5. Conclusions
The purpose of this research was to clarify the coupling effects of the spatial distribution of cypress and topography on surface runoff coefficient and propose a strategy of stand structure adjustment to reduce the surface runoff coefficient, and provide a theoretical basis and technical support for improving the water conservation function of cypress forests on steep slopes in southwestern China.
Among the key indicators reflecting the characteristics of the topography and the spatial distribution of cypress forest, topographic relief, surface roughness, runoff path density, contagion index, and stand density of cypress, all had a significant impact on surface runoff coefficient, and the impact effects were 0.245,- 0.272, 0.239, -0.311, -0.134, respectively. Significant interactions were found between the stand density of cypress and surface roughness, the stand density of cypress and runoff path density, and the contagion index and topographic relief, and the interaction coefficients were 0.773, -0.491, -0.775, respectively; Under the condition of the coupling of topography and spatial distribution of cypress, the surface runoff coefficient increased monotonically with the increase of the composite index of topography (topographic relief*runoff path density/surface roughness), and increased first and decreased later with the increase of the composite index of the spatial distribution of cypress (stand density of cypress*contagion index of cypress); The interaction between the spatial distribution of cypress and the topography enhanced the influence of topography on surface runoff coefficient, with an enhancement rate of 25.05%, and weakened the influence of the spatial pattern of cypress on surface runoff coefficient, with a weakening rate of 40.74%.
To reduce the surface runoff coefficient from a larger value (>0.5) to less than 0.3, two strategies of stand structure adjustment could be adopted, including only increasing the stand density of cypress, or increasing both the stand density and the contagion index of cypress, and which strategy should be adopted depended on the initial stand density of cypress. When the initial stand density of cypress was relatively low (<20 ind/100m2), the first step was to increase the stand density of cypress, and until the stand density of cypress reached to moderate level (20-50 ind/100m2), adjusting the spatial structure of cypress from relatively regular to relatively clumped could reduce the surface runoff coefficient to a greater extent.
In this study, the constructed SEM was highly consistent with the actual observation data and could simulate the causal relationship as well as the coupling mechanism among the topography, the spatial pattern of cypress, and surface runoff coefficient. The next stage of the research would be based on the SEM constructed in this study, combined with the determination of sediment yield and ecosystem service functions to expand the model, so that the optimal allocation for the spatial distribution of cypress was more targeted.