Introduction
Shrub encroachment (SE) has been a serious concern and extensively
studied by ecologists. According to our literature review, SE studies
have globally targeted different ecosystems i.e. arid (Eldridge et al.,
2011; Soliveres et al., 2014), semiarid (Gazol et al., 2012; Kulmatiski
et al., 2013; Foronda et al., 2019), subalpine grasslands (e.g. Ding et
al., 2019), temperate grasslands (e.g. Li et al., 2019), and mesic areas
(Briggs et al., 2005) such as savanna (Wiegand et al., 2005; Kulmatiski
et al., 2013; Stevens et al., 2017), steppe (e.g. Chen et al., 2014) and
prairie (e.g. Briggs et al., 2005). These global studies revealed that
soil properties are the primary dependent variables in response to the
extent of SE (see also Li et al., 2016).
Previous studies have shown that within landscape-level, the effects of
shrub on understory soil are varied, ranging from positive to negative
or neutral (Eldridge et al. 2011; Du et al., 2016). Many studies have
reported reductions in soil nutrient (e.g. Guidi et al., 2014), and
increasing soil erosion following SE (Parizek et al., 2002). Other
studies, however, have shown that SE can substantially enhance soil
nutrient acquisition, or amplify soil biota diversities (Maestre et al.
2009; Gómez-Rey et al., 2013; Throop et al., 2013; Blaser et al., 2014).
In patch-level effects, evidence has reported generally positive effects
of individual shrubs on their sub-canopy soil conditions (e.g. Ding et
al., 2019). For instance, the well-known term of “fertile island” has
been defined for shrubs due to positive effects of its individuals on
sub-canopy soil (Ren et al., 2008; Kondo et al. 2012). However, effects
of different species of shrubs on their sub-canopy soil characteristics
have remained poorly understood.
Maintaining the plant diversity is a major challenge in plant community
ecology (Gonzalez and Ghermandi, 2019). Although classical studies on
this issue have focused on the negative plant-plant interactions, more
recent studies have considered the importance of the positive
interactions, such as the nurse effect of larger plants on smaller ones
(e.g. Baldelomar et
al., 2019). Although at the landscape level, shrubs could reduce plant
diversity (Ratajczak et al., 2012), at the patch level, they are known
as nurse plants due to their positive effects on sub-canopy and
increasing plant diversity. Nurse plants are described as plants that
facilitate
the growth
and development of other plant species beneath
their canopies as
they offer
benign microhabitats which
are more favorable for
seed germination and/or seedling recruitment than
their surrounding environment. Thus, nurse plants have been mainly
considered for restoring vegetation in terrestrial ecosystems that
suffer harsh natural conditions such as arid and semiarid areas (Ren et
al., 2008). Shrubs, as nurse plants, alter vegetation properties under
their canopies with seed trapping, facilitating seed production by
understory plants, increasing soil seed bank, increasing soil moisture,
protecting the surface soil from erosion, and adding litter into the
soil (García-Sánchez et al., 2012; Erfanzadeh et al., 2014b; Mussa et
al., 2016; Erfanzadeh et al., 2019). Although the functions of shrubs
have been studied in different ecosystems around the world, the effect
of their nursing role in sub-canopy plants has been hardly compared
between different species of shrubs.
It can be supposed that depending on the structure and feature of shrubs
(erect or recumbent, dense or open canopy, having single or multiple
stems), they could play different roles in changing the soil properties
and vegetation characteristics we tested in this study. Accordingly, the
main objective of this study was to assess the effects of three species
of shrubs on soil nutrients and vegetation characteristics in semiarid
areas. We hypothesized that the extent to which shrubs affected soil and
vegetation is dependent on the species of shrubs.