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 have significant influences on habitat conditions. Shrubs have been shown to increase soil microbial functions (Chandregowda et al., 2018), enhance mycorrhizal colonization (Armenta Calderón et al., 2019), alter runoff and sediment yields (García Ruiz et al., 2013; Keesstra et al., 2016; Lu et al., 2019) and affect the soil seed bank (SSB) (Niknam et al., 2018; Funk et al., 2019). They increase SSB under their canopies by trapping seeds or increasing seed production by sub-canopy plants through ameliorating the environment (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 species of shrubs are different in their canopy architectures. Some shrub species have raised stems and some are attached to the ground. The canopy is dense in some and open in others. Therefore, it can be supposed that shrubs may have different performances in trapping seeds and could affect differently seed production by smaller species in their sub-canopy. As a result, different species of shrubs may alter SSB characteristics, differently. Nevertheless, our literature review showed that none of the studies has compared the effect of different species of shrubs on SSBs in semiarid regions.
In this study, we compared the effect of three dominant species of shrubs on sub-canopy SSB characteristics. Since the selected shrubs were different in their canopy traits, we supposed that they affect SSB characteristics, differently. In restoration perspective, we need to know whether these shrub 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. Unfortunately, planting of exotic shrubs (e.g. Atriplex canescens ) has been extensively occurred in arid and semiarid rangelands in Iran for restoration goals. Before starting the restoration activities in degraded sites by planting native shrubs, it is important to us to know different potentials of different shrub species in facilitation of herbaceous plant recovery through SSB. We 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 mezereumwith single-elongated main stems and, Ebenus stellata with procumbent canopy and multiple stems. We hypothesized that the density and species richness and diversity of SSB under the canopy of E. stellata would be higher than A. scoparia and D. mezereum .