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.