Seedling competition of Lasiurus scindicus and Rhazya stricta in response to water stress

Rhazya stricta is a widespread invading shrub in deteriorated rangelands in Saudi Arabia. Limited studies indicate that overgrazing by livestock puts this species at a competitive advantage. The objective of this study was to investigate the competitive relationships of R, stricta and Lasiurus scindicus, a valuable range grass, under three levels of soil water at seedling stage. Seeds of both species were sown in 500-mm high and 160-mm diameter tubes filled with soil either in monoculture or in mixtures at equal rates. When seedling emergence ceased, seedlings were thinned to 10 seedlings of monocultures or mixtures of the two species at equal rates. Soil water was brought back to capacity when depleted to 75%, 50% or 25% of field-water capacity. Results indicated that soil-water had very limited effect on plant growth and survival when averaged over species and species mixture treatments. Performance of the two species differed significantly. As inferred from the ratio of mixture to monoculture for both species, growth and survival of L. scindicus was favored by mixing while that of R. stricta was reduced. This indicates that under similar conditions, L. scindicus out-competes R. stricta. These results have practical implications on grazing management and revegetation of rangelands in Saudi Arabia. (C) 2001 Academic Press.