larger than that of soil of cooling sand yellow soil. In addition, the root system of Imperata cylindrical plant community is most developed, and its soil anti-shear strength is highest.