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Drought is common in Mediterranean-type climates. Water stress can have serious physiological consequences for plant fitness. Here we analysed the response of two alien invasive species, Senecio inaequidens DC. and S. pterophorus DC., and one native non-invasive, Senecio malacitanus Huter, in terms of photosynthesis, water relations and growth. The proportional reduction in growth as a result of water stress was smaller in S. malacitanus, followed by S. inaequidens and finally S. pterophorus. Variations in relative growth rate were related to differences in unit leaf rates, which are strongly correlated with photosynthesis. At a similar level of leaf relative water content (RWC), photosynthesis in S. inaequidens and S. malacitanus did not differ, whereas it was lower in S. pterophorus. S. malacitanus started to show a reduction in RWC later than the other species. The hypothesis that alien invaders have greater physiological tolerance to drought than native non-invaders is not supported by our results since S. malacitanus showed a more adaptive response to drought than the aliens and was also the most resistant of the three species to water shortage. Differences in invasiveness would therefore be explained by a combination of traits, including establishment capacity, competitive capacity and drought resistance, among others.

A good way to check hypotheses explaining the invasion of ecosystems by exotic plants is to compare alien and native congeneric species. To test the hypothesis that invasive alien plants are more competitive than natives, we designed a replacement series experiment to evaluate interspecific competition between three Senecio species representing the same bushy life form: two alien species (S. inaequidens and S. pterophorus, both from South Africa) and a native species from the south-east of the Iberian Peninsula and Maghreb (S. malacitanus). While S. inaequidens is widespread throughout western Europe and is expanding towards the south of Spanish–French border, the geographical distribution of the recently introduced S. pterophorus is still limited to north-eastern Spain. Plants from each species were grown in pure and in mixed cultures with one of their congeners, and water availability was manipulated to evaluate the effects of water stress on competitive abilities. Our results show that the alien S. inaequidens is the most competitive species for all water conditions. The native S. malacitanus is more competitive that the alien S. pterophorus in water stress conditions, but this situation is reversed when water availability is not limiting.