Dunes are dynamic systems naturally subject to morphological and ecological changes. Their spatial and temporal evolution is due to several physical and biological factors [1
] and to occasional disturbances (stochastic events) after which they tend to recover to a dynamic equilibrium [4
]. At the initial stage, the formation of the dune is mainly driven by the availability of sediment and wind energy that enable sand accumulation and, therefore, determines the type and distribution of vegetation (primary vegetation). At a later stage, the vegetation gains relevance for growing and stabilizing the dune, trapping sand with leaves, branches, and roots (secondary and tertiary vegetation) [8
Sediment distribution can be considered the result of the retention capacity of the dune environment and is due to a large number of factors, such as wind regimes, runoff, coastal current, wave energy, vegetation coverage, and coastal exploitation. The development and evolution of coastal dunes are led by the balance between the wind regime, sediment budget, and vegetation coverage [10
]. Key studies [15
] highlight that vegetation coverage plays a particularly important role, showing that only 15%–20% of vegetation coverage is sufficient to inhibit wind-blown sand transport [16
]. In vegetated environments, biostabilizations of sediments occur in preparation for the establishment and spread of the next serial stage of succession species [20
]. Morphometric factors related to ecological constraints, such as vegetation presence/absence and structure, are generally relevant in the coastal setting [15
]. Concurrently, the Anthropocene fingerprint, i.e., human activities [24
], has an effect on the morphological equilibrium of coastal areas and, consequently, on the distribution of water, sediments, and nutrients in coastal habitats [25
Thus, geomorphological processes along with vegetation presence and absence jointly determine the evolution of dune systems, generally known as land–surface eco-morphodynamics [26
]. In this context, the spatial relationship between sediment distribution and vegetation cover (vegetated and non-vegetated areas) within dunes is still a question that needs to be investigated, specifically finding spatial patterns [26
Most remote sensing studies have focused on detecting spatial patterns through the use of aerial sensors [15
] and lower-resolution images [31
], and the use of spatial variables derived from remote sensing imagery offers an ideal tool to study the relationships among vegetation type, patch distribution, and eventually crest and foot sinuosity of the dune system. In the last 20 years, some authors [33
] have provided an initial overview of the capabilities of remote sensing for coastal zone studies and, recently, other authors have focused on the estuarine environment [35
]. The focus in these articles is on both low-resolution remote sensing [33
] and the additional value of very-high-resolution hyperspectral data [23
]. Hyperspectral remote sensing is a powerful tool for the identification of earth surface materials due to the value of continuous spectral signals in terms of band contiguity and density, as argued by authors in previous research related to this topic [36
For vegetated dune fields, a number of key studies have adopted methods for analyzing color aerial photography and hyperspectral remote-sensing data. Such tools are essential to determine functional vegetation types and plant communities [15
], as well as for identifying mineral composition and texture in soil and sediment studies [36
The ability of airborne light detection and ranging (LiDAR) data to accurately represent altimetry over large areas has been demonstrated by a number of analyses [41
]. Airborne laser altimetry is largely used to map coastal geomorphology and the geomorphic process and for analyzing the distribution of sediment and vegetation spatial patterns [43
]. The typical dunes’ vegetation zonation is closely related to ecomorphological gradients from the shoreline to the inland regions, and the great potential in coastal systems monitoring is the availability of synchronous acquisition of LiDAR and hyperspectral sensors to explore the relationships between dune vegetation and local dune morphology [44
To improve the understanding of spatial patterns and relationships relevant to the sediment retention capacity exerted by the dune environment, an approach is developed in the present study for a linear correlation model based on sliding windows, to explore hypotheses that elevation, slope, vegetation cover typologies, and sinuosity are spatially correlated with sand cover abundance.
To test these hypotheses, vegetation and topographic characterizations are obtained using the FHyL (Field spectral libraries, airborne Hyperspectral imagery and LiDAR altimetry) method [30
] that enables multisensory integration of observations in emerged and submerged coastal environments.
Our scientific purpose is to develop a remote sensing methodology to examine coastal dune spatial patterns: More specifically, it is not well-known how sediment is distributed in relation to the vegetation pattern from the surf zone to the later structured vegetated stages. The spatially explicit ecomorphological patterns occurring in the area should highlight the signatures of how geomorphic features including Anthropocene patches, such as buildings, houses, roads, and other anthropic activities, interact with the vegetation distribution.
