Nasr H Gomaa1,2, Ahmad K Hegazy3, Arafat Abdel Hamed Abdel Latef4,5. 1. Department of Botany and Microbiology, Faculty of Science, Beni-Suef University, Beni-Suef 62521, Egypt. 2. Biology Department, College of Science, Jouf University, P.O. Box 2014, Sakaka 72341, Saudi Arabia. 3. Department of Botany and Microbiology, Faculty of Science, Cairo University, Giza 12613, Egypt. 4. Department of Biology, Turabah University College, Turabah Branch, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia. 5. Botany and Microbiology Department, Faculty of Science, South Valley University, Qena 83523, Egypt.
Abstract
Perennial shrub-annual plant interactions play key roles in desert regions influencing the structure and dynamics of plant communities there. In the present study, carried out in northwestern Saudi Arabia, we examined the effect of Haloxylon salicornicum shrubs on their associated understory annual species across four consecutive growing seasons, along with a record of the seasonal rainfall patterns. We measured density and species richness of all the annual species in permanent quadrats located beneath individual shrubs, as well as in the spaces between shrubs. During wet growing season H. salicornicum shrubs significantly enhanced the density and species richness of sub-canopy species, whereas in the relatively dry seasons they exerted negative effects on the associated species. In all growing seasons, the presence of shrubs was associated with enhanced soil properties, including increased organic carbon content, silt + clay, and levels of nutrients (N, P and K). Shrubs improved soil moisture content beneath their canopies in the wet growing season, while in the dry seasons they had negative effects on water availability. Differences in effects of H. salicornicum on understory plants between growing seasons seem due to the temporal changes in the impact of shrubs on water availability. Our results suggest the facilitative effects of shrubs on sub-canopy annuals in arid ecosystems may switch to negative effects with increasing drought stress. We discuss the study in light of recent refinements of the well-known "stress-gradient hypothesis".
Perennial shrub-annual plant interactions play key roles in desert regions influencing the structure and dynamics of plant communities there. In the present study, carried out innorthwestern Saudi Arabia, we examined the effect of Haloxylon salicornicum shrubs onpan> their associated unpan>derstory anpan>nual species across four conpan>secutive growing seasonpan>s, alonpan>g with a record of the seasonpan>al rainfall patterns. We measured density anpan>d species richness of all the anpan>nual species in permanpan>ent quadrats located beneath individual shrubs, as well as in the spaces between shrubs. During wet growing seasonpan> H. salicornicum shrubs signpan>ificanpan>tly enhanpan>ced the density anpan>d species richness of sub-canpan>opy species, whereas in the relatively dry seasonpan>s they exerted negative effects onpan> the associated species. In all growing seasonpan>s, the presence of shrubs was associated with enhanpan>ced soil properties, including increased organpan>ic pan> class="Chemical">carbon content, silt + clay, and levels of nutrients (N, P and K). Shrubs improved soil moisture content beneath their canopies in the wet growing season, while in the dry seasons they had negative effects on water availability. Differences in effects of H. salicornicum on understory plants between growing seasons seem due to the temporal changes in the impact of shrubs on water availability. Our results suggest the facilitative effects of shrubs on sub-canopy annuals in arid ecosystems may switch to negative effects with increasing drought stress. We discuss the study in light of recent refinements of the well-known "stress-gradient hypothesis".