Managing biotic interactions during early seagrass life stages to improve seed-based restoration

Seagrasses are declining globally, and effective restoration actions to promote the recovery of degraded meadows are urgently needed. Harnessing positive plant interactions during early life stages is considered a valuable strategy to improve terrestrial and coastal habitat restoration. Yet, its application to seagrass restoration is still in infancy, and very little is known on the role played by biotic interactions in shaping newly established populations. We assessed the feasibility of manipulating intraspecific and interspecific plant interactions to enhance seed-based restoration success using the seagrass Posidonia oceanica as a model. Specifically, we investigated in mesocosm whether seed germination increased with increasing seed density and whether increasing seedling density and planting a pioneer seagrass promoted facilitation among seedlings. To do this, seedlings were grown either as a single individual or in clumps of medium and high density, with and without Cymodocea nodosa, for 2 years encompassing their most critical growth stage. Germination of P. oceanica seeds was not affected by seed density. Posidonia oceanica seedlings planted at medium and high density showed higher survival than those planted individually but only in the presence of C. nodosa. Seedlings planted at medium and at high density with C. nodosa performed better than those grown at low density or without C. nodosa due to a positive joint effect of intraspecific and interspecific interaction. Cymodocea nodosa plants grown with P. oceanica seedlings at high density were larger than those grown alone, indicating a mutualistic relationship. Synthesis and applications. Our results show that in seagrasses positive interactions during early life stages can be promoted by planting seedlings in dense clumps in association with an early successional species. The incorporation of this novel nature-based approach in seed-based restoration could accelerate the recolonization of degraded seagrass habitats. In planning future seed-based restoration interventions, managers should assess both intraspecific and interspecific interactions established by seedlings of target species for identifying proper planting density/spatial configuration and potential benefactor species promoting facilitative mechanisms to maximize seedling planting success.