Abstract
Invasion by the exotic species Prosopis juliflora has become a major threat to native plants in Saudi Arabia as the species continues its spread into different regions of the country. Ziziphus spina-christi is a native tree that is common in Saudi Arabia. The objective of this study was to determine how both species would benefit from the availability of sufficient resources without competition. To gain a better understanding of growth under such conditions, a greenhouse experiment was conducted in which seedlings of both species were grown under favorable conditions for 6 months. During this period, growth performance was evaluated three times at intervals of 30, 90, and 180 days. Growth performance varied between the two species during one or more of the studied periods. Significant differences between the species were observed for root mass fraction, number of root tips, root to shoot ratio, height, stem diameter, stem dry weight, stem mass fraction, leaf area, leaf mass fraction, and chlorophyll a and b contents. The relative growth rate (RGR) and relative height growth rate were higher in P. juliflora at 30-90 days, whereas leaf area ratio and net assimilation rate were higher for Z. spina-christi at 90-180 days. Remarkably, the RGR for diameter in P. juliflora was almost double that in Z. spina-christi at 30-90 days and 90-180 days. The results obtained reflect a strategic difference in the biomass allocation to different plant components by the two species, whereby P. juliflora allocates higher biomass to the stems and Z. spina-christi allocates higher biomass to the roots.