A series of communities were established in situ to differentiate the effects of species richness, functional richness and functional group identity on invasibility of Mediterranean annual old fields. We monitored the demographic and vegetative parameters of two exotic annuals introduced as seedlings, Conyza bonariensis and C. canadensis. Community species richness and functional composition determined resistance to invasion by Conyza.Conyza bonariensis biomass decreased with increasing species richness. Legumes increased the biomass and consequently the net fecundity of both Conyza, while survival was favoured by Asteraceae. Communities with fewer Asteraceae and grasses increased the reproductive effort of C. bonariensis. A separate glasshouse experiment using the same species mixes revealed that establishment of Conyza decreased with increasing species richness or when grasses were present. Patterns of Conyza performance are interpreted in the light of measurements of ecosystem functional parameters, making it possible to formulate hypotheses about mechanisms limiting community invasibility.

At present, no efficient technique is available for cleaning up soils which are highly polluted by heavy metals. Limiting the movement of pollutants out of the contaminated area by creating a dense and persistent plant cover appears to be the more reasonable approach. In this context, phytostabilization is a technique that uses metallicolous plants to revegetate highly polluted soils.This paper presents the results of an experiment performed in situ using metallicolous ecotypes of four plant species native to the Mediterranean French region, and grown in different combinations at a polluted site over two years. The soils were highly polluted with zinc, cadmium and lead. The aim was to find the best species mixture in terms of cover, biomass and duration. The four species used were the biennial legume Anthyllis vulneraria, two perennial grasses, Festuca arvernensis and Koeleria vallesiana, and the perennial forb Armeria arenaria. Mixtures which included A. vulneraria, and especially when in combination with F. arvernensis, showed the highest values of cover and biomass. After flowering, the biennial individuals of A. vulneraria disappeared but subsequent germination and survival of seedlings occurred abundantly under the two grasses. Mixtures with A. arenaria showed the lowest values of cover and biomass. Soil nitrogen increased in the plots with A. vulneraria as well as the concentration of essential nutrients (N P K) in the aerial parts of the two grasses. In contrast, the concentration of metals (Zn Pb Cd) decreased in the aboveground biomass of the latter in the same plots.These results show that reciprocal facilitation effects can act in heavy metal polluted environments, and that phytostabilization efforts in the Mediterranean region can be improved by using mixtures including local metallicolous legume and grass species.

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