High mangrove density enhances surface accretion, surface elevation change, and tree survival in coastal areas susceptible to sea-level rise

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.

Facilitation shifts paradigms and can amplify coastal restoration efforts

“…Restoration of oyster populations has been found to be more successful with construction of reefs with higher oyster density (57). Likewise, densely planted mangroves can achieve higher survival rates in comparison with those loosely planted, especially at sites susceptible to sea-level rise (28,58). Although future experiments are required to test how facilitation in a clumped configuration can enhance the success of restoration of these coastal habitats, those past works together with ours suggest that harnessing positive interactions may significantly enhance those restoration efforts.…”

Mangroves as a protection from storm surges in a changing climate

“…Category 3 mangroves account for the remaining 9 %; these mangroves are the most vulnerable to SLR and are likely to be lost. However, we acknowledge that as pointed out by many previous research-understanding the locationspecific impact of SLR on mangroves must take into account local factors that affect the ecological dynamics of the ecosystem, such as the history of sea levels in regard to development of coastal gradients, relative geomorphic and sedimentologic homogeneity of the coast, coastal processes including tidal range and its stability, density of mangroves, availability of freshwater and sediment, and salinity of soil and groundwater (Kumura et al 2010). Our knowledge from the cross-country analysis like the one presented in this paper with the wetland migratory potential of the publicly available DIVA database remains far from complete and quantification of location-specific loss deserves further analyses.…”

Turning the Tide: How Blue Carbon and Payments for Ecosystem Services (PES) Might Help Save Mangrove Forests

“…In response mangroves may adapt by shifting further inland, but this will only be possible in areas where human settlements and agriculture occur at some distance from the coastline (Gilman et al 2008). Alternatively they may maintain surface elevation through soil building and sediment accretion, but such a response requires vigorous growth and a good supply of sediment (Kumara et al 2010). Where adaptation is impossible the habitat available to mangrove forests will shrink and the remaining forest may become less ecologically resistant and/or resilient (Alongi 2008).…”