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Linking the Atmosphere and Terrestrial biosphere carbon and water cycles using oxygen ISotopes

Final Report Summary - LATIS (Linking the Atmosphere and Terrestrial biosphere carbon and water cycles using oxygen ISotopes)

Quantifying terrestrial carbon storage and predicting the sensitivity of ecosystems to climate change relies on our ability to obtain observational constraints on photosynthetic and respiratory activity at large spatial scales (ecosystem, regional, continental and global) and over long periods of time (years to centuries). Currently, large-scale and long-term estimates of photosynthesis and respiration are principally derived from models that use parameterisations of carbon-climate feedbacks based on highly-uncertain climate sensitivities for photosynthesis and respiration. To constrain our estimates of photosynthesis and respiration and better represent these processes in vegetation models studying additional tracers influenced by these fluxes would provide powerful insights.

Carbon dioxide (CO2) molecules are naturally abundant in the environment and occur with slightly different masses of C (12C and 13C) and O (16O and 18O), called stable isotopes. In the case of carbon, photosynthesis typically selects the lighter 12CO2 to make sugars and biomass. As a consequence when ecosystems are actively photosynthesising during the day and over the summer the number of light CO2 molecules in the atmosphere declines considerably. In contrast when sugars or plant organic matter are metabolised and converted to CO2 they release light 12C back to the atmosphere, setting up a seasonally recurring wave in the concentrations of 12CO2 and 13CO2 that can be observed in the atmosphere. Because environmental conditions such as drought affect the rates of photosynthesis and respiration, subtle variations in the number of 12C molecules trapped in woody biomass or respired back to the atmosphere can provide insights on carbon cycling within plants and between ecosystems and the atmosphere.

The oxygen stable isotopes of carbon dioxide also carry additional information on the rates of photosynthesis and respiration. When CO2 moves into a leaf or soil column it rapidly swaps one of the oxygen atoms in CO2 with an oxygen atom in the surrounding water. In general soil water pools contain more light water molecules (more H216O) than leaves (more H218O). This difference occurs because light water molecules tend to evaporate quickly from the small water reservoirs in leaves, especially when the weather is warm and dry. Thus variations in the oxygen isotope composition of CO2 in the atmosphere are influenced by photosynthesis, respiration and evaporation. Because soil and atmospheric humidity affect the rates of evaporation, subtle variations in the number of 16O molecules trapped in woody biomass or exchanged with atmospheric CO2 can also provide insights on carbon and water cycling within plants and between ecosystems and the atmosphere. Thus the overall objective of the Marie Curie (MC) action LATIS was to define more precisely in numerical models the processes regulating the carbon and oxygen isotope signals arising in the atmosphere and plant organic material resulting from variations in weather over time that impact the measured exchange of water and CO2 between forests and the atmosphere.

Highlights of LATIS

The MC action LATIS has helped the young researcher forge a pathway to the forefront of tree ecophysiological research, deploying innovative gas exchange and stable isotope techniques in forested ecosystems and testing and developing original, process-based hydrological and ecophysiological models. The MC fellow worked closely with colleagues at the host institution, INRA Ephyse to develop and implement the necessary tools and theories to describe how seasonal fluctuations in photosynthesis, respiration and evapotranspiration are recorded in atmospheric CO2 [1,2,3] and the intra-annual carbon (13C) and oxygen (18O) stable isotope composition of wood cellulose. These advances open up new possibilities to obtain insights on past carbon and water fluxes in forested ecosystems as well as historical records of growing season length and past climate. In addition these benchmark datasets and state-of-the-art models are filling gaps in our understanding of isotopic exchanges between ecosystems and the atmosphere and improving the representation of CO2 exchange in the next generation of global models. Finally, LATIS helped increase the scientific profile of the fellow within the host institution and secure a permanent research scientist position at INRA.

New laser spectroscopy techniques to test and develop theories of the CO2 and water isotopic exchanges between forests and the atmosphere

Over the last three years, the MC fellow with the host institution has developed state-of-the-art laser technologies to continuously measure the CO2 isotopic exchanges (isofluxes) between ecosystem components (soil, stem, branch) and the atmosphere. This technological breakthrough provided for the first time high-precision continuous measurements of these ecosystem component isofluxes. The results of this work were included in three high-impact publications [1,2,3], and are the feature of a glowing commentary in New Phytologist (Subke and Ineson, 2010). This work demonstrated how variable the 13C and 18O of CO2 exchanged between ecosystem components and the atmosphere can be over different timeframes and enabled us to test current theories and predict these exchanges using a new version of the isotope-enabled soil-vegetation-atmosphere model, MuSICA developed as part of the LATIS project [1,2,3]. These techniques and datasets are improving our ability to model the influence of climate on the isotopic composition of plant carbon and water pools and atmospheric CO2 and should feature in a further four LATIS publications that are currently in preparation. Numerous invitations for oral presentations at the highest impact European and American geoscience conferences indicate the growing interest and international recognition for this research topic.

Integrating leaf and soil gas exchange findings at larger spatial and temporal scales

Stable isotopes can also integrate process dynamics over long temporal scales. The MC fellow continues to work closely with Dr Jérôme Ogée at the host institution incorporating a simple sub-model in MuSICA that successfully simulates the isotopic variations in both 13Ccellulose and 18O cellulose using mechanistic theory for the first time [4]. This work demonstrates that the cellulose composition in Maritime pine trees on the time-scale of days (~ten days) can be simulated using environmental drivers (Ogée et al., in prep).The isotopic signals observed in the cellulose and those simulated by the model highlighted the physiological sensitivity of these trees to droughts (2002-03, 2005-06) and wet years (1997 & 2007). Sensitivity analyses also indicate that this approach can help constrain the physiological response of trees to diffuse light or soil water limitations, currently two of the major challenges for constraining water and CO2 budgets (Ogée et al., in prep). This advance now provides us with an exciting new independent dataset to validate ecosystem models of carbon and water exchange at high resolution, alongside long-term eddy covariance measurements. In addition this model-data fusion can be developed to explore and interpret past climate patterns and plant function using tree cores.