Responses of serpentine plants to pine invasion: vegetation diversity and nickel accumulation in species with contrasting adaptive strategies

Introductions of non-native trees is one of the major threats to ecosystem integrity and biodiversity. Stands of maritime pine (Pinus pinaster Ait.) introduced decades ago represent a threat to the specialized plant communities of serpentine outcrops in Italy. This study investigates the effects of such invasions at the community and species level, based on vegetation sampling in three selected sites with comparable environmental conditions. Pine cover caused a decrease of α-diversity by lowering the species evenness of the community, though species richness was not negatively affected. Compositional changes between the two habitats were significant but not clearly associated with a decrease in taxonomic distinctness in the pine stands. As many as nine indicator species were found in the open vegetation, along with the obligate endemics Odontarrhena bertolonii and Armeria denticulata. Both of them declined in the pine stands. Here, an increase in the phytoavailable nickel fraction was associated with a decrease in total nickel concentration in the soil, via mobilization of the metal caused by lowering of pH induced by the conifer litter. The nickel-hyperaccumulator O. bertolonii was able to maintain high metal concentrations in the shoots despite a decrease in root concentration, resulting in a higher shoot/root ratio in the pine stands (~20). Conversely, shoot/root ratio in the non-accumulator Plantago holosteum was < 1 and not affected by the conifer, as well as its abundance in this anthropogenic habitat. Contrasting responses of the two species were likely due to their different sensitivity to modified light and soil conditions, whereas stability of shoot nickel-concentration in O. bertolonii did not support increased predation by natural enemies as one of the causes for its decline under the conifer. Progressive thinning of these stands is advocated to limit soil nickel mobilization and to restore a unique ecosystem with its endemic metallophytes.