Project highlights

  • Unravelling the effect of CO2 fumigation of a mature forests, looking both below and above ground processes
  • Research at BiFoR FACE- the only CO2 experiment in mature temperate forests
  • Work with a world-leading team on global change, ecology and biogeochemistry.


National pledges to reduce greenhouse gas emissions and limit global warming to 1.5 °C depend on the balance between carbon (C) emissions and sequestration. Globally, terrestrial vegetation acts as a large carbon sink, absorbing around one third of anthropogenic CO2 emissions. Trees are a fundamental component to this equation, absorbing CO2 during photosynthesis and allocating this C into their leaves, wood and roots. Trees also export large amounts of C as root exudates and into mycorrhizae, to maintain their supply of water and nutrients. Whilst much of this carbon returns to the atmosphere on short timescales, part of it remains within biomass and soil for decades or centuries.

Whether forests will continue to contribute to the global carbon sink under elevated CO2 concentrations (eCO2) is still uncertain. Trees grow faster under eCO2, but tree growth cannot increase indefinitely, partially because growth will be limited by other factors, such as nutrient availability. To an extent, trees can compensate that by allocating the extra C belowground, increasing root growth, exudation and mycorrhizal activity to explore the soil and obtain more nutrients and water. Experiments in which plants are exposed to eCO2 have shown greater C capture, notably by increasing root growth. However, most of our knowledge is based on experiments using seedlings or relatively young forests. There is much less evidence for eCO2 responses in mature trees, but an Australian experiment has shown that eCO2 enrichment does not result in more ecosystem-level C sequestration in a mature forest.

Understanding where the trees will allocate extra C under eCO2is crucial to understand the future of the global C sink. As with many underground processes, allocation to fine roots is particularly uncertain. This project makes use of the BIFoR FACE experiment, the only eCO2 experiment in a mature temperate forest in the world, to investigate above and below-ground C allocation, providing evidence for a crucial aspect of the response of forest ecosystems to eCO2.

CENTA Flagship

This is a CENTA Flagship Project

Case funding

This project is suitable for CASE funding


University of Birmingham


  • Climate and Environmental Sustainability
  • Dynamic Earth


Project investigator

  • Rob Mackenzie (UoB )



  • Adriane Esquivel-Muelbert (UoB )
  • Marie Arnaud (IFREMER )
  • Sami Ullah (UoB)
  • Matthew Wilkinson (Forest Research)
  • Richard Norby (University of Tennessee, US)
  • Ed Rowe (UKCEH)
  • Debbie Hemming (UK Met Office)

How to apply


This project will take place at the Free Air CO2 Enhanced (FACE) experiment as part of the Birmingham Institute of Forest Research (BIFoR). BIFoR FACE is one of the only FACE sites in the world. It exposes a 100-years old oak forest to elevated CO2 at around 550ppm, which is what we expect to reach by 2050. Thanks to this unique facility you will be able to investigate the influence of elevated CO2 on trees carbon allocation, including leaf and root productivity.

You will collect and images using cutting-edge minirhizotron techniques. These images will be vectored and converted into root production, which will be used to compare control and experiment plots, investigating the influence of eCO2 on root production. Additionally, you will analyse data on leaf area collected in the previous years of the experiment aiming to understand the effect of eCO2 on carbon allocation.

Training and skills

You will be trained to use state-of-the-art minirhizotron techniques, using a camera system specially designed to investigate underground dynamics with little disturbance to the soil. You will be working with a large amount of data and will receive training on statistical techniques and programming using the R language. You will present your work at high profile conferences including at least one international conference, to sharpen your communication skills and develop your network. You will also be among the very active BIFoR community, allowing you to discuss your work with scientists from all over the world.

Partners and collaboration

This project includes a CASE scholarship with Forest Research, providing financial support for your research and allowing part of the project to be developed at Alice Holt Forest.

