Article

Species interaction plays a central role in structuring plant communities. However, few studies have investigated the nature of species interaction among different life forms along a productivity gradient. The current study is aimed to investigate the nature of life forms interactions and its impact on production of annual grasses grown in semiarid Mediterranean areas. Competition treatments (absence/presence of neighbours) were applied in randomized complete block design over two growing seasons. Relative interaction index (RII) was used to measure competition intensity. RII standardizes the reduction in growth of one species due to presence of neighbour species. Competition reduced grass biomass production by approximately 15%. Competition role was eliminated under low productivity conditions. Results showed that competition may limit the direct effect of defoliation on dominant grass species. Further, competition effect on dominant annual grasses showed positive and negative effects in relation to site productivity and best explained by a sinusoidal model. This hypothesized sinusoidal model suggests that facilitation and competition are alternatively affecting grassland communities along productivity gradient. The nature of interaction changes with changing community productivity revealing a cyclic pattern. The reflection points where interaction switches from facilitation to competition may explain the mechanism of maintaining high species diversity at intermediate level of community productivity. The experimental defoliation exerted a pronounced effect on plant productivity and modified the nature of interaction between annual grasses and other growth forms. These mechanisms may explain the ability of Avena and Hordeum species to form persistent annual climax grasslands in semi-arid rangelands. These findings may suggest that Avena and Hordeum species may be used in revegetating degraded areas in the north-eastern part of Jordanian Mediterranean grasslands.