Abstract
Amount and distribution of rainfall are important factors influencing seedling growth and hence survival and establishment or range plants in arid areas. Litle work has been done on this respect on Saudi Arabian native range plants. Thus, this study was conducted to simulate the effect of amount of rainfall and the period between two effective rains on growth characteristics of seedlings for three populations of Hammada elegans, collected from Riyadh, Al-Qassim and Al-Jouf. A total amount of irrigation water equivalent to 100,200 or 400 mm average rainfall was applied over three months (wet period) every seven or fourteen days. Seedlings were then left to grow for another two months (dry period).
Results indicated that shoot height and root depth increased significantly as the amount of rainfall increased over 100 mm during the wet period. The higher average rainfall was significantly different from the other two averages during the dry period in this aspect. The increase in shoot height and root depth was also positively reflected on shoot and root dry weights. Rate of increase in rooting depth was 4-6 folds of rate of increase in shoot height indicating that seedling were putting more effort on root growth especially under the low rainfall to utilize deep soil moisture when the rainfall season is over. The rate of shoot growth increased by increase of rainfall during the wet period and continued to increase during the dry period while that of roots was not affected by the amount of rainfall. When the period between effective rains was increased from seven to fourteen days, only root depth was increased significantly at the age of 120 days and continued to increase during the dry period. Apart from the low average rainfall, seedling root growth was induced by increase in period between two effective rains. Plant populations differed significantly in some growth parameters in response to varying the amount of rainfall and the period between two effective rains. Generally, plant population from Al-Qassim area was the most effective one in utilizing available soil moisture followed by Riyadh population.