Studying the plant-plant interactions leads to the basic information that is useful for the restoration of degraded ecosystems. The type and intensity of interactions may change from negative to positive, depending on the environmental conditions. We studied physiological ecology aspects of the interaction types between an aridland nurse shrub, Artemisia Khorasania and a perennial grass Bromus kopetdaghensis in a semiarid rangeland, Quchan, Iran. Effects of shrub s allolepathic compound were assessed on the grass germination and seedling growth. Available soil water were measured in three soil depths at the vegetative, flowering and seed ripening growth stages of the grass. Grass seeds were sawn under the canopy of shrub, and their establishment and survival were measured. At the early growth season, higher soil moisture, and possibly a favorable light and temperature conditions, facilitated grass establishment under the canopy of shrub. By the end of growth season, lower soil moisture under the canopy led to high mortality of grass seedlings. Shrub also induced an allolepathic effect, which was started at the flowering stages of grass. Allelopathic compounds (phenol) had the least effect on the grass germination and establishment, because of a time difference in the phonological stages of the two plants. Therefore, results indicate a more positive and negative effects of shrub on the grass in the early and later stages of growth, respectively. Physiological measurements also confirm this idea; leaf proline and malundealdehids increased and that of chlorophyll a and chlorophyll b decreased for the grass, from the beginning towards the end of growth season. In conclusion, results of this research indicate a sequence of facilitation, allelopathy and competition that correspondingly occur between an aridland shrub and its understory grass within a single growth season.

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