Coastal dune ecogeomorphic states regulated by extrinsic factors

Abstract

Coastal dunes result from multiple interactions between biotic and abiotic factors. The complexity of the resultant dune ecogeomorphology will therefore be determined by the spatial and temporal variability of the involved factors and their interactions. This work explores the longshore variability of morphological features, plant community distribution and accumulation patterns of a dune segment (1.4 km-long) located at the downdrift end of a sandy peninsula in the Ria Formosa, Portugal. To understand the main drivers of the observed variability and the implications for dune morphological response, this information was combined with recent multidecadal shoreline evolution data (i.e., 60 years). The integrated results document significant differences in dune morphology, sedimentation patterns and plant zonation, with two distinct dune configurations or states identified in close proximity. One (western sector) shows a narrower dune system, vegetation cover characterised by pioneer species with low densities, and squeezed plant zonation. Conversely, the other (eastern sector) presents a wider dune system with a new foredune, a more developed plant zonation and relatively high vegetation density. Both states could be partially explained by the recent shoreline trends and inlet shifts, with stable to retreating trends in the western sector and shoreline progradation in the eastern one. Plant zonation and accumulation patterns suggest that the dune along the retreating sector is in a cycle of inland migration, encouraged by the reduced accommodation space and the low retention capacity of the vegetation across the dune stoss. Alternatively, observations along the prograding sector suggest that the greater accommodation space and the stabilising feedback between vegetation and topography promoted the seaward progradation of the system and the development of an incipient foredune. Outcomes support the importance of biogeomorphic feedbacks for the dune configuration. However, they also evidence that the role of vegetation within this feedback is rather passive and primarily regulated by physical factors, including regional (low precipitation and sediment transport potentials) and local conditions (e.g., variations in the sediment supply alongshore). Therefore, despite the undeniable role of vegetation in reinforcing dune topography, it is worth highlighting that local external forces may dominate dune response, inhibiting, allowing or reinforcing ecogeomorphic interactions in the long-term.This work is supported by projects 2022.05392.PTDC and PTDC/CTA-GFI/28949/2017, funded by the Portuguese Foundation for Science and Technology.