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.
Materials and methods