Phenological and reproductive responses of a semiarid shrub to pulsed watering

Abstract

Climate models forecast the decrease in annual precipitation and more extreme events (i.e. fewer rainy days and longer drought periods between rainfall events) in Mediterranean ecosystems which may have important consequences for plant community dynamics. Water availability is the most limiting factor for plant performance in arid environments, where it is often highly pulsed, and discrete rainfall events interspersed with drought periods are the important components of the annual water supply. Therefore, relatively small changes in rainfall frequency can have strong effects on the plant communities. We tested the effects of fewer rain events on inflorescence and fruit phenology, and reproductive success of a Mediterranean shrub species from semiarid SE Spain in a glasshouse trial. We found that by changing patterns of water supply, we strongly altered soil drying cycles with consequences for Phagnalon saxatile life history. Inflorescence and fruit phenology were not affected, and no changes in the number of inflorescences, fruits or fruit set were recorded. However, individuals growing under lower rainfall frequency produced bigger shoots, and heavier fruits and seeds. Bigger fruits produced a higher number of seeds that were heavier and more viable and, therefore, produced more seedlings than plants growing in the control treatment. Higher reproductive success achieved by Phagnalon individuals under low watering frequency may have important consequences for community dynamics if larger but less frequent rain events lead to an increase in soil water availability: natural selection favours individuals making the greatest contribution to the next generation. Such advantages could change community composition, the spatial distribution of species and their interactions.