The gross photosynthetic CO2 uptake is the largest component of the global carbon cycle. It is a crucial variable for monitoring and understanding global biogeochemical cycles along with 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 worldwide. Gross photosynthetic and ecosystem respiratory fluxes are typically inferred from the net CO2 exchange and used for benchmarking of terrestrial biosphere models, or as backbones for upscaling exercises. One major source of uncertainty in predictions of ecosystem-atmosphere feedbacks under climate change is the response of photosynthesis and respiration to changes in environmental conditions. Furthermore, transpiration estimates are highly uncertain at ecosystem to global scales, with estimates ranging from 20 % to 90 % of total evapotranspiration. The most important bottleneck in narrowing down 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 have enabled alternative approaches for constraining ecosystem-scale photosynthesis, respiration and transpiration. These include remote and proximal sensing of sun-induced fluorescence, as well as isotope flux measurements and quantum cascade laser-based trace gas measurements, including carbonyl sulfide as a proxy for GPP. Additionally, a variety of other approaches have been developed to directly assess the gross fluxes of CO2 and transpiration by using both process based and empirical models, as well as machine learning techniques.
In this session, we aim at reviewing recent progress made with novel approaches of constraining ecosystem gross primary productivity, respiration and transpiration. We will discuss their weaknesses and future steps required to reduce the uncertainty of present-day estimates. To this end, we invite contributions that use emerging constraints to improve the ability to quantify respiration and photosynthesis processes, transpiration and water use efficiency, from leaf to ecosystem and global scales.