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 .