Harnessing spatial nutrient distribution and facilitative intraspecific interactions in soft eco-engineering projects to enhance coastal dune restoration

Planting engineer species is widely used to restore coastal dune ecosystems, but the success of this nature-based practice is often low. Increasing nutrient availability through fertilization and using plant density promoting facilitative plant-plant interactions are increasingly advocated to improve dune restoration. However, how fertilizer application (homogeneous or heterogeneous spatial distribution) influences plant interaction intensity in dune species is still poorly known. We conducted a field-mesocosm experiment assessing the effects of spatial nutrient distribution and planting density on functional traits and intraspecific interactions in two dune plants with contrasting clonal growth architecture. Fragments of Calamagrostis arenaria (phalanx architecture) and Sporobolus virginicus (guerrilla architecture) were grown in monoculture either at low or at high density with the same total supply of fertilizer applied either heterogeneously or homogeneously. We hypothesized that these species would respond differentially to nutrient distribution and plant density. C. arenaria allocated less biomass below- than aboveground under homogeneous than heterogeneous conditions and produced less shoots at low than high density. Conversely, under homogeneous conditions, S. virginicus plants grown at low density had a greater rhizome, root, and total biomass than those at high density. For C. arenaria, intraspecific interactions on aboveground, root, and total biomass shifted from positive to neutral under heterogeneous conditions while those on rhizome biomass switched from neutral to negative. S. virginicus intraspecific interactions on all biomass variables shifted from negative to neutral. Our results demonstrate that nutrient distribution affected functional traits and intraspecific interactions differentially in phalanx and guerrilla species. Notably, in nutrient homogeneous substrates high planting density increased phalanx species performance but reduced that of guerrilla species due to competition. Instead, in heterogeneous substrates both species performed similarly at low and high density. These findings may help restoration practitioners in identifying suitable fertilization/planting schemes and implementing effective dune restoration programs.