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

Lead Research Organisation: University of Exeter
Department Name: Geography


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.

Planned Impact

TropOz will deliver a step-change in our ability to simulate O3 impacts on tropical forest productivity and carbon sequestration. Project deliverables will make it possible to provide credible assessments of the near-term impacts of changes in air quality (due to land-use and land cover, population and climate change) on tropical forests. These results would interest a diverse group of policy makers, land-managers and stake-holders interested in predictions of fundamental ecosystem services across tropical regions.
IPCC, IPBES: Our results are very relevant to the Intergovernmental Panel on Climate Change (IPCC), including both Working Group I (WGI) 'The Physical Science Basis' and WGII 'Impacts, Adaptation and Vulnerability'. Indeed, in the fifth Assessment Report (AR5) PI Sitch was a contributing author to two chapters (WGI 6 & WGII 4) which referred to ozone impacts on vegetation and the carbon cycle. Our results on the impact of O3 on global carbon sequestration will not only inform future IPCC assessments but also be relevant to the Intergovernmental Platform on Biodiversity & Ecosystem Services (IPBES).
UK Government: Our development of the Joint UK Land Environment Simulator (JULES), the land component of the Met Office Hadley Centre's UK Earth System Model (UKESM) will directly lead to improvements relevant to the ability of the UK to predict future climate. The Department for Environment Food & Rural Affairs (DEFRA) like all other governmental departments are expected to invest to help ODA countries (0.7% aid target), e.g. Brazil. Department for International Development (DFID) will be clear beneficiaries of our findings.
Governments in Tropical Countries: National and regional government concerned with the development of policies to facilitate adaptation to air quality and climatic change will benefit directly from our work. For example, the Malaysian Palm Oil Board (MPOB), a government agency that promotes and develops the palm oil industry in Malaysia will be very interested in results on O3 impacts on Oil Palm productivity. Project deliverables assessing the impacts of O3 on tropical forests as carbon sinks will be made on a timeframe (i.e. 2030) of relevance to country mitigation pledges under the Paris Climate Agreement. About a quarter of the 2030 mitigation potential in Countries' (Intended) Nationally Determined Contributions ((I)NDCs) comes from the land sector with afforestation and avoided deforestation (REDD+) key mechanisms potentially at risk from future O3 pollution.
Non-government organisations. Relevant NGOs include those whose goals relate to the conservation of tropical biodiversity and functioning tropical forests. Their activities include developing strategies to facilitate adaptation to air pollution and climatic change, helping to inform their supporters, the wider public, and lobbying governments. These will include bodies operating at national, state and intergovernmental levels (e.g. The Nature Conservancy, Global Carbon Project).
Private organisations and citizens with relevant economic concerns in relation to air quality, tropical forests, agriculture/land use. University of Exeter has established links with Permian Global (http://permianglobal.com/en) an investment firm dedicated to the protection and recovery of natural forests to mitigate climate change. Also as part of the project we will be assessing the O3 sensitivity of two economically important tropical tree species. Elaesi guineensis (Oil Palm), represents one of the largest growing cover types in the tropics with the global market forecast to exceed 72 million metric tons by 2020. Theobroma cacao (Cocoa) is the basis of a $98.3 billion industry with ~90% of bean production carried out by 6 million small-holders dependent on it as a cash crop (www.fao.org). Our results will therefore benefit both large agricultural enterprises and small-holder farmers across the tropics.


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Brown F (2022) The ozone-climate penalty over South America and Africa by 2100 in Atmospheric Chemistry and Physics

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Cernusak LA (2021) Understanding how ozone impacts plant water-use efficiency. in Tree physiology

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Collins W (2018) Increased importance of methane reduction for a 1.5 degree target in Environmental Research Letters