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.

Mangroves are intertidal ecosystems that are particularly vulnerable to climate change. At the low tidal limits of their range, they face swamping by rising sea levels; at the high tidal limits, they face increasing stress from desiccation and high salinity. Facilitation theory may help guide mangrove management and restoration in the face of these threats by suggesting how and when positive intra-and interspecific effects may occur: such effects are predicted in stressed environments such as the intertidal, but have yet to be shown among mangroves. Here, we report the results of a series of experiments at low and high tidal sites examining the effects of mangrove density and species mix on seedling survival and recruitment, and on the ability of mangroves to trap sediment and cause surface elevation change. Increasing density significantly increased the survival of seedlings of two different species at both high and low tidal sites, and enhanced sediment accretion and elevation at the low tidal site. Including Avicennia marina in species mixes enhanced total biomass at a degraded high tidal site. Increasing biomass led to changed microenvironments that allowed the recruitment and survival of different mangrove species, particularly Ceriops tagal.

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At present, aquaculture of black tiger shrimp (Penaeus monodon) in the Puttalam district of Sri Lanka is unsustainable with more than 90% of the former shrimp ponds laying abandoned as a result of shrimp disease and improper management. Between 1992-1998, over 50% of the mangrove cover was removed in order to establish shrimp aquaculture. This severely affected the functioning of coastal ecosystems, reducing their provision of useful services and increasing coastal vulnerability to climate change. Changes in the hydrology and polluted residual matter make the soil unsuitable for other agricultural purposes without costly restoration. The root of the problem was inappropriate management and a focus on short term economic gains. Despite this previous failure, the national development plan, Mahinda Chinthana, now promotes aquaculture as an avenue for development. This will only be possible if aquaculture is regulated in a sustainable and well-managed manner, and does not increase local vulnerability to climate change effects. The current paper discusses the possible costs and benefits in applying the concept of Climate Compatible Development (CCD) to shrimp aquaculture in Puttalam, Sri Lanka. It suggests how the sector can support mitigation of Green House Gas (GHG) emissions and adaptation to climate change effects, whilst stimulating development that will also benefit rural societies. Mechanisms that could enable this change include subsidies, insurance and bank loans which will also facilitate investment by foreign private enterprises and subsequent export. The development of a specific aquaculture policy and master plan would facilitate the process further.

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1Understanding the effects of seedling density on sediment accretion, biogeochemistry and

is at the forefront of global mangrove conservation. It is the first country to officially protect all its remaining mangrove forests and has embarked on an ambitious plan to restore 10,000 ha of wetland during the United Nations Decade of Ecosystem Restoration. One incentive for this conservation effort is a recognition, based on research mostly done elsewhere, of the importance of mangroves for carbon sequestration and storage. However, a lack of data on Sri Lankan mangrove carbon pools, especially on soil organic carbon, has been recognized as a major impediment to national climate change mitigation strategies. The current work examined both above and below-ground carbon stocks of five important mangrove forests in Sri Lanka (Rekawa, Puttalam-Kalpitiya, Pambala-Chilaw, Batticaloa and Negombo) which are situated in the three major climate zones (dry, intermediate and wet) and therefore sample the main climatic drivers of spatial variability. Above-ground carbon, below-ground root carbon and soil carbon stocks of mangroves in Sri Lanka ranged from 75.5 to 189.1 Mg C ha − 1 , 7.9 to 14.3 Mg C ha − 1 and 643.6 to 1253.6 Mg C ha − 1 , respectively. The highest total mangrove carbon stock was recorded from the Rekawa lagoon which is in the intermediate climate zone (1455.4 Mg C ha − 1 ) while the lowest was found in the Batticaloa lagoon in the dry zone (734.7 Mg C ha − 1 ). Soil carbon stocks were substantially higher in the places where vegetation biomass and stand densities are high. Soil comprised 83-90% of the total mangrove carbon stocks at all sites, highlighting the large potential for release into the atmosphere as carbon dioxide if these habitats are disturbed. Overall, our study contributes important data that broadens our current understanding of how mangrove organic carbon pools vary spatially and with climatic zone.