High mangrove density enhances surface accretion, surface elevation change, and tree survival in coastal areas susceptible to sea-level rise |
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Authors: | M P Kumara L P Jayatissa K W Krauss D H Phillips M Huxham |
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Institution: | (1) School of Life Sciences, Edinburgh Napier University, Edinburgh, UK;(2) Department of Botany, University of Ruhuna, Matara, Sri Lanka;(3) US Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd, Lafayette, LA 70506, USA;(4) Environmental Engineering Research Centre, School of Planning Architecture and Civil Engineering, Queen’s University of Belfast, Belfast, UK |
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Abstract: | Survival, growth, aboveground biomass accumulation, sediment surface elevation dynamics and nitrogen accumulation in sediments
were studied in experimental treatments planted with four different densities (6.96, 3.26, 1.93 and 0.95 seedlings m−2) of the mangrove Rhizophora mucronata in Puttalam Lagoon, Sri Lanka. Measurements were taken over a period of 1,171 days and were compared with those from unplanted
controls. Trees at the lowest density showed significantly reduced survival, whilst measures of individual tree growth did
not differ among treatments. Rates of surface sediment accretion (means ± SE) were 13.0 (±1.3), 10.5 (±0.9), 8.4 (±0.3), 6.9
(±0.5) and 5.7 (±0.3) mm year−1 at planting densities of 6.96, 3.26, 1.93, 0.95, and 0 (unplanted control) seedlings m−2, respectively, showing highly significant differences among treatments. Mean (±SE) rates of surface elevation change were
much lower than rates of accretion at 2.8 (±0.2), 1.6 (±0.1), 1.1 (±0.2), 0.6 (±0.2) and −0.3 (±0.1) mm year−1 for 6.96, 3.26, 1.93, 0.95, and 0 seedlings m−2, respectively. All planted treatments accumulated greater nitrogen concentrations in the sediment compared to the unplanted
control. Sediment %N was significantly different among densities which suggests one potential causal mechanism for the facilitatory
effects observed: high densities of plants potentially contribute to the accretion of greater amounts of nutrient rich sediment.
While this potential process needs further research, this study demonstrated how higher densities of mangroves enhance rates
of sediment accretion and surface elevation processes that may be crucial in mangrove ecosystem adaptation to sea-level rise.
There was no evidence that increasing plant density evoked a trade-off with growth and survival of the planted trees. Rather,
facilitatory effects enhanced survival at high densities, suggesting that managers may be able to take advantage of high plantation
densities to help mitigate sea-level rise effects by encouraging positive sediment surface elevation. |
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