2. Study Area
The coastal stretch (Figure 1
) is a microtidal sandy beach–lagoon system in the Circeo National Park (Central Italy) characterized by a single continuous dune ridge, approximately 22 km long, running parallel to the shoreline. Channels and inlets that connect the coastal lagoons naturally and artificially interrupt the continuity of the system. The beach–dune system of the Circeo National Park is the remnant of a barrier island that separated the coastal plain from the open sea during the Holocene [46
]. Longshore sediment transport toward the SE is a consequence of the main NW-coming wind (Mistral). As a consequence, both the elevation and amplitude of the emerged beach–dune system increase from north to south to the maximum elevation of 28m above sea level (including surface objects) and to the maximum width of 250 m in the southern part of the area [48
The dunes are mostly colonized by specialized coastal vegetation typical of the Mediterranean areas and characterized by a linear floristical, chorological, and phytosociological zonation from the sea landward [20
]. In the 1950s, the dynamics of the system (i.e., the dynamics orthogonal to the shoreline) were blocked by the construction of the coastal road at the dune crest, and its natural morphological evolution was further stopped by increased runoff phenomena related to the soil sealing.
Due to the different exposures to wind and insolation, the dune lee side (facing the coastal lagoons) is characterized by the presence of well-structured trees and shrub vegetation, while the stoss side facing the sea is dominated by xerophytic bushes and halophytes, which can retain the sand through specialized root systems [49
]. The emerged beach, mostly in the southern part of the area, is subject to management practices during summer for a better exploitation. In the northern part, the most exposed to coastal erosion, various coastal defense and nourishment interventions were carried out in the last 35 years to protect buildings and guarantee tourist activities [51
The glossary used to describe the dune system metrics is graphically explained in Figure 2
The unique contribution of remote sensing to the study is provided by the novelty of the processing approach that combines the hyperspectral data with parameters obtained from LiDAR. It is rare having a classification of trees, herbaceous, and shrubs classes, and the study proves that the integration of hyperspectral airborne and field sensors via spectral mixture analyses can support this kind of classification. By inspecting vegetation and sand cover obtained by processing hyperspectral data and the LiDAR DSM, it is possible to understand how fragmented the sand landscape is when there is also presence of vegetation and anthropic cover, and how topography is related to the vegetation and the sand distribution.
Considering the results obtained for morphological characterization of the dune system [63
], the foredune is a highly dynamic environment reactive to external drivers, both natural- (i.e., marine- and weather-related) and human-driven. Unless marine forces favor the southern portion of the stretch in terms of elevation and width, the trees, herbaceous, and shrub cover are directly and inversely related to the dune and foredune width and the profile underlying the surface, but with variable magnitude. It is already known that the limited variation of the foot dune elevation is influenced by the action of waves that, due to winter storms, periodically erode the dunes where the beach width is limited and the system is more vulnerable [64
]. Beach width has the primary role in protecting the backshore habitats from the direct impact of surges during storms, but the effect of aerosols on vegetation is highly variable depending on vegetation type and assemblages (Figure 8
The intersections of a variety of ecomorphological parameters have largely been used to extend and extrapolate vegetation presence and variability within the morphological dunes [10
]. Coastal dune system classification traditionally uses orthogonal analyses of the ridges moving from the foot of the hind dune (lee side) to the shoreline. Here, thanks to the spatial continuity of data offered by the hyperspectral and LiDAR acquisition, there is a mixture of orthogonal transects and moving windows that enables the derivation of both discrete and continuous metrics describing the actual ecogeomorphological relations of the dune environment (perpendicular and parallel to the dune ridge).
The width profile underlying the surface and elevation show many correlations with the cover percentages and landscape indexes, even if the sand cover percentage does not follow the behavior observed for the profile underlying the surface moving north to south (Figure 7
, Figure 12
). As an unexpected result, we observed a weak linear correlation for the sand cover percentage with the patchiness and the fragmentation of the landscape.
Several studies have demonstrated the effect of vegetation density on the width of the sedimentation for time periods of weeks [11
], but in the present study, the sand cover LSI is positively correlated with the morphological parameters and with the trees, shrub, and herbaceous LSI, meaning double evidence in terms of sediment retention. On the one hand, the volume of the dune is still a proxy of the sediment amount; on the other hand, the cover classes percentage, unless anthropic, is positively related to an increasing edge effect.
Sediment transport from north to south determines increasing size of the dunes, and such morphological evidence has also been investigated in the submerged beach of the same area, where nearshore sandy bars have been recognized and classified [66
], suggesting further relations to be explored.
