Introduction
Arid and semi-arid ecosystems occupy 36% of the land area of the globe and shrubs function as foundation species within these ecosystems (Yang and Williams, 2015). A foundation species in ecology was described as species with significant impacts on the structure and functioning of an ecosystem (Lortie et al., 2017). It has been frequently called shrubs as fertile islands, since, they can strongly influence habitat conditions. Shrubs have been shown to affect the quality and quantity of soil microorganisms (Wardle and Zackrisson, 2005), control the soil erosion and runoff amounts (Zuazo and Pleguezuelo, 2008; García Ruiz et al., 2013; Keesstra et al., 2016) and influence the soil seed bank (SSB) (Erfanzadeh et al., 2014; Niknam et al., 2018; Funk et al., 2019). They can be facilitators and alter SSB under their canopies by trapping seeds or enhancing seed production by sub-canopy plants through ameliorating the environment (Li et al., 2011; García-Sánchez et al., 2012; Mussa et al., 2016).
Study on SSB is important, since, it is one of the most important functional parts of any plant community and can be significant components in the process of rehabilitating degraded lands (Mohammed and Denboba, 2020). Bakker (1989) identified SSBs as non-mature seeds buried in soil that can replace existing vegetation when they are degraded.
In semiarid regions, shrubs are able to change SSB characteristics. According to some reports SSB density was much higher under the shrubs than the surrounding areas (Pugnaire and Lázaro, 2000; Marone et al., 2004). In overgrazing sites, particularly, shrubs accumulated large and diverse SSBs beneath their canopy which were different in composition from seed banks of the open matrix (Dreber and Esler, 2011). This significant effect are induced by the ability of shrubs in seed trapping and providing suitable microclimate and conditions for seed production by other plant species (Erfanzadeh et al., 2014). However, different shrubs with different canopy architectures (erect stems or recumbent, open or dense canopy) could have different efficiency in trapping seeds and could affect differently seed production by herbaceous species and therefore, they play different roles in changing the characteristics of SSB (density, species richness and diversity) in semiarid environments. Nevertheless, none of the previous studies has focused on the effect of different species of shrubs on affecting SSB characteristics.
In this study, we selected three dominant shrub species and determined SSB density, species richness and diversity and, similarity between SSB and above-ground vegetation (AGV) in their understory. Since the shrubs were different in their canopy traits, we assumed that they affect SSB characteristics, differently. Moreover, in semiarid areas, shrubs are cleared and damaged by humans for many purposes including agriculture, oil and gas production, and sustainable energy developments (Lortie et al., 2017); we need to know whether these foundation species can recover and consistently facilitate the abundance and diversity of other plants through SSB and whether this effect is different between different species of shrubs with different crown features and architectures. It can be supposed that dwarf and procumbent shrubs have higher ability to trap seeds comparing with erect stem and free canopies. In this study, three dominant shrubs with different features and architectures in the canopy were selected, i.e. Amygdalus scoparia and Daphne mezereum with single-elongated main stems and, Ebenus stellatawith procumbent canopy and multiple stems. We hypothesized that the density and species richness and diversity of SSB under the canopy ofE. stellata would be higher than A. scoparia and D. mezereum .