Quantifying gas-phase losses of carbon from intact and degraded mangrove ecosystems.

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Mangrove ecosystems are some of the most productive in the world, storing carbon at rates as high as tropical rainforests. Although mangroves represent only ~0.4 % of all forest, they contribute ~10% of the carbon that is permanently removed from the atmosphere & locked up by the coastal ocean. Mangroves are also beneficial to stabilising land at coastal margins, particularly during extreme events such as tropical storms & tsunami, and there is evidence that as sea level rises with global warming, mangroves can adjust to mean tidal heights, maintaining this protective role. Furthermore, mangroves are the fundamental basis of the livelihoods of many people in developing countries, providing, for example, fuel wood, building materials & fisheries. An unfortunate consequence of their productivity, however, is that many mangroves have been overexploited & degraded such that they can no longer sustain themselves as forests, and many of the ecosystem services they have provided in the past have now been lost. In other areas, mangroves have been cleared for conversion to permanently flooded fisheries, rice paddies, or for urban development, with the consequence that global mangrove coverage is now 65% of what it was 20 years ago. This trend is continuing.Payment for ecosystem services (PES) schemes, either through the voluntary sector or through proposed global mechanisms such as REDD (reduced emissions from deforestation and degradation), seek to reduce or reverse the over-exploitation of natural ecosystems by compensating local stakeholders for income forgone and removing (short-term) financial incentives for overexploitation. One of the main directions that PES schemes are taking is to value the carbon stored & locked up by ecosystems in terms of its financial value to actors with a statutory (or voluntary) commitment to reducing carbon emissions. To achieve this, the amount of carbon stored in, or by, an ecosystem is measured, and compared to the carbon stored in, or by, an alternative land use (or degraded version of the same ecosystem). The difference is the ecosystem's carbon value, and this is converted into an equivalent amount of CO2, the greenhouse gas (GHG) most responsible for global warming.Although methane (CH4) emissions are much lower than CO2 emissions, CH4 is an important GHG because, over 100 years, each molecule produces as much global warming as 23 CO2 molecules; CH4 contributes ~ 20% of the total radiative forcing. CH4 is mainly produced when organic matter (e.g. leaves) is decomposed by micro-organisms living in anaerobic (oxygen-free) conditions such as those found in the guts of ruminants (cows, sheep) and, crucially, in the flooded soils of wetland ecosystems. Globally, emissions from wetlands contributes around 76% of total CH4 emissions, but uncertainties around this figure are large because we do not have a full understanding of how CH4 production and consumption (by bacteria that oxidise CH4 to produce CO2) balance one another in soils that are permanently, regularly or occasionally flooded. Even less is known about how the degradation, or permanent conversion, of wetland ecosystems affects CH4 emissions. For example, do mangrove trees increase CH4 emissions by supplying organic litter to anaerobic soils & providing channels for CH4 transport (arenchyma - used to supply roots with oxygen), or do they reduce (or increase) CH4 emissions by drying out the surface soil (or preventing it drying out) quickly as the tide recedes, thereby changing the rate of CH4 oxidation?This project will measure the transport of CO2 and CH4 between the atmosphere and mangrove forests at different stages of degradation and restoration, and determine how the balance between these two forms of carbon emission is driven by biological & environmental factors. With this information, this project will provide a more accurate estimate of the GHG mitigation value of mangrove degradation, conversion & restoration.