Understanding effects of rainfall pattern on plant interactions is a prerequisite for anticipating the effects of climate change on communities and ecosystems. We tested effects of growth season rainfall pattern on the individual performance, and interaction between, arid land plants. Same amount of water was applied, either as pulses of high frequent- small size or pulses of low frequent- large size on natural vegetation of an arid rangeland, in Nasr-Abad, Yazd, Iran. Effects of rain size were more significant than those of rain frequency. There were linear increases in performances of both annuals (density) and perennials (current year growth), and a shifts in plant interaction (from facilitation to competition) as the magnitude of pulse sizes increased linearly from 0, to 10 and 20 ml per event. Accordingly we suggest that rainfall size represent a significant niche dimension for richness maintenance in arid ecosystems. Large rain events create a greater opportunity for perennials to increase their biomass or reproduction yield, and for annuals to increase establishment in open spaces. However under small rain events, facilitative effects by perennial shrubs, and decreased competition intensity, may maintain plant diversity under harsh climatic conditions of arid ecosystems.

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