Understanding how plants have responded to recent environmental changes is essential for a reliable projection of future changes in the terrestrial carbon and water cycles by the end of the 21st century. Plant water use efficiency (WUE) – the ratio of carbon uptake (photosynthesis) to water loss (transpiration) – is a key metric of the exchange of CO2 and water between the vegetation and the atmosphere. Rising atmospheric CO2 concentration tend to increase carbon uptake and reduce stomatal conductance (in the absence of other limitations), resulting in increased WUE. However, the magnitude of recent changes in WUE remains hugely uncertain. Land surface models differ greatly in their representations of terrestrial carbon uptake and water loss, resulting in major unresolved discrepancies in predictions of past and future CO2 uptake, vegetation cover and WUE.
The aim of the project was to develop the use of stable carbon isotopes (∆13C) in the UK land surface model (JULES) to improve the representation of the key processes regulating the coupled carbon and water cycles and their responses to environmental changes in the model and to investigate past and future changes in ∆13C and WUE over the globe.
The research helped answer fundamental questions:
How have ∆13C and WUE changed over the past century? Overall, ∆13C decreased until 1965, then increased slightly at least until 2010. In contrast, WUE increased over the entire period but at a higher rate since 1966.
What were the main drivers of these changes? Prior to 1965, ∆13C declined due to the negative effect of atmospheric CO2 on ∆13C. After 1965, when the rate of increase in CO2 accelerated, ∆13C started to increase with rising CO2. CO2 had a weaker effect on WUE after 1965. Changes in temperature and soil water availability influenced ∆13C and iWUE more strongly after 1965.
What are the long-term consequences of future socio-economic decisions on coupled carbon and water cycles? Model projections suggest that the increase in WUE reported at least since 1965 will accelerate in the future if CO2 emissions continue to increase at the same rate as over the past decade. The higher the increase in CO2 in the future, the greater the uncertainties about the evolution of the coupled carbon and water cycles.
