The contribution of trees to tropical wetland methane emissions

甲烷（CH4）是一种重要的温室气体，其吸收太阳能量的能力是二氧化碳的25倍。因此，人们对主要在湿地浸水土壤中产生甲烷的过程以及导致甲烷排放到大气中的过程非常感兴趣。本研究关注的是增加CH4向大气排放量的过程，特别是从土壤向大气转移CH4的新机制。一般认为，浸水土壤中产生的CH4通过三个过程的组合释放：1)通过充满水的孔隙扩散，2)通过气泡的突然释放，3)通过适应浸水土壤的类草植物茎的内部空间释放。我们提出，湿地树木的茎也为CH4从潮湿土壤向大气的转移提供了一个重要的管道，这一可能性迄今几乎完全被忽视。这个项目建立在这个团队收集的公开数据的基础上，这些数据表明成熟的温带湿地桤木确实通过它们的树干排放CH4，这一发现得到了日本最近另一项关于白蜡树的研究的证实。这是一个重要的发现，因为湿地是向大气排放甲烷的最大单一来源，而这些生态系统中有60%是森林。我们现在有额外的未公布的数据，这些数据是在2011年春季收集的（在呼吁截止日期前10周），显示婆罗洲热带泥炭沼泽森林树木排放的甲烷占所有生态系统甲烷排放量的65%，是目前泥炭地表面量化排放量的两倍。目前，在森林湿地工作的研究人员通常只测量土壤表面释放的CH4，因此我们断言，这些生态系统释放的CH4总量被严重低估了。过去的这种疏忽也可以解释为什么估算一个地区甲烷排放量的不同方法很少达成一致。通过卫星或大气测量获得的甲烷排放估计值往往大于基于地面观测的估计值。这在热带森林地区尤为明显。我们发现树木能促进土壤中CH4的释放，这可能解释了这种差异，部分原因是许多森林地区的土壤要么是季节性的，要么是永久性的，这意味着土壤中应该存在丰富的CH4。我们认为，潮湿土壤中产生的CH4通过树木运输和排放的方式有两种：1)通过树木中充满空气的组织作为气体，这些组织形成了一种适应，使氧气能够从大气中转移到生长在缺氧涝渍土壤中的树木的根部；2)溶解在树液中，当树木的水分通过树干和树叶的气孔蒸腾损失时，树液被释放到大气中。在拟议的研究中，我们将测量热带湿地的甲烷排放，主要是在婆罗洲，也在巴拿马，使用技术来帮助区分树木排放路线，并建立它们对生态系统甲烷通量的贡献，使用更大规模的微气象方法测量。我们还将测量两种自然存在的碳“版本”（同位素）的比例：相对罕见的重同位素碳-13和更轻的更常见的碳-12。土壤和树木排放的CH4中这些碳同位素的比例提供了关于CH4如何产生以及如何在树木中移动的有价值的信息。我们的研究将是第一个对热带湿地树木排放的多年研究，这将首次确定这些生态系统对大气甲烷浓度的真正贡献。

Methane (CH4) is an important greenhouse gas that is ~25 times more powerful than CO2 at trapping the Sun's energy. There is therefore considerable interest in the processes involved in CH4 production, principally in waterlogged soils in wetlands, and the processes that lead to its emission to the atmosphere. This study is concerned with processes that enhance the amount of CH4 emitted to the atmosphere, in particular, a novel mechanism for transferring CH4 from soil to the atmosphere. It is generally thought that CH4 produced in waterlogged soils is emitted by a combination of three processes: 1) by diffusion through water-filled pores, 2) by abrupt release of bubbles, and 3) through internal spaces in the stems of grass-like plants which are adapted to live in waterlogged soils. We propose that the stems of wetland trees also provide an important conduit for the transfer of CH4 from wet soils to the atmosphere, a possibility that to date has been almost entirely overlooked. This project builds on published data gathered by this team which showed that mature temperate wetland alder trees indeed emit CH4 via their trunks, a finding that is corroborated by one other recent study of ash trees in Japan. This is an important finding because wetlands are the largest single source of CH4 emissions to the atmosphere and 60% of these ecosystems are forested. We now have additional unpublished data that was collected in the spring of 2011 (10 weeks before the call deadline) which show that tropical peat swamp forest trees in Borneo emit 65% off all ecosystem methane emissions and twice as much as emissions currently quantified from the peatland surface. At present, researchers working in forested wetlands typically measure only CH4 emitted from the soil surface and thus we assert that the total amount of CH4 being released from these ecosystems is being grossly underestimated. This oversight in the past may also explain why different ways of estimating CH4 emissions for a region rarely agree. Estimates of CH4 emission obtained from satellite or atmospheric measurements are often greater than estimates based on observations made at ground level. This is particularly evident in forested tropical areas. Our finding that trees enhance venting of CH4 from soil is a possible explanation to account for the discrepancy, in part, because soils in many of the forested areas are flooded either seasonally and in many cases permanently, which means an abundance of CH4 should be present in soils. We suggest that there are two ways by which CH4 produced in wet soils may be transported and emitted through trees: i) as a gas through air-filled tissue in trees that has formed as an adaptation to enable transfer of oxygen from the atmosphere to the tree's roots which are growing in oxygen-poor waterlogged soil, and ii) dissolved in sap and then liberated to the atmosphere when tree water is lost by transpiration through pores in tree stems and leaves. In the proposed study we will measure CH4 emissions from tropical wetlands, principally in Borneo but also in Panama using techniques to help distinguish the tree emission routes and establish their contribution to ecosystem methane flux as measured using larger scale micro-meteorological methods. We will also measure the ratio of two naturally occurring 'versions' (isotopes) of carbon: the relatively rare heavy isotope carbon-13 and the lighter more common carbon-12. The ratio of these isotopes of carbon in CH4 in the soil and in tree emissions provides valuable information about how CH4 is produced and how it moves through the tree. Ours will be the first multi-year study of tropical wetland tree emissions which should, for the first time, establish the true contribution of these ecosystems to the atmospheric methane concentration.