Effect of cushion plants on the soil seed bank in overgrazed semiarid regions

Evaluation of soil seed bank (SSB) in relation to biotic environmental factors could be important in degraded areas, since SSB is one of the major sources that facilitates the recovery of degraded plant communities after disturbances such as grazing, flooding and drought. The aim of this study was to investigate the role of Astragalus myriacanthus and Acantholimon spinosum on SSB characteristics. Soil sampling was carried out in four different positions (upslope edge, downslope edge, center and outside) of each cushion in semiarid mountainous regions in Iran. Then, SSB composition and density, species diversity and richness of SSB in each position were estimated using the germination method. The results of the nonmetric multidimensional scaling showed that the separation of species composition of SSB in four different positions was not possible in any of the cushion species. Nevertheless, the results indicated that the lowest of SSB density, species diversity and richness were observed in the outside of the two cushions. In both cushions, A. myriacanthus and A. spinosum, the mean SSB density (1,606.4 and 646.5 seeds/m(2), respectively) was significantly higher in the upslope edge. Totally, the mean density of SSBs in A. myriacanthus (903.6 seeds/m(2)) was significantly higher than that of A. spinosum (360.6 seeds/m(2)). We concluded that the cushion plants can act as seed traps and therefore could facilitate recovery of degraded sites in the steep-hilly areas, while, the possibility of seed penetration into the soil of different directions of cushion might be significant.