Remote sensing tools to parse the acclimation response of saltmarshes to flooding stress: upscaling perspectives in lagoon systems

Sea level rise is considered a prominent aftermath of the ongoing global warming, which is expected to seriously treat the worldwide coasts. Among coastal environments, saltmarshes harbor plant communities particularly sensitive to the increase of flooding. Although saltmarshes might contrast sea level rise by accretion and niche shifting, sea rise rates and the coastal squeeze phenomenon undermine the acclimation capacity of such plant communities. For these reasons, parsing the underlying mechanisms of the response of saltmarshes to flooding is of outmost importance to foresee the future scenarios for these important ecosystems. In this light, linking different ecological scales using an upscaling approach might provide new insight into the ecological processes involved. 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 growth (i.e. shoot annual growth, dry biomass, dry matter content, plant height) and physiological traits (i.e. pigments and secondary metabolite content) in response to flooding gradient. Concurrently we carried out a UAV (Unnamed Aerial Vehicle) multispectral survey, in order to obtain remote sensing-derived vegetation indices (i.e. NDVI - Normalized Difference Vegetation Index, LCI - Leaf Chlorophyll Index, ARI - Anthocyanin Reflectance 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 remote sensing-derived indices to be related to the analyzed plant traits, showing promising perspectives for the upscaling plant flooding stress response. In particular, NDVI was mainly linked to individual annual plant shoot elongation while the other indices were also related to stem pigments and secondary metabolites content. 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, proving to be a promising method for long-term monitoring of these plant communities.