Habitat modification by marram grass negatively affects recruitment of conspecifics.

Ecosystem engineers alter their environment often benefiting their own survival and growth yielding self-reinforcing feedbacks. Moreover, these habitat modifications have been found to facilitate recruitment of conspecifics for some species, while for others engineering inhibits recruitment. Whether dune grasses facilitate or inhibit recruitment of conspecifics is yet unknown. Here, we investigated how habitat modification by European marram grass (Ammophila arenaria) through embryonic dune development affects recruitment from seeds and marine dispersed rhizome fragments. Specifically, we tested at three locations with different dune morphologies how habitat modification affected natural seed and rhizome presence and shoot emergence from plots in which seeds or rhizome fragments were added. In addition, we investigated how sediment burial (i.e., the main effect of habitat modification by dune grasses) affected germination and emergence in a controlled experiment. Results show that regardless of habitat modification or beach width, seeds and rhizomes were absent in natural conditions. Habitat modification negatively affected shoot emergence from seeds (8 × less) and rhizomes (4 × less) and was negatively related to sediment dynamics. Furthermore, fewer seedlings were found with higher elevations. In controlled laboratory conditions, the highest seedling emergence was found with slight burial (0.5-3 cm); both germination and seedling emergence decreased as seeds were buried deeper or shallower. Overall, habitat modification by marram grass negatively affects recruitment of conspecifics through increased sediment dynamics and elevation. Consequently, storm events or eradication programs that include removal of adult vegetation-which leads to an unmodified system-might benefit new recruitment from seeds or clonal fragments.

Habitat complexity drives food web structure along a dynamic mangrove coast.

Structurally complex habitats, such as mangrove forests, allow for rich assemblages of species that benefit from the provided space, volume and substrate. Changes in habitat complexity can affect species abundance, diversity and resilience. In this study, we explored the effects of habitat complexity on food web networks in four developmental stages of mangrove forests with differing structural complexities: climax > degrading > colonizing > bare, by analyzing food web structure, stable isotopes and habitat complexity. We found that food webs became gradually more biodiverse (species richness: +119 %), complex (link density: +39 %), and robust (connectance: -35 %) in climax versus bare stages with increasing complexity of the mangrove forest (i.e., number of trees, leaf cover, and pneumatophore densities). This study shows that habitat complexity drives food web network structure in dynamic mangrove forests. We recommend restoration practitioners to use this food web network approach to quantify habitat restoration successes complementary to traditional biodiversity metrics.

Sediment availability provokes a shift from Brownian to Lévy-like clonal expansion in a dune building grass.

In biogeomorphic landscapes, plant traits can steer landscape development through plant-mediated feedback interactions. Interspecific differences in clonal expansion strategy can therefore lead to the emergence of different landscape organisations. Yet, whether landscape-forming plants adopt different clonal expansion strategies depending on their physical environment remains to be tested. Here, we use a field survey and a complementary mesocosm approach to investigate whether sediment deposition affects the clonal expansion strategy employed by dune-building marram grass individuals. Our results reveal a consistent shift in expansion pattern from more clumped, Brownian-like, movement in sediment-poor conditions, to patchier, Lévy-like, movement under high sediment supply rates. Additional model simulations illustrate that the sediment-dependent shift in movement strategies induces a shift in optimisation of the cost-benefit relation between landscape engineering (i.e. dune formation) and expansion. Plasticity in expansion strategy may therefore allow landscape-forming plants to optimise their engineering ability depending on their physical landscape.

Resilience of beach grasses along a biogeomorphic successive gradient: resource availability vs. clonal integration.

Coastal ecosystems are often formed through two-way interactions between plants and their physical landscape. By expanding clonally, landscape-forming plants can colonize bare unmodified environments and stimulate vegetation-landform feedback interactions. Yet, to what degree these plants rely on clonal integration for overcoming physical stress during biogeomorphological succession remains unknown. Here, we investigated the importance of clonal integration and resource availability on the resilience of two European beach grasses (i.e. Elytrigia juncea and Ammophila arenaria) over a natural biogeomorphic dune gradient from beach (unmodified system) to foredune (biologically modified system). We found plant resilience, as measured by its ability to recover and expand following disturbance (i.e. plant clipping), to be independent on the presence of rhizomal connections between plant parts. Instead, resource availability over the gradient largely determined plant resilience. The pioneer species, Elytrigia, demonstrated a high resilience to physical stress, independent of its position on the biogeomorphic gradient (beach or embryonic dune). In contrast, the later successional species (Ammophila) proved to be highly resilient on the lower end of its distribution (embryonic dune), but it did not fully recover on the foredunes, most likely as a result of nutrient deprivation. We argue that in homogenously resource-poor environments as our beach system, overall resource availability, instead of translocation through a clonal network, determines the resilience of plant species. Hence, the formation of high coastal dunes may increase the resistance of beach grasses to the physical stresses of coastal flooding, but the reduced marine nutrient input may negatively affect the resilience of plants.