Does tidal phase determine successful plant colonisation of atoll lagoon islands in the Maldives?

Vegetation colonisation is an important process in the geomorphic development of atoll lagoon islands and understanding how plants establish may help model how these islands will respond to eustatic sea-level rise. The recent morphodynamics of two lagoon island forms, Maaodagalaa, a sand cay, and Mahutagalaa, a forested island, in Huvadhoo Atoll, Republic of the Maldives, provided an exceptional opportunity to examine processes of plant colonisation over annual to decadal time scales. Maaodagalaa sand cay migrated on the reef platform between June 2019 and February 2020 resulting in a new plant-free surface close to sea-level. During this period the new island surface was colonised by 10 species of marine-dispersed plant species. Seedlings were found across the highest surfaces formed in that period (0.62 m above local datum and 0.48 m above the highest tides recorded between 1987 and July 2020). An inundation event in February 2020 stranded debris up to 0.50 m above spring high tide, 15 m inland of the northern margin of the island. Our results indicate wave events, resulting from local, fetch-limited waves, in combination with spring high tides, can strand seeds of marine-dispersed plants on sand cays and around the margins of forested islands. Seedling survival is enhanced if seeds are stranded during peak perigean spring high tides during a local 7-month spring tide cycle. Lagoon islands in the Maldives are still being formed and colonised by plants despite local and ongoing sea-level rise.