The multi-year impacts of the 2015/2016 El Nino on the carbon cycle of tropical forests

地球系统科学的一个关键挑战是了解生物圈在改善或加速全球大气变化方面所起的作用。全球碳循环尤其令人感兴趣：陆地生物圈似乎正常地充当大气二氧化碳的汇，但在极端条件下，特别是在厄尔尼诺事件期间，似乎从汇转变为大气中二氧化碳的净来源。热带陆地生物圈尤其主导着碳循环的年际变化，而且这种变化似乎在近几十年来有所增加。这表明热带陆地生物圈可能正在接近一些关键的转变，这可能导致它成为气候变化的加速器。虽然热带碳循环对气候扰动的敏感性经常被模拟，最近根据大气质量预算和遥感进行估计，但几乎没有直接的实地验证厄尔尼诺对热带碳循环的影响。地球目前(2015/2016)正在经历几十年来最强烈的厄尔尼诺现象，热带地区气温异常高，热带美洲、非洲和亚洲许多地区干旱。与之前强烈的厄尔尼诺现象不同，我们现在已经在短期资金的支持下，在整个热带地区建立了一个详细的碳循环过程研究的全球网络。这样的机会可能在几十年内不会再次出现，我们有充分的条件抓住它。我们的地块在季节和每月的时间尺度上监测森林的碳循环，并横跨热带地区(在伯利兹、玻利维亚、秘鲁、巴西、加纳、加博姆、埃塞俄比亚和马来西亚)。它们也跨越了人类改造的梯度，从古老的生长到不同强度的伐木、火灾发生、农林业和包括飓风在内的其他自然干扰。这提供了一个独特的机会，可以直接从经验上了解厄尔尼诺现象发生前后碳循环过程的变化，包括生产力、分配、自养和异养呼吸。我们建议通过这次厄尔尼诺现象监测热带地区的80个样地，直到2017年底，以月度或季节分辨率进行监测。我们处于独特的地位，可以通过这次事件以及之后的几年密切跟踪森林的碳循环，可能提供一个明确的答案，即热带森林的碳循环响应对生物圈碳汇的年际变化有多大贡献，以及哪些组成部分的过程决定了这种年际变化。这些信息将被用来验证和改进陆地生物圈模型，包括位于英国气象局地球系统模型中的Jules陆地表面模型。在这项提议中，我们将利用这个网络来回答几个关键问题：1.当地对干旱的即时和多年碳循环反应是什么，以及整个热带地区这些反应的一致性如何？2.气温上升约1摄氏度如何影响干旱和干扰的梯度上的碳循环过程？3.生产力和呼吸的哪些组成部分主导着年际变化，这能告诉我们什么是关于短期变化和长期弹性之间的对应关系？4.是否有证据表明，通过非结构性碳水化合物储备的下降，森林生产力(NPP)在整个事件期间得以维持？5.人为修改(伐木、树木生长、过去的火灾)和自然干扰(飓风、大象)如何影响碳循环对厄尔尼诺的敏感性？6.火灾对在这次厄尔尼诺事件中燃烧的巴西林地的碳循环有什么影响？

A key challenge in Earth System science is to understand the role the biosphere plays in ameliorating or accelerating global atmospheric change. The global carbon cycle is of particular interest: the terrestrial biosphere appears to act normally as a sink for atmospheric carbon dioxide, but under extreme conditions, in particular during El Niño events, appears to switch from being a sink to a net source of carbon dioxide to the atmosphere. The tropical land biosphere in particular dominates the interannual variability of the carbon cycle, and this variability appears to be increasing over recent decades. This suggests that the tropical land biosphere may be approaching some critical transition, that may result in it becoming an accelerator of climate change.While the sensitivity of the tropical carbon cycle to climate perturbations has frequently been modelled and more recently estimated from atmospheric mass budgets and remote sensing, there has been almost no direct field validation of the impact of the El Nino on the tropical carbon cycle. The Earth is currently (2015/2016) experiencing its strongest El Niño event for many decades, with unusually high temperatures across the tropics and drought in many parts of the tropical Americas, Africa and Asia. Unlike the case during previous strong El Niños, we now have established a global network of detailed carbon cycle process studies across the tropics, supported by a patchwork of short-term funding. Such an opportunity may not come again for decades, and we are well-placed to seize it. Our plots monitor forest carbon cycling at seasonal and monthly timescales, and span across the tropics (in Belize, Bolivia, Peru, Brazil, Ghana, Gabom, Ethiopia, and Malaysia). They also span gradients of human modification ranging from old growth through different intensities of logging, fire occurrence, agroforestry and other natural disturbance including hurricanes. This provides a unique opportunity to attain a direct empirical understanding of how carbon cycle processes change over and after an El Nino, including productivity, allocation, autotrophic and heterotrophic respiration. We propose to monitor 80 plots across the tropics through this El Nino and beyond until end 2017, at monthly or seasonal resolution.We are uniquely placed to closely track the carbon cycle of forests through this event and for several years after, potentially providing a definitive answer to how much tropical forest carbon cycle responses contribute to the interannual variation of the biosphere carbon sink, and which component processes determine this interannual variation. This information will be use to validate and improve land biosphere models, including the JULES land surface model that sits within the UK Met Office Earth System Model.In this proposal we see to utilise this network to answer several key questions:1. What are the local immediate and multi-year carbon cycle responses to drought, and how consistent are these across the tropics?2. How does a ~1 degree C rise in temperature affect carbon cycle processes across gradients of drought and disturbance? 3. Which components of productivity and respiration dominate interannual variability, and what can this tell us about the correspondence between short-term variability and long-term resilience?4. Is there evidence that forest productivity (NPP) is maintained through the event via a decline in non-structural carbohydrate reserves?5. How do human-modification (logging, tree crops, past fires) and natural disturbance (hurricanes, elephants) affect carbon cycle sensitivity to the El Niño? 6. What are the impacts of fire on the carbon cycle of our forest plots in Brazil, that have burned in this El Niño?