Periodic Reporting for period 1 - ECAW-ISO (Past, present and future Exchanges of CArbon and Water between the vegetation and the atmosphere: new insights from analysis and modelling of stable carbon ISOtope data)
Reporting period: 2019-10-01 to 2021-09-30
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.
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.
Firstly, I gathered a large network of stable carbon isotopes (δ13C) data from leaves and tree rings to estimate changes in ci/ca and hence in WUE. I then investigated how an optimal stomatal model could be improved to reproduce ci/ca variations derived from the global isotopic dataset by examining the response of ci/ca to soil water stress in the theoretical model. I showed that despite contrasting hydraulic strategies, the stomatal responses of angiosperms and gymnosperms to soil water tend to converge, in agreement with the theory of optimality and that the impact of soil water on ci/ca results in a reduction of 2% of the average ci/ca values around the world. The paper was published in 2020.
Secondly, I incorporated several formulations to describe the discrimination against 13C (Δ13C) and stomatal functions in JULES. I tested model predictions for the different configurations of JULES against an updated compilation of isotopic data. I also evaluated the implication of the stomatal models used in JULES for the prediction of carbon and water fluxes at the ecosystem scale. This allowed me to identify which model assumptions in JULES were valid and therefore which model configurations I should use to predict carbon and water exchanges between vegetation and the atmosphere in JULES. I found that the least cost hypothesis model showed the lowest biases with the isotopic measurements and led to improved predictions of canopy-level carbon and water fluxes. I then used the new version of JULES to 1) understand global changes in Δ13C and WUE over the past decades, 2) determine the contributions from photorespiratory and mesophyll effects to the global Δ13C trend and, 3) identify regions where gross primary production was very sensitive to temperature changes. I found that Δ13C stays relatively constant globally over 1979-2016 while WUE increased by 21%. Photorespiratory effect increasing global Δ13C while the mesophyll effect decreased global Δ13C. These predictions contrasted with previous findings based on atmospheric carbon isotope measurements. This work has been the subject of an article published in 2021.
Finally, I explored the impact of plausible scenarios of CO2 emissions by 2100 on changes in Δ13C and WUE across the globe. I first reconstructed global interannual variations in Δ13C and WUE over 1901-2010 using 386 tree-ring isotopic measurements and compared the historical reconstructions with simulations from models widely used in the literature. I also predicted historical variations in Δ13C and WUE using linear mixed-effect (LME) models trained on the isotopic observations. I then projected the changes in Δ13C and WUE for the end of the 21st century following the Shared Socioeconomical Pathways (SSPs). I showed that all models tend to overestimate Δ13C and underestimate WUE variations as reconstructed by tree rings, very likely because they do not incorporate long-term plant adaptation and acclimation to environmental changes. The uncertainties in the future projections of Δ13C and WUE were larger for the business-as-usual scenario (SSP5-8.5) and lower for the greener growth paradigm (sustainable development SSP1-2.6). A paper is in preparation and will be submitted soon.
Secondly, I incorporated several formulations to describe the discrimination against 13C (Δ13C) and stomatal functions in JULES. I tested model predictions for the different configurations of JULES against an updated compilation of isotopic data. I also evaluated the implication of the stomatal models used in JULES for the prediction of carbon and water fluxes at the ecosystem scale. This allowed me to identify which model assumptions in JULES were valid and therefore which model configurations I should use to predict carbon and water exchanges between vegetation and the atmosphere in JULES. I found that the least cost hypothesis model showed the lowest biases with the isotopic measurements and led to improved predictions of canopy-level carbon and water fluxes. I then used the new version of JULES to 1) understand global changes in Δ13C and WUE over the past decades, 2) determine the contributions from photorespiratory and mesophyll effects to the global Δ13C trend and, 3) identify regions where gross primary production was very sensitive to temperature changes. I found that Δ13C stays relatively constant globally over 1979-2016 while WUE increased by 21%. Photorespiratory effect increasing global Δ13C while the mesophyll effect decreased global Δ13C. These predictions contrasted with previous findings based on atmospheric carbon isotope measurements. This work has been the subject of an article published in 2021.
Finally, I explored the impact of plausible scenarios of CO2 emissions by 2100 on changes in Δ13C and WUE across the globe. I first reconstructed global interannual variations in Δ13C and WUE over 1901-2010 using 386 tree-ring isotopic measurements and compared the historical reconstructions with simulations from models widely used in the literature. I also predicted historical variations in Δ13C and WUE using linear mixed-effect (LME) models trained on the isotopic observations. I then projected the changes in Δ13C and WUE for the end of the 21st century following the Shared Socioeconomical Pathways (SSPs). I showed that all models tend to overestimate Δ13C and underestimate WUE variations as reconstructed by tree rings, very likely because they do not incorporate long-term plant adaptation and acclimation to environmental changes. The uncertainties in the future projections of Δ13C and WUE were larger for the business-as-usual scenario (SSP5-8.5) and lower for the greener growth paradigm (sustainable development SSP1-2.6). A paper is in preparation and will be submitted soon.
This fellowship delivered major theoretical and methodological advances and provided the critical datasets and model outputs needed to understand and project the fate of the terrestrial coupled carbon and water cycles by 2100 under a range of plausible future scenarios. The results of the project have equal significance for ecophysiologists and modellers across the international community.
1) JULES and UKESM users
The new isotopic modelling capability and implementation of the least cost hypothesis in JULES are now available to all JULES users through the MOSRS server. A better representation of the coupled carbon and water cycles in JULES can impact the atmospheric component of the UK Earth System Model, leading to improved atmospheric simulations for Earth System modellers.
2) Land surface model developers
Only a few land surface models have incorporated stable carbon isotopes into their biospheric component, and those that have, have relied on simple formulations to describe the isotopic fractionations and neglected post-photosynthetic fractionations. Scientists developing models can now benefit from the project by using the new formulations implemented in JULES during the fellowship to incorporate or improve the same processes in their models.
3) The wider research community
My work contributed to a better understanding of the interactions between carbon and water cycles in the land-atmosphere continuum and their impacts on the climate. I also provided recommendations for future studies that will be of direct use to researchers. A new understanding of the interactions between forest trees and the atmosphere can help to reduce uncertainties about how forest ecosystems will respond to climate change and in turn affect climate trends in the future.
The large dataset compiled during the project and the model outputs have been made available in public repositories for use by any research group.
1) JULES and UKESM users
The new isotopic modelling capability and implementation of the least cost hypothesis in JULES are now available to all JULES users through the MOSRS server. A better representation of the coupled carbon and water cycles in JULES can impact the atmospheric component of the UK Earth System Model, leading to improved atmospheric simulations for Earth System modellers.
2) Land surface model developers
Only a few land surface models have incorporated stable carbon isotopes into their biospheric component, and those that have, have relied on simple formulations to describe the isotopic fractionations and neglected post-photosynthetic fractionations. Scientists developing models can now benefit from the project by using the new formulations implemented in JULES during the fellowship to incorporate or improve the same processes in their models.
3) The wider research community
My work contributed to a better understanding of the interactions between carbon and water cycles in the land-atmosphere continuum and their impacts on the climate. I also provided recommendations for future studies that will be of direct use to researchers. A new understanding of the interactions between forest trees and the atmosphere can help to reduce uncertainties about how forest ecosystems will respond to climate change and in turn affect climate trends in the future.
The large dataset compiled during the project and the model outputs have been made available in public repositories for use by any research group.