Résumé
We have studied in southern Spain, the root growth and the associated water uptake rates and patterns of two mediterranean species growing close to each other: Vulpia spp. and Phalaris aquatica annual and perennial grasses respectively. Our results indicate that: (1) During the early stage of regrowth of Phalaris at the end of the drought period, the dormant roots begin to elongate and to exploit soil moisture before the new root system which arises from the base of the buds become established. During the growing period, the deep roots of Phalaris continue its extension into the moister layer of the soil. From seedling emergence, the roots of Vulpia settle quickly in the top soil layer. The maximal development of its root system ends in January. (2) An important factor in the survival of Phalaris is the ability of its root system to supply water during the summer. This behaviour contrasts with that of the Vulpia spp. which are unable to exploit subsoil moisture and die as soon as the surface moisture is exhausted. (3) Phalaris and some other perennial grasses have evolved mechanisms to withstand in the drier mediterranean environment. So, the existing predominance of annual species in the grassland of southern Spain cannot be explained only by climatic reasons.