Alien plant invasion during early succession stages of dune systems is driven by soil properties

Biological invasion is nowadays recognised as one of the major threats to biodiversity (1). This is particularly true for coastal habitats, where dunes are considered one of the most invaded habitat worldwide. Many studies linked the success of alien plant invasion in dune ecosystems to human disturbances, but less is known about the role of soil properties in plant invasion. Soil properties can directly affect plant dune colonization and the final community composition (2,3). This is expected to be linked to the interplay of soil and the plant functional response, generating important plant-soil feedbacks able to reverberate on the community structure and composition. Plant-soil interactions are thus expected to be crucial in explaining the invasion processes, but their role in dune alien colonization in mostly unknown. We performed a manipulative experiment in a barrier island of the Marano and Grado’s lagoon, Northern Adriatic Sea. The whole plant community of backdune was erased by a soil miller in the selected plots to trigger a new ecological succession and test the mechanism of alien plant invasion during the early stages of plant colonization. In 8 experimental bocks, we altered soil properties by adding salt, nitrogen and organic matter (i.e. peat) and combining those treatments in 1 m2 plots with a factorial design (i.e. 8 replicates × 8 treatments = 64 plots). We recorded the emergence of seedlings with a camera system every 15 days. At the end of experiments, we recorded the plant community composition and measured the following traits: plant height, species cover, number of individuals, leaf pigments, SLA. Moreover, those traits were calculated for the overall community and for the key species (Cakile maritima Scop.) We also estimated the decomposition rate of the soil using the Tea Bag Index. In addition, the same parameters were collected in 8 reference plots (i.e. surrounding unaltered plant community). The results showed that most of the treatments decreased the species richness of alien plants, in particular where soil salt content was enhanced. Moreover, some treatments had positive effects on the native plant cover and decreased the overall number of alien individuals, potentially reducing the initial propagule pressure due to the soil seed bank and, hence, changing the further plant community trajectories. This study provides new information on conservation and management efforts in this ecologically sensitive area, giving new insight into the dynamics of biological invasion and the impacts on native ecosystems.