Plant-plant interactions are known as the main biotic drivers of the vegetation dynamics. Therefore, investigating such processes is beneficial for the applied vegetation management. A filed experiment was conducted in Baharkish rangeland, Quchan, Iran, during spring and summer 2007. Ecophysiological aspects of facilitation and competition were studied between two aridland shrubs, A. meschedensis Bunge and A. raddeanam Czernjak and one perennial grass, B. kopetdaghensis Krasch. Soil and plant sampling were carried out for shrubs and the grass that were either growing alone or the grass was growing under the canopy of shrubs. In spring (May), soil humidity weight was higher under the "shrubs + grass" than the "grass-only" site. By the beginning of summer (July) grass consumed the common soil water and rapidly terminated its yearly growth. Therefore in August and September, soil humidity weight was lower under the "shrubs + grass" than "shrub-only" sites. Photosynthesis rate of B. kopetdaghensis was sharply reduced from the beginning towards the end of growth season, but was not varied between the different plant combinations. Leaf proline measurement in July indicated higher stress for Br. kopetdaghensis that were growing under shrubs than those of open areas. In conclusion, we found facilitation effects of shrubs on the grass at the early times of growth season, but it shifted into the competition for water during summer times. The outcome of plant interaction was positive for the grass but negative for the shrubs, especially A. meschedensis.

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