The supervisory team in Birmingham include experts in atmospheric science (Dr Mackenzie), forest ecology (Dr Esquivel-Muelbert) and biogeochemistry (Dr Ullah). You will count with a great team of external supervisors specialised on biogeochemistry and modelling (Dr Ed Rowe, UKCEH), forestry and global change (Dr Wilkinson, FR), belowground carbon dynamics and minirhizotron (Dr Arnaud, IFREMER, France) and forest dynamics (Prof. Norby, University of Tennesee, US).

Further details

Applications need to be submitted via the University of Birmingham postgraduate portal,, by midnight 11.01.2021. Please first check whether the primary supervisor is within Geography, Earth and Environmental Sciences, or in Biosciences, and click on the corresponding PhD program on the application page.

This application should include

  • a brief cover letter, CV, and the contact details for at least two referees
  • a CENTA application form
  • the supervisor and title of the project you are applying for under the Research Information section of the application form.

Referee’s will be invited to submit their references once you submit your application, but we strongly encourage applicants to ensure referees are aware of your submission and expecting a reference request from us. Students are also encouraged to visit and explore the additional information available on the CENTA website.

This is a CENTA Flagship Project

These have been selected because the project meets specific characteristics such as CASE support, collaboration with our CENTA high-level end-users, diversity of the supervisory team, career development of the supervisory team, collaboration with one of our Research Centre Partners (BGS, CEH, NCEO, NCAS) or student co-designed project. These characteristics are a CENTA priority. Studentships associated with Flagship projects will be provided exactly the same level of support as all other studentships.

Possible timeline

Year 1

Literature review, design of hypothesis, first field season. Training in statistical techniques, forest ecology and biogeochemistry. Analysis of existing leaf traits trait data. Participation on the BIFoR annual meeting.

Year 2

Analysis of existing data on roots production and second field season. Writing of first thesis chapter. Research stay at the Forest Research (optional). Potential visit to the Amazon FACE experiment. Attending a scientific meeting. Poster presentation on the BIFoR annual meeting.

Year 3

Analysis integrating above and belowground carbon allocation. Participation on international conference. Thesis writing-up.

Further reading

Jiang, M., Medlyn, B. E., Drake, J. E., et al. 2020. The fate of carbon in a mature forest under carbon dioxide enrichment. Nature, 580, 227-231.

Medlyn, B. E., Zaehle, S., De Kauwe, M. G., Walker, A. P., Dietze, M. C., Hanson, P. J., Hickler, T., Jain, A. K., Luo, Y., Parton, W., Prentice, I. C., Thornton, P. E., Wang, S., Wang, Y.-P., Weng, E., Iversen, C. M., Mccarthy, H. R., Warren, J. M., Oren, R. & Norby, R. J. 2015. Using ecosystem experiments to improve vegetation models. Nature Climate Change, 5, 528-534.

Ellsworth, David S., Anderson, Ian C., Crous, Kristine Y., Cooke, J., Drake, John E., Gherlenda, Andrew N., Gimeno, Teresa E., Macdonald, Catriona A., Medlyn, Belinda E., Powell, Jeff R., Tjoelker, Mark G. & Reich, Peter B. 2017. Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil. Nature Climate Change, 7, 279-282.

Norby, R. J., Ledford, J., Reilly, C. D., Miller, N. E. & O’neill, E. G. 2004. Fine-root production dominates response of a deciduous forest to atmospheric CO<sub>2</sub> enrichment. Proceedings of the National Academy of Sciences of the United States of America, 101, 9689-9693.


The outcomes of the PhD are unlikely to be greatly affected by restrictions that may be imposed by the COVID-19 pandemic. Although this project has a fieldwork component, it can be completed under COVID restrictions. Full risk assessments and safety procedures have been implemented for working at the BIFoR site. On-site technicians are also trained to take the measurements if needed. The other components of the work, such as data analysis and literature review, can be done remotely.