Foredune initiation and early development through biophysical interactions

Coastal dunes result from complex interactions between sand transport, topography and vegetation. However, uncertainty still persists due to limited quantitative analyses, integrating plant distribution and morphologic changes. This study aims to assess the initiation and maintenance of feedback processes by analysing the early development stages of incipient foredunes, combining data on the evolution of the plant cover and communities and dune morphology. Over three years, the monitoring of a newly formed dune (1 ha plot) reveals the progressive plant colonisation and the episodic accumulation of sand around vegetated areas controlled by sediment availability. Distinct colonisation rates were observed, influenced by inherited marine conditions, namely topography and presence of beach wrack. Berm-ridges provided elevations above the critical threshold for plant colonisation and surface roughness, aiding sediment accumulation. Beach wrack above this threshold led to rapid expansion and higher plant concentration. In the initial stages, vegetation cover significantly influenced sediment accumulation patterns, with higher accumulation around areas with high plant cover and low slopes or around areas with sparse vegetation but milder slopes. As the dune system matured and complexity grew, the link between vegetation cover and accumulation became nonlinear. Mid to low coverages (5-30 %) retained most of the observed accumulation, especially when coupled with steep slopes, resulting from positive feedbacks between vegetation, topography and sand transport. As foredune developed, vegetation cover and diversity increased while inherited morphologies grew vertically, explaining the emergence of dune ridge morphological types. Flat surfaces lacking wrack materials experienced a three-year delay in colonisation and sand accumulation, leading to the formation of terrace-type incipient foredunes. These observations underline feedback processes during the early stages of dune formation, with physical feedbacks primarily driving initiation and biophysical feedbacks prevailing in subsequent colonisation stages.

Effects of invasive Rosa rugosa on Baltic coastal dune communities depend on dune age

Rosa rugosa Thunb. (Japanese Rose) is one of the most invasive species in Europe. It spreads spontaneously in coastal areas of western, central and northern Europe, posing a threat to dune habitats, including those indicated in the EU Habitats Directive as particularly valuable. R. rugosa has already been reported to displace native plants and alter soil properties. However, little is known about how these effects are mediated by the habitat context or the invader condition (health, ontogenetic stage). This study addressed that gap by examining vegetation and soil in 22 R. rugosa-invaded sites, half of which were in yellow dunes and the other half in grey dunes, i.e. two habitats representing the earlier and later stages of dune succession. The study was conducted on the Hel Peninsula (Poland’s Baltic coast). R. rugosa had a significant impact on dune vegetation, but the impact was strongly dependent on the habitat type. In the yellow dune sites, R. rugosa outcompeted most resident plant species, which translated into a strong decline in their total cover and richness. The invasion was almost not accompanied by changes in soil properties, suggesting that it affected the resident vegetation directly (through space takeover and shading). In the grey dunes, R. rugosa caused a shift in species composition, from that characteristic of open communities to that typical of forests. In this habitat, a significant increase in the soil organic layer thickness under R. rugosa was also observed, which means that both direct and indirect effects of the invasion on the vegetation should be assumed. Finally, a negative relationship was found between the total chlorophyll content in R. rugosa leaves and the parameters of resident plant communities, showing that the invasion effects can vary not only across habitats, but also with the condition of the invader. The results may have practical implications for managing R. rugosa invasions in coastal sand dune systems. Since R. rugosa accelerates grey dune succession, protecting this habitat may be more urgent and, at the same time, more complicated than protecting dunes at the earlier stages of development.

Are Ecosystem Engineering Traits Fixed or Flexible: A Study on Clonal Expansion Strategies in Co-occurring Dune Grasses

Many vegetated coastal ecosystems are formed through ecosystem engineering by clonal vegetation. Recent work highlights that the spatial shoot organization of the vegetation determines local sediment accretion and subsequently emerging landscape morphology. While this key engineering trait has been found to differ between species and prevailing environmental conditions, it remains unknown how the interplay of both factors drive shoot organization and therefore landscape morphology. Here, we compared the spatial shoot organization of young, clonally expanding plants of the two dominant European dune grass species: sand couch (Elytrigia juncea) and marram grass (Ammophila arenaria) across a range of coastal dune environments (from Denmark to France). Our results reveal that, on average, sand couch deployed a more dispersed shoot organization than marram grass, which has a patchy (Lévy-like) organization. Whereas sand couch exhibited the same expansion strategy independent of environmental conditions, marram grass demonstrated a large intraspecific variation which correlated to soil organic matter, temperature and grain size. Shoot patterns ranged from a clumped organization correlating to relatively high soil organic matter contents, temperature and small grain sizes, to a patchy configuration with intermediate conditions, and a dispersed organization with low soil organic matter, temperature and large grain size. We conclude that marram grass is flexible in adjusting its engineering capacity in response to environmental conditions, while sand couch instead follows a fixed expansion strategy, illustrating that shoot organization results from the interaction of both species-specific and environmental-specific trait expression.