N. Loftus1,2*, E. Kahn2 , K. Sanchez2, A. Baniaga2 , D. Jacobs2
1 Yale School of the Environment, United States; 2 University of California Los Angeles, United States
* Corresponding author: natalie.loftus@yale.edu
Introduction
Spotted mangroves (Rhizophora stylosa) act as keystone species in their native environments by trapping sediment and sequestering carbon with extensive aerial prop roots. R. stylosa were introduced to Moʻorea, French Polynesia in 1930 and have since spread along the island’s coasts. Our project examines how stands of R. stylosa have grown, altered sediment composition, and affected infaunal invertebrate communities as they have aged.
Objective and Methods
We sampled seven sites around the island, determined stand age from literature and satellite imagery, measured stand area from satellite imagery, and measured tree dimensions (height, diameter at breast height, and prop root length). For the two largest trees in a stand, we took sediment cores at the base of the trunk, at the farthest edge of the prop roots towards the water, and 3 m into the water from the edge of the prop roots. We sieved these samples into six size classes (4 mm - 125 µm), measured sediment composition by volume, and identified infauna larger than 2 mm.
Results
Although spotted mangrove stand area increases linearly with age, individual tree sizes level off as they approach a maximum. The interior cores had higher Shannon diversity index for infauna compared to the samples taken at the edge of the prop roots, though Shannon diversity did not significantly change with age. Grains from the interior were dominated by mangrove roots, and they had a larger mean and median, were less right-skewed, and more platykurtic than edge grains. The number of calcium carbonate grains decreased with age for all core locations. The recent establishment of R. stylosa stands on Moʻorea is well documented, which allows us to present novel findings of spotted mangrove growth rates and their progressive alteration of the sedimentary environment.
References
Langer, M.R., Lipps, J.H., (2006), Assembly and persistence of forminifera in introduced mangroves on Moorea, French Polynesia. Micropaleontology, 52, 4, 343-355.
Meyer, J.Y., Taureau, F., Bisarah, L., Madi Moussa, R., Gorchakova, E., Caillaud, A., (2021), Introduced mangroves in the Society Islands, French Polynesia (South Pacific): invasive species or novel ecosystem?. Biological Invasions 23, 8, 2527-2539.
Smith, J.M.B., (1996), Perspectives on an Invasion of Paradise: Rhizophora stylosa on Moorea, French Polynesia. Australian Geographical Studies, 34, 1, 81-87.
Taureau, F., Bisarah, L., Caillaud, A., Gorchakova, E., Meyer, J.Y., (2019), Surveillons la mangrove de Polynesie francaise ensemble. Université des Nantes, Etude cartographique dans les îles de la Société, 1-37.
