Abiotic drivers of mudflat colonization by mangroves in Indonesia: a knowledge base to support ecological restoration

Mangrove seedlings are vulnerable to physical disturbance during their first days of development and require a disturbance free period (i.e., a Window of Opportunity; WoO) to allow initial root development and provide sufficient anchorage strength to resist dislodgment from subsequent flooding events. To understand the WoO periods required by Avicennia marina and Avicennia alba pioneer mangroves and their interactions with salinity, we designed a factorial mesocosm experiment with six WoO (days without flooding) treatments, at 10‰ and 30‰ salinity under semidiurnal simulated tides. For both species, the highest probability of establishment was recorded at 3-day WoO periods or higher. Salinity had a significant negative effect on establishment, delaying establishment onset by up to 5 days for both species. Avicennia alba had higher vulnerability to salt stress, with a probability of establishment below 50% at 30‰ for WoOs shorter than 3 days. Additional field monitoring of seedling establishment along four 100 m long transects showed higher increments in seedling density during periods of 3-day WoO occurrences. Information on natural pioneer species WoOs is key to support natural establishment of mangroves as ecological restoration projects are increasingly moving away from active planting. Our results can support Indonesia’s aim to restore 600,000 mangrove ha by: (1) identifying seasons adequate for restoration actions (i.e., hydrological rehabilitation or manual diaspore dispersal), (2) providing a knowledge base on ideal hydrological conditions for natural regeneration, and (3) highlighting that natural regeneration in ecological restoration is predictable and hence an alternative to mangrove planting.