Emerging constraints of photosynthesis (including chlorophyll fluorescence), respiration, and transpiration at ecosystem to global scales
Gross photosynthetic CO2 uptake is the single largest component of the global carbon cycle and a crucial variable for monitoring and understanding global biogeochemical cycles and fundamental ecosystem services. Nowadays routine measurements of the net biosphere-atmosphere CO2 exchange are conducted at the ecosystem scale in a large variety of ecosystem types across the globe. Gross photosynthetic and ecosystem respiratory fluxes are then typically inferred from the net CO2 exchange and used for benchmarking of terrestrial biosphere models or as backbones for upscaling exercises. Uncertainty in the responses of photosynthesis and respiration to the climate and environmental conditions is a major source of uncertainty in predictions of ecosystem-atmosphere feedbacks under climate change. On the other hand transpiration estimates both at ecosystem to global scales are highly uncertain with estimates ranging from 20 to 90 % of total evapotranspiration. The most important bottleneck to narrow down the uncertainty in transpiration estimates is the fact that direct measurements of transpiration are uncertain and techniques like eddy covariance measure only the total evapotranspiration.
During the last decade, technological developments in field spectroscopy, near surface remote sensing, isotope flux measurements and quantum cascade lasers have enabled alternative approaches for constraining ecosystem-scale photosynthesis, respiration and transpiration. On the other hand a variety of approaches have been developed to directly assess the gross fluxes of CO2 and transpiration by using both process based and empirical models, and machine learning techniques.
In this session we aim at reviewing recent progress made with novel approaches of constraining ecosystem gross photosynthesis, respiration and transpiration and at discussing their weaknesses and future steps required to reduce the uncertainty of present-day estimates. To this end we are seeking contributions that use emerging constrains to improve the ability to quantify respiration and photosynthesis processes, transpiration and water use efficiency, at scales from leaf to ecosystem and global. Particularly welcome are studies reporting advancements and new developments in CO2 and evapotranspiration flux partitioning from eddy covariance data, the use of carbonyl sulfide, stable isotopes approaches, with a special attention on sun-induced fluorescence, whose studies at various scales have flourished in the last years, addressing a large panel of fundamental challenges. The assessment of photosynthesis from space-based measurements of fluorescence will also be the focus of the FLEX mission, currently under implementation by the European Space Agency. This session welcomes presentations contributing to a better knowledge on vegetation fluorescence, including its measurement, modelling, interpretation, new instruments and applications from local to global scales. Modelling studies which enhance our fundamental understanding of ecosystem-atmosphere CO2 exchange at global scale or make use of these emerging new constraints in data assimilation schemes are also welcome.
Linda Kooijmans, Albert Porcar-Castell, Chao Zhang, Jon Atherton, Wu Sun, Kukka-Maaria Erkkilä, Enpei Li, Antje Gaertner, Erik van Schaik, Kadmiel Maseyk, Ivan Mammarella, Timo Vesala, Huilin Chen, and Maarten Krol