Discussion
Our results showed that among 22 plant families in SSB, the highest
number of species belonged to Asteraceae. Previous studies showed that
these plants were also widely present in the SSB (e.g. Gomaa et al.,
2012). One of the possible reasons for the increase of these plants in
SSB is the abundant seed production and morphological characteristics of
the seeds. Species of Asteraceae with small seed size and wing shape,
light and easy dispersal provide conditions for the presence of their
seeds in the SSB (Harper, 1977; Hong et al., 2012). Forbs were the most
abundant plants in the SSB composition. Our results showed that number
of forb species was higher (33, 27, 24 and 17 species beneath E.
stellata , D. mezareum , A. scoparia and control,
respectively) than grasses (6, 7, 5 and 5 species beneath E.
stellata , D. mezareum , A. scoparia and control,
respectively) in the study area. In accordance with the results of our
study, Bertiller et al. (2011), Parlak et al. (2011) and Tessema et al.
(2012) reported that forbs had the highest number of species in SSB.
Higher number of forbs in the AGV might be a reason for increasing the
seeds of these species in the SSB. In contrast, woody plant species
(trees and shrubs) were scarcely found in the greenhouse. Although,
Teketay and Granström (1997) and Chaideftou et al. (2009) attributed the
lack of woody species in the SSB to the lack of mature species in the
AGV, in our study, this cannot be the reason because woody species in
the AGV were frequent. Many factors are involved in reducing the density
and richness of woody species in the SSB in an area. These could include
the larger size of the seeds, the higher amount of predations and seed
dormancy (Esmailzadeh et al., 2011). Some studies have shown that
breaking seed dormancy of woody plants requires special conditions and
if these conditions are not provided, these plants will eventually be
removed from the SSB (Chaideftou et al., 2009).
In general, this study showed that the SSB density and species richness
and diversity under the shrubs was higher than control and this
differentiation was more pronounced for forbs. Previous studies (e.g.
Marone et al., 2004) showed that the seed density of forbs were often
higher under woody plants and positively correlated with the cover of
woody vegetation, whereas the seed density of grasses were less
associated by woody vegetation. Our results are consistent with some
previous studies (e.g. Erfanzadeh et al., 2014) and disagree with others
(e.g. Mndela et al., 2020). Positive effects of shrubs on SSB are
exerted through direct and indirect ways. They increase buried seeds in
soil by directly trapping seeds or by indirect mechanisms through an
intermediary animal or plant species (Bullock and Moy, 2004; Giladi et
al., 2013). Shrubs significantly influence the movement wind or water
around their canopy (Hoffman et al., 2013) and thus can trap seeds or
act as barrier for movement (Giladi et al., 2013). Shrubs can indirectly
facilitate seed arrival by acting as a perching site for seed-carrying
birds (Debussche and Isenmann, 1994) or as cache for granivorous rodents
(Beck and van der Wall, 2010) and ants (Vergara-Torres et al., 2018).
Additionally, shrubs can indirectly increase SSB by facilitating the
plants that are able to increase seed production or viability and
vigority of produced seeds (Pugnaire and Lázaro, 2000). Shrubs provide a
suitable conditions for growing, flowering and seeding of herbaceous
plants under their canopies through modifying the physical and chemical
properties of soil with litter and root exudation, improving soil
micro-relief, decreasing direct sunlight, increasing soil moisture,
protecting the surface soil from erosion and adding organic matter into
the soil (Ruiz et al., 2008; Barness et al., 2009; Olvera-Carrillo et
al., 2009; Li et al., 2011; Sylvain and Wall, 2011; García-Sánchez et
al., 2012).
However, different effects of different shrubs on SSB characteristics
were observed in increasing SSB density under the canopy of A.
scoparia and species richness and diversity under the canopy ofE. stellata. Previous studies showed that the size of a shrub
could impact the arrival of seeds (Pugnaire and Lázaro, 2000) because
larger shrubs can provide greater facilitative effects. Larger shrubs
can intercept more solar radiation (Maestre and Cortina, 2005), have
higher soil nutrients (Zhang et al., 2015), or lower evapotranspiration
(Kidron and Gutschick, 2013) creating a favourable microclimate for seed
production, particularly by annuals (Filazzola et al., 2019). As a
result, comparing to the other shrubs, taller and larger canopy inA. scoparia might increase SSB density through higher seed
production by plants. The seeds of some annuals were found at strong
frequent under A. scoparia , i.e. Bromus tectorum ,Galium aparine and Veronica anagalis . However, procumbent
canopy in E. stellata might increase species diversity and
richness of SSB. It might be that attached crown cover to the ground inE. stellata physically obstruct more seeds and enhance species
diversity and richness in SSB. Our results showed that the seeds of many
species were found under E. stellata , while they were absent in
control or under other shrubs, e.g. Polygonum dumosum , Poa
sinaica , Tragopogon jezdianus and Dianthus orientalis ,Tianthus crinitus . Briefly, indirect effect of A. scopariaon seed production of few plant species (some annuals) and direct effect
of E. stellata on seed trapping of many species resulted these
significant differences of SSB density, species richness and diversity
between shrubs.
Similarity between the seed bank and the AGV was generally low in three
shrubs and control. The low similarity between the AGV and the SSB in
our and other studies is usually due to the fact that some species were
present in the vegetation, while they were absent from the seed bank,
and vice versa (e.g. Valkóa et al., 2014; Erfanzadeh et al., 2016).
However, the similarity between the AGV and the SSB was lowest in the
control. In this area, many species, such as Acantholimon
scorpius and Ebenus stellata , were absent from the seed bank
while they were present in the AGV. Most of these species were
perennial, and these, especially shrubby ones, are well-known for their
transient seed bank (Thompson et al., 1997). Moreover some annuals such
as Bromus tectorum and Galium aparine were found in the
SSB in control and under three shrubs. These species were present in the
AGV under the shrubs while absent in the AGV of control. At the sampling
time, some annuals in the AGV might be grazed or dried and ended their
phenological stages in the control due to higher availability to grazers
or solar radiation, temperature and wind speed comparing with under the
canopies of
shrubs.