Foredune initiation through plant-topography interactions

Coastal dunes result from complex interactions between sediment supply, topography and vegetation. However, feedback processes remain uncertain, in part due to the lack of quantitative analyses, integrating plant distribution and morphologic changes. This work seeks to assess how feedback processes are initiated and maintained by analysing the early development stages of a coastal dune, combining data on the evolution of its biotic (plant density and plant communities) and abiotic (morphology) components. For that, we study the establishment and early evolution of a coastal dune (1 ha plot) between 2019 and 2022, using high-resolution images and photogrammetry from Unmanned Aerial Vehicle (UAV) surveys. Results show a gradual plant colonisation with an increasing number of species as the colonised area expanded and plant community composition shifted. Plant distribution appears controlled by environmental conditions (elevation, distance to shoreline and debris presence), explaining three colonisation patterns: 1) over inherited morphological features (ridges) with debris, 2) over nutrient-rich debris lines, and 3) at flat surfaces (platform) with limited sediment supply and debris material. Colonisation initiated along the ridge crests with pioneer and builder species. The ridges also captured and retained most of the aeolian sediment transported under favourable conditions. Plant communities increased their abundance through time and partially shifted to binder plant species. Colonised stranded debris lines displayed greatest plant densities. However, reduced fetch distances inhibited sediment transport and sand accumulation. The platform was the latest area to colonise with pioneer plants shifting to binders and very limited sediment supply. Accumulation of sand was promoted by surface roughness mainly due to changes in slope (3-6°) during the during first months of dune formation, when vegetation was very scarce, and due to the combined influence of slope and plant density at later stages, as plant density increased, gaining relevance. Sand accumulation contributed to reinforce the topography of the inherited morphologies during both phases. Still, low densities (5-20%), more common during early stages, had a higher contribution to the total accumulation of sand in the plot. Builder and perennial pioneer plants were more efficient, while annual pioneer and binders captured less sediment. Interestingly, highly efficient builders colonised areas prone to sand burial, ensuring their success in trapping sand, while binders occupied more protected areas, lacking active sediment supply. Overall, this work elucidates the feedback processes between sediment supply, topography and vegetation during the very early stages of dune formation, with physical feedbacks dominating the onset and biophysical ones dominating a subsequent colonisation stage. The results highlight the relevance of inherited morphologies to the early evolution of a dune and likely to its final configuration, conditioning future responses of the system to disturbances.

This work is supported by FCT – Fundação para a Ciência e a Tecnologia, I.P, through the projects 2022.05392.PTDC, 2021.04286.CEECIND/CP1672/CT0001, UIDP/00350/2020, UIDB/00350/2020 and LA/P/0069/2020.