Abiotic factors impacting seedling regeneration of Rhizophora mangle in Turneffe Atoll, Belize

Background/Question/Methods

Mangroves play a key ecological role in nearshore ecosystems and provide a number of ecosystem services to surrounding communities.  Mangrove rehabilitation from both natural regeneration and planting of propagules has had limited success necessitating further research into the factors limiting successful reestablishment.  A clear understanding of the biophysical factors limiting mangrove regeneration is needed to improve mangrove restoration success in areas where mangroves have been removed.  Turneffe Atoll, east of the Meso-American Barrier Reef, is undergoing significant land development including removal of Rhizophora mangle mangroves. Restoration will become necessary as the coastline begins to erode without mangrove forest protection.  A field observational and experimental study was conducted comparing intact and disturbed sites on the east side of the Atoll. Disturbed sites were areas where mangroves had been cleared previously (2-50 yrs) and natural seedling resettlement has occurred but forest regeneration has failed.  We compared mangrove seedling growth rates in intact forest and disturbed areas.  To investigate the factors limiting seedling success, we conducted a multi-factorial experiment including nutrient addition and/or wave protection treatments, treatment controls, and field controls to seedlings in the disturbed areas. We measured growth, leaf production, branching and prop root development in response to the various treatments.

Results/Conclusions

Rhizophora mangle seedlings in the intact forests showed significantly higher growth in height and leaf production compared to seedlings in the disturbed areas.  Pore water phosphate levels were 2-3 times higher in the intact areas compared to the disturbed areas indicating that nutrients may limit seedling growth in cleared areas. The combined nutrient addition + wave protection treatment significantly increased growth rates in the disturbed areas. Overall seedling mortality was 6% lower in the intact forest areas compared to the disturbed areas.  Seedlings grew more vigorously near established mangroves suggesting a strong link between growth rates and location of seedlings relative to existing mangrove forests.  These results indicate that established mangrove forests provide favorable conditions such as increased nutrient availability and shelter from waves for settled seedlings. Planting seedlings near established mangrove forests could be an effective strategy to expand/reestablish mangrove forests.  If seedlings plantations near established mangrove forests are not an option, it may be necessary to apply nutrient additions and/or shelter from waves to increase survival rates.