Unmanned aerial systems‐based monitoring of the eco‐geomorphology of coastal dunes through spectral Rao's Q

Abstract Question: Does spectral diversity captured by unmanned aerial systems (UAS) provide reliable information for monitoring the eco-­geomorphological integrity of Mediterranean coastal dune ecosystems? Can this information discriminate between two coastal areas with low (LP) and high (HP) human pressure? Location: Tyrrhenian coast, Central Italy. Methods: By processing UAS images, we derived the normalized difference vege- tation index (NDVI) and topographic variables at high spatial resolution (0.5 m) for 150 m wide strips starting from the coastline inland on two representative coastal tracts under low and high human pressure. We mapped the sea–­inland heterogene- ity applying Rao's Q index to the plant biomass (NDVI) and geomorphology variables (elevation and slope). Since Rao's Q index can be calculated in a multidimensional space, we summarized the variability of these three variables into a single eco-­ geomorphological layer. We then inspected and compared how the Rao's Q index values for plant biomass, geomorphology and eco-­geomorphology change as a func- tion of the distance from the sea between the two coastal sites. Results: Rao's Q heterogeneity values vary along the sea–­inland gradient of well-­ preserved sites (LP). The maximum eco-­geomorphological heterogeneity was found at intermediate distances from the sea and decreased toward the inner sector where the dune geomorphology was more stable and vegetation more homogeneously dis- tributed. Instead, Rao's Q heterogeneity values featured constant low values along the gradient on the HP site, highlighting a simplified eco-­geomorphological gradient related to the high human pressure. Conclusions: Using UAS, the eco-­geomorphological gradient of coastal dunes can be quantified at a very fine spatial resolution over management-­relevant extents. Rao's Q index applied to sensing imagery successfully captured the differences in the eco-­geomorphological heterogeneity along the sea–­inland dune gradient and among sites with different levels of anthropic pressure. This approach supports frequent surveys and is particularly suitable for spatial monitoring of key coastal functions and services.