Assessing the biogenic CO2 fluxes in urban green areas using an observation-constrained modelling approach

Numerous cities are developing ambitious climate action plans, reducing fossil fuel CO₂ emissions and increasing carbon removals through urban greening. Monitoring their progress towards carbon neutrality requires novel observational and modelling approaches. A major challenge is the accurate attribution between the anthropogenic CO₂ emissions and the biogenic CO₂ fluxes. Widely applied methods in natural ecosystems, such as source partitioning using eddy covariance observations or ecosystem model simulations, are hampered by the extreme heterogeneity of the urban environment. Within ICOS-Cities project, intensive field ecophysiological and meteorological observations were implemented in several green areas of Zurich, Munich, and Paris. These included continuous tree sap-flow, soil temperature/moisture, and local meteorology observations, as well as regular field campaigns of tree leaf area index and chamber gas exchange measurements at soils, lawns and tree leaves. This study presents a new CO₂ assimilation and respiration model, which is calibrated, forced and constrained by the available in-situ observations, simulating hourly tree and lawn CO₂ fluxes, representative of the observation areas. The photosynthetic CO₂ assimilation model is constrained based on sap-flow estimated stomatal conductance for trees, and an empirical estimation of stomatal conductance based on vapour pressure deficit and soil moisture for lawns. Plant and soil respiration is modelled based on calibrated versions of the Arrhenius equation, considering also soil moisture in the case of soil respiration. The model is evaluated in an urban forest in Paris using eddy covariance data. The simulated fluxes are compared between locations and cities to assess the carbon sequestration potential of the urban green areas and identify the effects of the local environment and management practices on the urban biospheric processes. The first results indicate that old parks with reduced impervious land cover show high carbon sequestration and are less vulnerable to summer drought.