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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.

A good way to check hypotheses that explain the invasion of ecosystems by exotic plants is to compare congeneric alien and native species. To test the hypothesis that alien invaders grow faster than natives, the maximum relative growth rate and its components were compared in controlled growth conditions between four Senecio species, two aliens introduced from southern Africa (S. inaequidens and S. pterophorus) and two European natives (S. malacitanus and S. jacobaea). The four species colonize similar habitats, but the frequency and abundance of their populations and their distribution ranges differ. The two aliens showed a higher relative growth rate than the natives, and although there were differences between species for leaf area ratio, leaf dry matter content, and dry matter partition between stems, leaves, and roots, no clear pattern was detected to explain the differences in growth rates: several combinations of the components of the relative growth rate can give similar results. The higher relative growth rate of the alien species, combined with other ecological and life-history traits, may enhance their invasive capacity.

Three series of seed addition experiments in natural sites and in common experimental conditions are described to compare the effect of disturbance, water availability, and habitat type on the emergence and establishment of the two alien congeners Senecio inaequidens D.C. and Senecio pterophorus D.C. and the native Senecio malacitanus Huter. The emergence of the three species was favored by disturbance, while establishment was enhanced by water availability. Habitat also had a dramatic effect on emergence, with more seedlings germinating under the shrubs and in the forest. However, only the plants that emerged in the open annual grassland survived until the adult stage, some of which flowered, mostly S. inaequidens. The emergence of the alien S. pterophorus was lower than that of the other species, but once established it produced significantly more biomass than the others. The effect of disturbance and water availability on emergence and establishment does not explain the differences in invasive capacity between the two aliens and the native; however, some traits, such as the biomass production of S. pterophorus and the early flowering of S. inaequidens, can contribute to the capacity of the two aliens to colonize new habitats.Key words: Senecio, alien invasive, emergence, disturbance, water addition, seed or site limitation.

Field survey in Catalonia (Spain) has increased the known distribution and the number of populations of the South African daisy Senecio pterophorus DC. in the Iberian Peninsula and mainland Europe. The species colonizes relatively disturbed habitats such as river beds, road borders and disturbed helm-oak communities with no limitation in light and water. The analysis of population structure related to habitat type has revealed that in riparian and ruderal areas populations are large and well-established, with a large number of members per age class. However, in disturbed forests populations have only a few scattered adults. Senecio pterophorus also shows great morphological plasticity related with habitat type: in open environments plants are shorter, adopt a spherical habit, and have smaller leaves than in forests, where they are taller and have leaves twice in size. This species may be considered as invasive in the Iberian Peninsula and mainland Europe, where it has rapidly spread in recent years. Furthermore, we suggest that S. pterophorus may be a threat to native species and habitat diversity as occurs in Australia, where the species displaces the native plants and hybridizes with some native Senecio species.