Nurse shrubs may increase the establishment and survival of their understory plants, by providing a favorable microclimate or by protecting them against herbivores. Therefore they are considered as safe sites for the restoration projects. However, several studies have indicated contrasting effects of nurse shrubs. We conducted series of field studies, to find the most common type of shrubs\' facilitation in the arid or semiarid rangeland, in the normal or dry years, in the north or south facing aspects, and for different nurse shrubs, from 2003-2010 in the extensively grazed rangelands on Iran. Results generally indicated higher soil moisture, soil fertility and protection against herbivores but lower sun irradiation and evapo-transpiration, under the canopy of shrubs than in open areas. Canopy facilitation generally increased the species diversity and richness of naturally growing plants. Protection against herbivores was a common facilitation under the all environmental conditions; only being dependent on the canopy structure of the nurse shrubs. On the other hand, shrubs\' facilitation for soil moisture was reduced from the beginning towards the end of growth season; it was higher under medium than the sever drought conditions; besides being higher in the semiarid than the arid rangelands. In conclusion, in the extensively grazed rangelands of arid and semiarid areas, protection against herbivory is a more common and persistent facilitation effect of aridland shrubs than their effect on the soil moisture and fertility. The contrasting reports on the role of canopy facilitation might be because of paying much attention on the soil moisture and fertility, whereas neglecting the protection against herbivory.

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