Vegetation patterns thus affect sand deposition, and conversely, the amount of deposition also affects the vegetation patch distribution [1
]. It is worth noting that the lag analyses weight the border effect introduced by the width and length ratio of each lag. The uncertainty, due to the fact that the transect line border does not match with the natural borders of the polygon patches used for fragmentation metric calculation, was estimated to be between 3.5% and 5.5% on average.
The vegetation cover distribution of the foredune has a discontinuous pattern due to anthropic features such as the coastal road overlapping the dune crest and the walkways and paths crossing the dune. These features are triggering factors for instability phenomena such as natural blowout openings, erosion furrows, and runoff (Figure 14
In the foredune, there is a variable distribution of shrubs that, in a specific part of the stretch, are almost absent (Figure 8
). This can be the indicator for blowouts and for a less stabilized portion of the stretch, where the vegetation does not respect an along-shore gradient, as expected. Within the foredune, the herbaceous cover percentage and its distribution increases southward and is inversely correlated with shrub percentage.
The dune slope facing the sea shows more discontinuous vegetation cover compared to the slope facing the lagoons. High vegetation cover is detectable close to the coastal road but generally does not have more than 75% of abundance. We did not find a strong correlation between the dune slope and dune cover. The foredune slope is covered by vegetation typical of the not-yet-consolidated dune, forming a discontinuous layer of elevation of a few decimeters on top of the topography only in the central part. Moving south, these vegetation formations are characterized by the presence of pioneer juniper, which represents the transition between herbaceous phytocoenoses and those of the Mediterranean coastal vegetation of consolidated dunes.
Moreover, the foot sinuosity along the dune ridge is naturally related to different vegetation typologies and wind-blown effects, but here, the inverse correlation between the foredune anthropic cover and sinuosity values is driven by the presence of linear fences that separate the public beaches from private properties. In the southern part of the stretch, the foredune foot has a higher sinuosity value due to the higher width of the beach and the dune elevation, but anthropic cover artificially prevents natural sinuosity that is associated with the presence of pioneering vegetation between 15.5 and 17.5 km (Figure 13
). In general, the whole foot system has a very highly scattered pattern in terms of sinuosity associated with beach cleaning and management activities (mainly in the summer period) and private villa garden maintenance (all the year). This leads to signature evidence of the Anthropocene because the presence of anthropic structures is mainly referable to the construction of the coastal road on the dune crest and houses in the southern part of the dune. It is argued that the Anthropocene is set in the mid-20th century when the “Great Acceleration” and “world-wide distribution” of several markers of human activities are found in the atmosphere and on land (radioactive fallout from the Trinity test in 1945, increase in aluminum, tarmac, and concrete production, decrease in many species) [68
]. Surprisingly, we also found evidence of anthropogenic influence on the beach–dune system morphology in the study area where, close to the dune foot, sandy areas are dominant and are exposed with patchy distributions; moving south, the sand is interspersed with plant communities. Nevertheless, anthropic cover is more abundant in the southern foredune and the lower sinuosity is extended along the entire stretch, highlighting the fingerprint of human activities (e.g., removal of pioneering species at the foredune foot and levelling of the emerged beach before and during the summer).
The effective and innovative contribution of the present study to the traditional approaches based on ecomorphological monitoring is in the use of landscape ecology indexes to map and explain land cover spatial patterns [26
The synergy among airborne hyperspectral-derived and altimetry-derived data, and in situ measurements for the vegetation and sand cover distributions, support the idea that the whole system scale can be captured in its entirety, preserving the details of the individual components. This approach will help future cross-scale monitoring activities because of its reliability in terms of data collection and method to process these data. It integrates the traditional method of extracting dune metrics with innovative theories of feedback between landscape ecology and geomorphology.
The results obtained have enabled us to produce thematic maps and calculate parameters and indices that describe and quantify the spatial distribution of sand cover within the coastal dune landforms. Significant north–south gradients related to beach and dune width, elevation, and the profile underlying the surface, as well as fragmentation of the landscape, were observed in the study area. The ecomorphological patterns have highlighted the signatures of vegetation and sand distribution as leading components of the dune landscape.
The significant advance exposes a valuable procedure to determine the sediment retention bionetwork stressed particularly by the intensive coastal dune exploitation and, subsequently, the Anthropocene fingerprint due to human activities.