Introducing the study area
This study was conducted in the rangelands of Chenarnaz, Yazd province, Iran (30° 03′ 51″ N - 30° 05′ 89″ N; 53° 00′ 16″ E - 54° 01′ 23″ E) (Fig. 1). The average altitude is 2200 m asl. The average annual temperature is 17.5°C and the average rainfall is 250 mm, which has a semiarid climate based on Domarten index.
Sheep and goats are the dominant grazers in the region (ca. four heads of sheep and/or goats per ha) during the year. Heavy grazing has led to locally exposed soil. In addition, the previous examination of erosion status in the study area showed that environmental and human factors had a role in the occurrence of soil erosion and vegetation degradation. Human activities such as intensive harvesting of medicinal plants and the use of pharmaceutical products have reduced the density of plant species and created empty gaps in the AGV in some places (Gravand et al., 2016). Therefore, herbaceous revegetating bare soil and restoration of degraded sites using of native plants is a priority. SSB has been considered one of the major natural sources that facilitates the recovery of degraded vegetation (Shang et al., 2016). We carried out this study to quantify the potential of different species of shrubs to increase the SSB associated with them and thus their potential use for restoration. Therefore, it is important to identify firstly shrubs that associated with larger and richer SSB and secondly, consider restoring shrub species which have higher potential as SSB reservoir accompany with higher palatability for grazing. Thereupon, three shrubs together with surrounding herbaceous vegetation (hereafter called control) were selected:
A) Amygdalus scoparia Spach (Rosaceae family) is a wild species of almond that occupies large areas in many parts of central Iran and its neighbouring countries. The extraction and use of the oil from theA. scoparia is of interest due to their fatty acids composition that is comparable to those of olive oil (Sorkheh et al., 2016). The plant is attractive for grazing animals due to its shade, fruits and high palatability of leaves. It is a deciduous large shrub that grows to a height of up to 6 m, having a single-elongate main stem. It produces numerous long and green branches. Fruits are drupes and are 1 to 1.5 cm long and 0.5 cm wide. They are ripened and dehiscent at the end of July (Mozaffarian, 2012).
B) Daphne mezereum L. (Thymelaeaceae family) is a rounded-upright deciduous shrub with an erect and bushy habit that typically grows to 1.5 m tall. All parts of this plant are poisonous to humans if ingested, especially the fruits, sap and bark. Therefore, this shrub is unpalatable for grazing animals. Nevertheless, fruits are attractive to birds with no resulting ill effects (Mozaffarian, 2012). This species is found globally in dry and semi-dry areas and, in the provinces located in the central Iran.
C) Ebenus stellata Boiss. (Fabaceae family) is a thorny shrub with a height of 30-120 cm, having short and oblong-leafy branches and ternate leaves that are alternate and covered with dense silk flakes. This species grows in large part of Iran including Kerman, Yazd, Esfahan, Fars and Hormozgan provinces and some dry and semi-dry regions of world (its native range is Oman and Iran to India). The canopy structure is open with compact thorny branches that it is difficult to graze livestock (Mozaffarian, 2012) (Fig. 2).