Effects of flooding stress on the saltmarsh halophyte Salicornia fruticosa (L.) L.: upscaling perspectives

One of the major aftermaths of global warming is the ongoing global sea level rise, which is expected to seriously treat the worldwide coasts. Among coastal environments, saltmarshes are particularly sensitive to the increase of flooding and frequency of storm surges. Indeed, although saltmarshes respond with a sediment accretion and coastal niche shifting, sea rise rates and the coastal squeeze phenomenon jeopardize the acclimation capacity of saltmarsh species and plant communities. For these reasons, monitoring and studying the response of saltmarshes to flooding is crucial to understand the fate of these fragile ecosystems. For a comprehensive understanding of these processes, it is necessary to link different ecological scales using an ecological upscaling approach. We analyzed main traits of plant community and the growth of the key species Salicornia fruticosa (L.) L. in 9 saltmarshes along the flooding gradient (Marano and Grado lagoon, northern Adriatic Sea). In particular, we considered community (i.e. species richness, dry biomass, dry matter content) and individual traits (i.e. shoot annual growth, dry biomass, dry matter content, plant height) analyzing the effect of flooding. Simultaneously we carried out a UAV (Unnamed Aerial Vehicle) multispectral survey, in order to obtain remote sensing-derived vegetation indices (e.g. NDVI Normalized Difference Vegetation Index) for the upscaling of plant responses. We found that the flooding gradient produced a significant decrease of plant biomass and growth, affecting both plant traits and plant community features. We also found NDVI effective to predict flooding effect only on the dry biomass of the community. In contrast, flooding intensity shaped the relationship between NDVI and the individual plant growth. In particular, the annual growth was satisfactorily explained by NDVI in the areas less subjected to flooding, whereas lack of correlation between these two parameters was observed for sites frequently flooded. Our findings shed new light on the potential use of the remote sensing tool for the understanding of the response of saltmarshes vegetation to the future increase of the sea level. Further attention should be paid to species morphological traits and additional ecophysiological responses (e.g. secondary metabolites, soil redox status) in order to fully understand such crucial processes at the plant community scale.