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Wind-induced mechanical stimulation and water shortage are important stresses in arid/semiarid regions. However, little is known about the combined effects of mechanical stimulation and water availability. Since the effects of high water availability on biomass and allocation are opposite to those from mechanical stimulation, it is hypothesized that high water availability suppresses the effects of mechanical stimulation. To test this hypothesis, seedlings of Lolium perenne were subjected to two levels of brushing (non-brushing versus brushing 60 seconds d(-1)) and two levels of water availability (200 ml d(-1) versus 400 ml d(-1)). Brushing had no effects on the total biomass of L. perenne, indicating that brushing is not a stressful factor. However, brushing significantly decreased plant height and dramatically increased the root/shoot ratio. The differences in biomass allocation between brushed and non-brushed plants at high water availability were significantly smaller than those at low water availability, showing that the effects of brushing are suppressed by high water availability. These results suggest that L. perenne may have evolved some traits to cope with frequent mechanical disturbance.