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 scind icus, 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 t ubes filled with soil either in monoculture or in mixtures at equal rates. When seedling emergence ceased, seedlings were thinned to 10 seedlings of m onocultures or mixtures of the two species at equal rates. Soil water was b rought back to capacity when depleted to 75%, 50% or 25% of field-water cap acity. Results indicated that soil-water had very limited effect on plant g rowth and survival when averaged over species and species mixture treatment s. Performance of the two species differed significantly. As inferred from the ratio of mixture to monoculture for both species, growth and survival o f 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 an d revegetation of rangelands in Saudi Arabia. (C) 2001 Academic Press.