Abstract: Human activities are strongly affecting ecosystems worldwide, altering abiotic factors and often triggering massive habitat invasions, as in the case of coastal dunes. Moreover, biotic interactions with native dune species can either facilitate or hinder the invasion process. In order to curb the invasion of alien plants, it is therefore important to understand the interplay between biotic and abiotic factors during the colonization process. Our experiment investigated the cascading effects of soil stress, plant growth, and the functional traits of the key species Cakile maritima, on the alien and native plant community. In an island of the Marano’s lagoon, Northern Adriatic Sea, we mechanically removed the vegetation in the back dune, triggering a new ecological succession. In the site we created a soil stress gradient by altering main soil properties (i.e., salt, nitrogen, and organic matter) with a randomized block design. Soil properties directly affected the plant functional traits of C. maritima and the diversity and composition of the whole community. Moreover, the cover, height and functional traits of C. maritima showed a direct effect on native and alien species populations, likely competing with other native species, but only when soil conditions ameliorate, leaving free niches for the alien species colonization. These results showed a direct effect of soil on sand dune plant succession and diversity, but this was also indirectly mediated by the key species response. This study provided new information on the mechanisms of the coastal dune biological invasions, suggesting that induced soil stress can be effective to combat alien plant proliferation while maintaining native stress tolerant species.

Key words: Biological invasion, Cakile maritima, coastal dunes, functional traits, plant-plant interactions, soil properties