An assessment of the relationships between the surface soil properties and components of slope and vegetation in the Upper Wadi Bishah Basin, Saudi Arabia
This study, the first of its kind undertaken in Saudi Arabia, investigates and evaluates the condition of soil and vegetation and the relationship between them, as well as their relationship with slope components in the Upper Wadi Bishah basin, south-west Saudi Arabia. It is based mainly on quantitative and laboratory analysis, and provides a scientific basis for exploitation of the basin's slopes and conservation of its soil and vegetation. The findings reveal that floristic diversity of the research area is low. Only 62 perennial species belonging to 49 genera and 28 families were recorded in the upper Wadi Bishah basin. Of these, only 8 species or 12.9% of all species recorded, account for 89% of the absolute frequency of species in the basin. Although the vegetation density in this basin is low (2.71/ 100 m²), it is considered relatively high compared with other areas in Saudi Arabia. However, great diversities in density and distribution of vegetation were found between slope segments, and also generally between the south-west and north-east parts of the basin. The floristic composition and distribution of main plant groups were found to reflect the condition of soil and the topographical variety in the wadi basin. Also, it was found that over-grazing, particularly in north-east of the basin, is reflected in deterioration and degradation of vegetation and soil. In terms of soil properties, this study demonstrates quantitatively that the soil of south-west Wadi Bishah basin is generally loamy sand, shallow and somewhat rich in moisture, organic matter, organic carbon, nitrogen and phosphorus, and lacking in CaC0₃, potassium, electrical conductivity and pH, compared with the north-east of the basin. In the north-east of the Wadi Bishah basin, the soil is mostly sandy loam to sand, deep and very lacking in moisture, organic matter, organic carbon, nitrogen and phosphorus. Also, this region is affected by salinity problems. On the vertical level, deeper soils were found in the lower slopes and shallow soils found in the middle and upper slopes. Soil moisture, potassium and electrical conductivity increase down-slope and decrease in the middle and upper slopes. Soil organic matter, CaC0₃ and pH decrease going along the slope units from the top to the bottom. This study has revealed that slope angle and slope gradient have negative significant relationships with soil depth, sand content, potassium, soil pH and electrical conductivity, and positive significant relationships with moisture, silt content, organic matter, organic carbon and nitrogen. Slope length has inverse significant relationships with organic matter, organic carbon and nitrogen, and positive significant relationships with soil depth and potassium. None of the soil properties are associated significantly with slope form. Vegetation cover value and vegetation density are significantly correlated with almost all soil properties. Only electrical conductivity and clay content are not significantly associated with vegetation density. It is clearly demonstrated by this study that 51%, 27%,14%,56%,56%,12%, 45%, 6%, 6%, 31 % and 1 % of the variation in soil depth, moisture, texture, organic matter, organic carbon, CaC0₃, nitrogen, phosphorus, potassium, soil pH and electrical conductivity respectively relate to the function (or to the variation) of slope angle, slope gradient, slope length, vegetation cover value and vegetation density.