Edinburgh Napier University

  Survival, growth, aboveground biomass production, belowground biomass production, sediment accretion, soil surface elevation dynamics, sediment carbon and C/N ratio, sediment N and P, sediment 13C and 15N, sediment texture and macrofauna community structure, 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 Palakuda, Puttalam Lagoon, Sri Lanka. The first three parameters were also studied at a replicated experiment at Rekawa Lagoon, Sri Lanka. For both Palakuda (1,171days) and Rekawa (702 days) sites, the highest tree density (6.96 seedlings m-2) showed significantly higher % survival: means (±S.E.) of 93.4 (± 1.1) and 91.2 (± 1.38) respectively. The measures of individual trees (tree height, stem diameter, number of leaves, leaf area, number of branches, number of prop roots ) did not differ among treatments (p>0.05) for both sites. In contrast, the aboveground biomass responded significantly to planting density with higher plantation densities equating to greater biomass: 10772 ± 24 and 9904 ± 18.1g dry weight m-2 for Palakuda (1,171days) and Rekawa (702 days) respectively. The belowground biomass at Palakuda (1370 days) varied significantly between the densities with the highest belowground biomass (105.41 ± 6.98 g m-2) in the highest tree density. The same tree density had the highest numbers of fine roots m-2 for efficient absorption of nutrients. Higher tree densities accumulated more N in their sediments while the sediment phosphate was not different between the densities. Sediment ‰ 13C, ‰ 15N and C/N ratios ranged between -16.41 to -14.58, 2.80 to 2.40 and 9.32 to 7.85 respectively and were independent of the planting density. The 13C values indicated a potential mix of seagrass C and mangrove C in sediments of the treatments; the highest tree density (6.96 seedlings m-2) had significantly higher % sediment carbon (0.68 ± 0.04) compared with the unplanted controls (0.46 ± 0.05). Rates of surface accretion 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. The community structure of the sediment macrofauna was unchanged between the treatments; that is likely to be the result of unchanged sediment texture and unavailability of mangrove derived carbon as the major food source. This study demonstrated the role of higher mangrove densities in enhancing the rates of sediment accretion and surface elevation processes that may be crucial in mangrove ecosystems’ 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 during their early 1171 days of growth. While these potential processes need further research, the enhanced survival at high densities suggests the potential to use high plantation densities to help mitigate sea-level rise effects by encouraging positive sediment surface elevation. Higher biomass production in higher densities would also help faster sequestration of atmospheric carbon dioxide.