A circumpolar water use efficiency network to decipher CO2-driven changes of the hydrological cycle

• Applicant: Professor Dr. Jan Esper
• Subject Area: Physical Geography
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 560404722

The unprecedented increase of atmospheric CO2 concentrations affects the ratio of photosynthesis (An) to stomatal conductance (gs) of land plants causing multiple, inter-linked effects in the climate system. Estimating changes of this ratio, referred to as intrinsic water use efficiency (iWUE), is critically important to the carbon and water cycles in a warming world and has been done in several pioneering studies using stable isotope and plant growth data. However, none of the employed datasets was originally developed to quantify iWUE, and the derived trends differ substantially in magnitude and even sign. Here I propose to create such a dataset that is specifically designed to estimate iWUE across the Northern Hemisphere temperate and boreal forests and differentiate the relative contributions of An versus gs to narrow uncertainties under projected global warming. Such a project became feasible only because (i) a unique herbarium of 100 circumpolar tree sites is currently developed providing the necessary wood material, and (ii) a laboratory has been established at a counterpart institution capable of measuring tree-ring stable isotopes at unprecedented rates and quality. This setup will be used to produce long 13C and 18O timeseries and compare these with tree-ring width and density measurements to disentangle the influences of climate and rising atmospheric CO2 concentrations on the changing ratio between An and gs and resultant transpiration across the world’s largest land biome. Empirical estimates of terrestrial iWUE and growth trends will be applied to process-based vegetation models and used to optimize parametrizations of earth system models to improve understanding of land biosphere-atmosphere couplings and projected hydroclimatic changes. Conducting this research appears extremely timely as CO2 emissions rebounded to highest levels and laboratory and field techniques evolved to a stage to realistically achieve such a goal.

• DFG Programme: Reinhart Koselleck Projects