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Climate change may cause profound effects on terrestrial ecosystems. Changes in rainfall patterns may have large effects on a wide range of biological processes such as seed germination, seedling establishment, plant growth, community composition, and population and community dynamics. Climate change models for the Mediterranean region forecast reduced annual precipitation and more extreme rain events (i.e., fewer rainy days and longer drought periods between rainfall events), along with seasonal changes. We experimentally addressed the response of a semiarid Mediterranean community to higher aridity and changes in seasonal rainfall patterns in two glasshouse experiments in which we manipulated water supply. We simulated a delay in the onset of autumn rainfalls (i.e., a longer summer drought period), decreased watering amount and frequency as predicted by climate models, and manipulated the seasonality of water supply. We found that delayed watering led to decreases in plant community productivity and to delays in flowering time, in terms of both date and number of days of water supply. Decreased watering amount and frequency, and accentuated seasonality, also diminished biomass and individuals recruited, but did not change flower phenology. Species diversity was not affected by watering delays; however, it was reduced by changes in frequency, amount and seasonality. Overall, these data underline the need to consider rainfall patterns as an important element that might alter community dynamics and ecosystem structure and functioning. Therefore, the analysis of climate change consequences must not depend on climatic means-based scenarios but must take into account expected seasonal changes in rainfall quantity and frequency. (C) 2009 Rubel Foundation, ETH Zurich. Published by Elsevier GmbH. All rights reserved.