Critical Hydraulic Thresholds for Tree Mortality in Southern Amazonia

Climate Change in the Southern Amazon

The Southern Amazon faces the greatest climatic threat of all Amazon regions. This region is drier and warmer than ‘core’ areas of the Amazon and has been subject to the most pronounced drying and warming trends. Furthermore, it is also the region of the Amazon with greatest tree mortality increases and where atmospheric measurements suggest forests no longer actas carbon sinks but as net sources of carbon to the atmosphere (Gatti et al. 2021). Given that Southern Amazon is at the front line of the Amazon’s battle against climate change, we urgently need to understand how resistant its forest species are to climate stress.

Drone flight over VCR-2 study plot in Nova Xavantina, Mato Grosso, Brazil

Integration with Ecosystem-scale Drought Experiment

This PhD builds directly upon a new 1-ha rainfall exclusion experiment to better understand the physiological survival limits of southern Amazon trees. It is now clear that the process of hydraulic failure plays an important role in drought-induced tree mortality (Rowland et al. 2015). Water is transported from the soils to the canopy under tension. As drought ensues and the soil dries, the tension in the xylem vessels that transport water intensifies and this can lead to the formation of air bubbles (embolism) in xylem vessels, disrupting water transport to the canopy and ultimately resulting in tree death. Although we understand this process in general terms, one critical current knowledge gap is that we don’t know the thresholds of embolism formation that result in the death of tropical trees. This lack of understanding constitutes a key uncertainty for accurately modelling tree mortality under climate change (Hammond et al. 2019).

Measurements at the new drought experiment will enable key indicators of hydraulic function (e.g. leaf water potentials and sap flux) to be tracked from the beginning of our imposed drought all the way to the death of the tree to quantify how loss of xylem conductance translates into mortality risk.   This PhD focuses on making use of these new field measurements to develop a new drought-induced mortality formulation that can be used to predict the impacts of future drought on the functional composition and carbon storage of southern Amazon forests.  The student will use a unique trait-based model specifically developed to simulate Amazon forests and their responses to environmental change (Fyllas et al. 2014), which includes a state-of-the-art description of plant water transport.

Up to now, drought experiments have only been set up northeastern Amazonia, where annual rainfall is almost twice that of our study site and where changes in climate have been much less pronounced than in southern Amazonia. Given their ecotonal nature and the rapid climate change experienced in southern Amazonia (Tiwari et al. 2020, Reis et al. 2022), we expect trees in this region to be much closer to their climatic limits.  Thus, we expect them to experience more rapid and accentuated mortality under imposed drought than observed in experiments in northeastern Amazonia.

Research Opportunities

This PhD is designed to address major global change questions of relevance to tropical forests including: 1) How close are forests in southern Amazonia to hydraulic thresholds that result in tree mortality?, 2) How do plant functional traits influence drought mortality risk in southern Amazonia?  While we envisage that this PhD will have a strong vegetation modelling focus, there will also be opportunities for fieldwork in southern Amazonia with project partners Prof. Beatriz Schwantes Marimon and Prof.  Ben Hur Marimon Junior.  In Leeds, the PhD student would be jointly supervised by Prof. David Galbraith and Prof. Emanuel Gloor and would be based in the Ecology and Global Change (EGC) cluster.  EGC is a dynamic, world-leading research cluster with a strong focus on understanding the ecological and biogeochemical functioning of tropical forests.  We anticipate that this studentship will lead to high-impact manuscripts in leading academic journals.

References

Fyllas N, et al. 2014. Analysing forest productivity using a new individual and trait-based model. Geoscientific Model Development 7:1251-1269.

Gatti L, et al. 2021. Amazonia as a carbon source linked to deforestation and climate change. Nature 595:388-93.

Hammond W, et al. 2019. Dead or dying?  Quantifying the point of no return from hydraulic failure in drought-induced tree mortality.  New Phytologist 223:1834-43.

Reis S, et al. 2022.  Climate and crown damage drive tree mortality in southern Amazonian edge forests. Journal of Ecology 110:876-88.

Rowland L, et al. 2015. Death from drought in tropical forests is triggered by hydraulics and not carbon starvation. Nature 528:119-122.

Tiwari R, et al. 2021. Photosynthetic quantum efficiency in southeastern Amazonian trees may already be affected by climate change. Plant, Cell and Environment 44:2428-39.