Understanding below-ground controls on drought-induced mortality of tropical trees
Tropical forests play a wide range of beneficial roles for humans including regional climate moderation, carbon capture, clean water provision and, possibly most importantly, preserving an immense richness of biodiversity. These forests are under threat from human activity as well as a warming and, in some places, drier future climate. Indeed in several regions of the globe, including Amazonia, increasing tree mortality trends are being observed (e.g. Brienen et al. 2015, Bauman et al. 2022) and trends tend to be larger in regions affected by human changes of the land cover, specifically forest to pasture conversion.
Enhanced tree mortality in the tropics, particularly in marginal zones, is often related to drought. Understanding how drought causes tree death is being intensely studied and a lot of progress has been made. Nonetheless the exact sequence of events which leads to death in response to drought and increasing water vapour deficit is not yet fully understood which makes future predictions how the forests will fare difficult. A wide range of processes are involved including for example stomatal responses to environmental conditions like increasing vapour pressure deficit, decrease in productivity and hydrocarbon reserves, loss of conductivity of the vascular system (xylem) leading to organ desiccation, decoupling of fine roots from soil and fine root death (Carminati and Javaux, 2020), which ultimately can lead to death of vital organs and eventually death of trees.
The overall purpose of the PhD is to obtain a better understanding of the sequence of events leading to tropical tree death under drought. It will employ experimental greenhouse drought experiments and potentially also work in the field, specifically a 1 ha forest plot drought experiment which will start early 2024. The experimental and field work will be located in the region of Nova Xavantina, Mato Grosso as well as possibly Riberao de Cascalhera, Mato Grosso where the 1 ha forest drought experiment will be located. These regions have been warming particularly rapidly and precipitation is a limiting factor for tree growth, thus it is a region where forests are particularly at risk. The focus of the project will be on understanding the role of the soil root interface as part of the whole tree hydraulics system, nonetheless it will include accompanying measurements of the full plant hydraulic system and vital state of organs to obtain an improved understanding of the processes at the soil root interface under drought leading to tree death.
The overall objective is to determine and understand the sequence of events leading to tropical tree death under drought conditions. The project will take advantage of existing temperature controlled greenhouses in Nova Xavantina, Mato Grosso and from early 2024 onwards of a 1ha drought experiment in Riberao Cascalhera, Mato Grosso. A range of eco-physiological measurement tools and techniques are available and well developed, while others will need to be acquired, which will be part of the challenge. The project will profit from a long-standing collaboration with University of Mato Grosso in Southern Amazonia.
A likely sequence of objectives will include
- Acquisition of the knowledge / development of the skills needed to characterise and quantify belowground soil-root interface processes related to water uptake by roots and vital state of tree organs. This may involve tools to measure soil water potential gradients around roots and rootlets, tracer experiments with isotope tagged water, determination of vital state of fine and coarse roots and stem cambial cells.
- Designing, organising and performing a set of greenhouse experiments and concomitant measurements of functioning of the tree to determine the sequence of drought responses leading to mortality (or possibly recovery) of a selected set of Amazon forest tree species. It will involve selecting drought scenarios, producing measurement protocols of all measurements to be taken. Depending on choices of the student it will furthermore involve developing similar protocols for application to droughted trees of the 1 ha drought experiment.
- Performing the experiments both in greenhouses and in the field and analysing and publishing the results.
Potential for High Impact Work
How tropical forests will be able to cope with changing climate conditions as a result of climate change and fragmentation caused by human activity, is of considerable interest in the science community from a scientific point and also because of the concerns about the future of tropical forests. This project aims to provide important information at a fundamental level of how tropical forests may be affected by drought and thus is important for realistic predictions. Because of its significance the project has the potential for high impact results and thus publications.
Training and Supervision
The student project will be led by Emanuel Gloor and David Galbraith of the University of Leeds, together with Beatriz Schwantes and Ben Hur Marimon from University of Mato Grosso, Nova Xavantina. The PhD work will be embedded in several ongoing projects in Mato Grosso funded both by NERC (UK Environmental Research funding) and Brazilian funding agencies, and will build on previously funded projects. The student will join a lively group of Masters and PhD students, both at the Ecology and Global Change cluster Leeds and the ecology laboratory of Beatriz Schwantes and Ben Hur Marimon, Nova Xavantina. Both institutions are well recognized in their fields of study.
We welcome motivated students with a keen interest in understanding how tropical forests may respond to increasingly hotter and more variable climate. As the project will involve experimental work ideally candidates, besides keen interest in the subject area, will have excellent skills with experimental work both under controlled conditions and the field. The student should also enjoy working in South America.
Bauman et al. (2022) Tropical tree mortality has increased with rising atmospheric water stress, Nature, https://doi.org/10.1038/s41586-022-04737-7
Arend et al. (2021) Rapid hydraulic collapse as cause of drought-induced mortality in conifers, PNAS 2021 Vol. 118 No. 16 e2025251118
Koerner (2019) No need for pipes when the well is dry—a comment on hydraulic failure in trees, Tree Physiology 39, 695–700, doi:10.1093/treephys/tpz030
Carminati and Javaux (2020) Soil Rather Than Xylem Vulnerability Controls Stomatal Response to Drought, Trends in Plant Science, 25(9), 868-879
Tiwari et al. (2019) Photosynthetic quantum efficiency in south-eastern Amazonian trees may be already affected by climate change, Plant Cell Env. , DOI: 10.1111/pce.13770