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

红树林生态系统是世界上最具生产力的生态系统之一，其碳储存率与热带雨林一样高。虽然红树林仅占所有森林的0.4%，但它们贡献了约10%的碳，这些碳被永久地从大气中移除并被沿海海洋锁定。红树林也有利于稳定沿海边缘的土地，特别是在热带风暴和海啸等极端事件期间，有证据表明，随着全球变暖，海平面上升，红树林可以调整平均潮汐高度，保持这种保护作用。此外，红树林是发展中国家许多人生计的基本基础，提供例如燃料木材、建筑材料和渔业。然而，其生产力的一个不幸后果是，许多红树林已被过度开发和退化，以至于它们不再能作为森林自我维持，它们过去提供的许多生态系统服务现在已经丧失。在其他地区，红树林被砍伐，用于永久性的淹没渔业、稻田或城市发展，其结果是全球红树林覆盖率现在是20年前的65%。这一趋势仍在继续，生态系统服务付费计划，无论是通过自愿部门，还是通过拟议的全球机制，如降排（减少毁林和退化所致排放量），都力求减少或扭转对自然生态系统的过度开发，办法是补偿当地利益攸关方损失的收入，并消除对过度开发的（短期）财政奖励。生态系统服务付费制度的一个主要方向是，根据对具有减少碳排放法定（或自愿）承诺的行为者的财务价值，对生态系统储存和锁定的碳进行估值。为了实现这一目标，测量生态系统中或由生态系统储存的碳量，并将其与替代土地使用（或同一生态系统的退化版本）中或由替代土地使用（或同一生态系统的退化版本）储存的碳量进行比较。不同之处在于生态系统的碳价值，这被转换成等量的二氧化碳，二氧化碳是造成全球变暖的主要温室气体。虽然甲烷（CH 4）的排放量远低于二氧化碳的排放量，但CH 4是一种重要的温室气体，因为在100年的时间里，每一个分子产生的全球变暖相当于23个二氧化碳分子; CH 4占总辐射强迫的约20%。甲烷主要是在有机物（如树叶）被生活在厌氧（无氧）条件下的微生物分解时产生的，如反刍动物（牛、羊）内脏中的微生物，更重要的是，湿地生态系统的淹水土壤中的微生物。在全球范围内，来自湿地的排放量约占CH 4总排放量的76%，但这一数字的不确定性很大，因为我们并不完全了解CH 4的生产和消耗（通过氧化CH 4产生CO2的细菌）如何在永久，定期或偶尔被淹没的土壤中相互平衡。关于湿地生态系统的退化或永久性转化如何影响CH 4排放量，人们所知更少。例如，红树林是通过向厌氧土壤提供有机废弃物和提供CH 4运输渠道（砂质组织-用于向根部提供氧气）来增加CH 4排放量，还是通过在潮水退去时迅速使表层土壤干燥（或防止其干燥）从而改变CH 4氧化速率来减少（或增加）CH 4排放量？该项目将测量大气和红树林在不同退化和恢复阶段之间的CO2和CH 4传输，并确定生物和环境因素如何驱动这两种形式的碳排放之间的平衡。有了这些信息，本项目将对红树林退化、转化和恢复的温室气体减排价值提供更准确的估计。

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.