Death by double stress: how fire and drought interact to influence tree death in savannas

• Applicants: Professor Dr. Steven Higgins; Professor Dr. Tobias Ullmann
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems ; Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 458911709

There is a consensus among climate scientists that the frequency and intensity of droughts will increase across the global distribution of savannas as a result of global warming. This increase in drought frequency is expected to result in an increase in tree mortality, and such consequences are already being observed in many parts of the world. Despite this, the mechanisms of tree mortality and especially how savanna trees will respond to drought remains poorly understood. In our existing DFG project (Death by Dehydration) we have failed to induce mortality in a rain exclusion experiment, most likely because savanna trees are highly adapted to drought, especially those of shorter duration. Because savannas are disturbance driven systems, we argue that drought impacts cannot be interpreted in isolation but need to be interpreted in conjunction with fire impacts. Recent pioneering research has suggested that mortality after fire is the consequence of hydraulic failure (analogous to a loss of water during drought), rather than cambial damage (which would compromise growth). As a result, we hypothesise that sequences of drought and fire events collectively impair the hydraulic and carbon status of plants which in turn affect mortality. To test this, we first set out to better understand the ecophysiological strategies savanna trees use to avoid mortality when exposed to drought and/or fire by measuring important functional traits linked to tree hydraulics. We also propose two new experiments: 1) A controlled experiment with pot grown plants to determine the interactive effect of fire and drought on whole plant mortality, and 2) A field experiment that will be used to estimate fire induced mortality in naturally growing savanna trees. Finally, we propose to adapt and extend the existing drought experiment to include a drought-fire interaction. We include state-of-the-art remote sensing technologies in this renewal proposal as this will allow us to generate time series that quantify in multiple-dimensions plant responses to both drought and fire. The resulting remote sensing models will allow us upscale to landscape and regional products. This multi-faceted approach, combined with an excellent team of international collaborators will allow us to quantify how fire and drought interact to influence savanna tree mortality. Such information will be invaluable for planning global change adaptation strategies for savanna ecosystems.

• DFG Programme: Research Grants