Ozone impacts on tropical vegetation; implications for forest productivity (Trop-Oz)

在低层大气中，臭氧是一种重要的人为温室气体，也是一种空气污染物，每年造成数十亿欧元的植物生产力损失。自1850年以来，由于车辆、工业过程和森林燃烧产生的化学物质排放，地表臭氧翻了一番。虽然陆地生态系统(主要是森林)目前通过储存约四分之一的人类排放的二氧化碳(CO2)来减缓全球变暖，但这可能会受到影响森林生长的臭氧浓度上升的破坏。这反过来会导致更多的二氧化碳留在大气中，加剧全球气候变化。热带雨林提供了全球近一半的植物生产力，正是在这些热带地区，我们可能会看到本世纪人类人口的最大扩张。例如，位于亚马逊雨林中心的马瑙斯在过去25年中经历了人口激增，居民数量翻了一番，达到略高于200万人。在人口不断增长的同时，我们看到城市化和高强度农业区的臭氧前体排放也在扩大。不断变化的空气污染对热带森林的全球影响可能是深远的。在2007年的开创性工作中，皮西奇和他在英国气象局和生态与水文学中心的同事们首次发现了臭氧污染对热带森林的巨大潜在风险，以及这如何反过来加速全球变暖。然而，他们的研究给研究界带来了两大挑战：1)这种影响的规模高度不确定；因为他们的全球模型研究是基于对温带和北方物种的植物臭氧敏感性数据的推断。该项目将通过提供第一套全面的臭氧对植物功能和热带树木生长影响的测量来解决这一问题。此外，由于臭氧、二氧化碳和水都是通过植物气孔在大气和叶片之间交换的，较高水平的二氧化碳为植物提供了减少气孔开放的机会，这反过来又会导致减少对臭氧的吸收和损害。该项目将首次调查气候变化(二氧化碳和温度)对热带森林物种臭氧响应的潜在协同或拮抗影响。2)全球植被模型的一个基本挑战是准确地表示高度生物多样性的生态系统的结构和功能；全球模型通常只能表示一组有限的广义植物功能类型(如常绿乔木、C4草本植物等)。然而，最近对植物功能性状数据(如叶片营养浓度、叶片大小和形状)的收集和合成使得在全球模型中能够更好地表示生态和植物功能。包括项目合作伙伴Johan Uddling在内的一组科学家最近提出了一种基于叶片功能特征的温带和北方树种对臭氧敏感性的统一理论。我们处于独特的地位，可以推进这项工作，在热带森林物种中测试这一理论，并在区域和全球范围内测试这一理论的影响。将臭氧敏感性和基本植物功能特征之间的关系纳入我们的全球植被模型Jules(联合英国陆地环境模拟器)，将导致我们评估空气质量对热带森林生产力的影响和碳固存后果的能力发生阶段性变化。该模型将应用于热带森林中臭氧热点位置，结合观测到的植物特征信息和臭氧浓度，我们将能够外推单一植物功能类型(PFT)范例。Jules的全球运行还将使我们能够调查未来臭氧浓度、土地利用变化和气候变化情景对热带森林生产力和全球碳汇的影响。

In the lower atmosphere ozone (O3) is an important anthropogenic greenhouse gas and is an air pollutant responsible for several billion euros in lost plant productivity each year. Surface O3 has doubled since 1850 due to chemical emissions from vehicles, industrial processes, and the burning of forests. While land ecosystems (primarily forests) are currently slowing down global warming by storing about a quarter of human-released carbon dioxide (CO2) emissions, this could be undermined by rising O3 concentrations impacting forest growth. This in turn would result in more CO2 left in the atmosphere adding to global climate change. Tropical rainforests are responsible for nearly half of global plant productivity and it is in these tropical regions that we are likely to see the greatest expansion of human populations this century. For example, Manaus, in the centre of the Amazon rainforest has seen a population boom in the last 25 years, with the number of residents doubling to just over 2 million people. Alongside this growing population, we see the expansion of O3 precursor emissions from urbanization and high-intensity agricultural areas. The global impacts of changing air pollution on tropical forests are potentially profound. In his seminal work in 2007, PI Sitch and colleagues at the Met Office and Centre for Ecology and Hydrology, were the first to identify the large potential risk to tropical forests from O3 pollution, and how that could in turn accelerate global warming. However, their study presented two major challenges for the research community: 1) the scale of this effect is highly uncertain; as their global modelling study was based on extrapolating plant O3 sensitivity data from temperate and boreal species. This project will address this by providing the first comprehensive set of measurements of O3 effects on plant functioning and growth in tropical trees. Also, as both O3, CO2 and H2O are exchanged between the atmosphere and leaves through a plants stoma, higher levels of CO2 provide plants the opportunity to reduce their stomatal opening, which in turn leads to reduced O3 uptake and damage. This project will for the first time investigate the potential synergistic or antagonistic impacts of climate change (CO2 and Temperature) on O3 responses in tropical forest species. 2) a fundamental challenge in all global vegetation modelling is to accurately represent the structure and function of highly biodiverse ecosystems; global models are generally only able to represent a limited set of generalized plant functional types (e.g. evergreen trees, C4-grasses etc). However, recent collection and synthesis of plant functional trait data (e.g. leaf nutrient concentrations, leaf size and shape) have enabled improved representation of ecology and plant function in global models. A group of scientists, including project partner Johan Uddling, have very recently proposed a unifying theory for O3 sensitivity in temperate and boreal tree species based upon leaf-functional traits. We are in a unique position to take this work forward to test the theory in tropical forest species, and to test the implications of this at the regional and global scale. The inclusion of the relationship between O3 sensitivity and basic plant functional traits in our global vegetation model, JULES (Joint UK Land Environmental Simulator), will lead to a step-change in our ability to assess the impact of air quality on tropical forest productivity and consequences for carbon sequestration. The model will be applied at O3 hotspot locations in tropical forests and together with observed plant trait information and O3 concentrations we will be able to extrapolate beyond the single plant functional type (PFT) paradigm. Global runs of JULES will also enable us to investigate the implications of future O3 concentrations, changes in land-use, and climate change scenarios on the tropical forest productivity and the global carbon sink.