Experimental field analysis of factors affecting the growth and restoration potential of Sporobolus maritimus

Salt marshes are coastal habitats providing many ecosystem services (e.g., coastal protection and carbon sequestration). Unfortunately, marshes have strongly degraded worldwide from climatic and local human impacts, which fostered many projects for the restoration of salt marshes and their ecosystem services. Yet, salt marsh restoration is difficult and the success varies. Specifically, in some areas, like the Northern Adriatic, restored marshes experienced invasion by Sporobouls anglicus and loss the native Sporobolus maritimus. Considering the strong effect of these ecosystem engineering species on many functions (e.g. sediment stability), it is highly important to understand what prevents the spread of the native Sporobolus in contrast to the invasive one. According to previous studies, the two species occupy the same environmental niche, without competing directly. Interestingly, restored marshes occupied by S. anglicus, were sandier than natural marshes, therefore we conducted a manipulative field experiment comparing the mediation effect of sediment on the colonizing capacity of the two species in new, restored marshes . In a natural marsh of the Venice Lagoon (Italy), we transplanted S. maritimus and S. anglicus in plots with either natural, muddy or sandy, restored sediment and delimited control plots with the two species growing naturally and un-manipulated. After 2 months, we measured the plant biomass and several morphological trait. Our results revealed S. maritimus. to be strongly affected by the transplantation treatment, with almost no survival in neither muddy or sandy sediment . Surprisingly, the few surviving shoots where in the sandy sediments, suggesting higher sand contents to actually favour this species. In comparison, S. anglicus suffered, but less from transplantation especially in sandy treatments. Altogether, these results indicate that the higher sand content of sediment might favour the plant growth of both species (likely linked to faster increased draining of sand, reducing sediment waterlogging). Because S. anglicus is more resistant to transplantation, we suggest that this species has higher capacity to spread and colonise vegetatively, through rhizomes dispersions, potentially explaining it´s higher capacity to colonise new, restored marshes.