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 .