On Spatial Pattern of Species Richness in Plant Communities Along Riparian Zone in Xiangxi River Watershed

There are two approaches adressing the relationships between environmental factors and biodiversity, namely, experimental manipulation of number of species within a community and survey of biodiversity along a natural gradient at large scale. In this study, we employed the latter method to investigate the variability of species richness along the altitude gradient in the Xiangxi River Watershed. Xiangxi River originates in the southern slope of Mt. Shennongjia of western Hubei, and runs for 94km into the Yangtze River. Its mean discharge is 65.5m 3/s. From high to low elevation, vegetation types along the river are composed of needle-leaved forest, needle- and broad-leaved mixed forest, evergreen and deciduous broad-leaved mixed forest, and evergreen broad-leaved forest. In 2000, we sampled 40 plots, each with the size of 100m×10m and 100m apart from its neighboring plots along the gradient of altitude. In each plot, plant species richness was gauged by counting all species present, and environmental factors including slope, aspect, altitude, breadth of stream and height above stream were also recorded. Statistical analysis demonstrated that total species richness (Y 1), arborous species richness (Y 2) and herbaceous species richness (Y 3) were significantly related to altitude. The relationships could be fitted into the parabola function, Y 1=-0.00001X 2+0.0357X+6.0921(R 2=0.536, p0.01), Y 2= -0.000004X 2 +0.012X-1.5709 (R 2=0.577, P0.01), Y 3=-0.000003X 2+0.0105X+5.5894 (R 2=0.324, P0.05), respectively. The highest species richness occurred in the transitional zone between mountain evergreen forest and deciduous broad-leaved forest at the altitude of 1500~1800m. There were great variations of shrub and liana species richness from plot to plot, but no significant relationships with altitude were found at watershed scale. This study indicated that , at large scale, altitude plays an important role in species richness, and spatial heterogeneity resulting from seasonal flooding and attributable to micro-landform determines the species diversity at a small (local ) scale. The results support the prediction that overall species diversity reaches a peak in the midreaches of a pristine river.