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

1. 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. 2. We assessed the feasibility of manipulating
intra- 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 two years encompassing their most critical growth stage. 3.
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 intra- and interspecific
interaction. Cymodocea nodosa plants grown with P. oceanica seedlings at
high density were larger than those grown alone, indicating a mutualistic
relationship. 4. 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 intra- 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.