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Estimate the benefits from vegetation

Urban vegetation is more than just green scenery; it actively influences the urban climate and the carbon balance of a city. In the context of GHG inventories, trees, parks and green areas absorb and release carbon in complex ways, creating time-varying fluxes that must be understood to further improve the accuracy of inventories and to optimize park management between various goals.

With scientific tools, cities can now quantify how much their green spaces contribute to carbon exchange. In this service, we explore how these scientific methods reveal the hidden role of vegetation and how this knowledge helps cities design climate strategies that reflect the real impact of their natural assets.


The dynamics of urban ecosystems

The simplistic view that all urban green areas are CO2 sinks is often inaccurate. Their carbon contribution is not static; they can function as net sinks or sources depending on management, weather, and environmental factors. ICOS Cities observations across parks and forests highlighted critical factors influencing the carbon balance:

  • Photosynthesis vs. respiration: Vegetation acts as a sink through photosynthesis (absorbing CO2) but is a source through respiration (releasing CO2). Soil respiration, which is highly temperature and moisture-dependent, can be a major source, sometimes offsetting the absorption.
     
  • Water stress: Drought is a critical factor. Most urban parks and forests in Zurich, Paris, and Munich were not watered during summer and experienced severe water stress. That led to a significant reduction in photosynthesis and rapid drying of lawns, undermining their function as carbon sinks.
     
  • Management practices: Actions like intensive lawn mowing or soil disturbance can increase soil respiration and release stored carbon, pushing the balance towards a CO2 source.
     
  • Heterogeneity across green areas: Denser urban forests and older, larger trees generally showed more resilience and greater net CO2 absorption than exposed, young plantings.

Urban greenery is only a reliable carbon sink if it is strategically designed, managed, and supported under climate change (e.g., through strategic species selection, targeted irrigation, and minimized soil disturbance).


Measurement and modeling

Scientists assessed the role of vegetation by combining extensive field observations with an ensemble of carbon cycle models (SUEWS, VPRM, diFUME, JSBACH).

Field measurements: Techniques included traditional eddy covariance towers, alongside innovative and scalable methods like sap flow measurements (tracking water use in individual trees) and soil respiration chambers.

Model ensemble: No single model can accurately capture the heterogeneity of urban ecosystems. Using an ensemble of models, coupled with diverse measurements, provides the most reliable picture and allows for robust uncertainty quantification.

Separating signals: A major scientific challenge is isolating the biogenic CO2 flux (from nature) from the anthropogenic one (from human activity). Advanced atmospheric transport models are vital for this separation.
 

 Key takeaways

City Administrators
Treat green areas as active infrastructure. Implement evidence-based management (e.g., irrigation plans, minimal soil disturbance) to maximize their climate benefits.
Businesses
Understanding the real carbon function of green spaces can improve sustainability assessments and help design effective Nature-Based Solutions (NBS).
Researchers
Combining multiple approaches and datasets is crucial to distinguish natural fluxes from anthropogenic emissions for accurate urban carbon accounting.



Dive Deeper

Ahongshangbam, J., Kulmala, L., Soininen, J., Frühauf, Y., Karvinen, E., Salmon, Y., Lintunen, A., Karvonen, A., and Järvi, L.: Sap flow and leaf gas exchange response to a drought and heatwave in urban green spaces in a Nordic city, Biogeosciences, 20, 4455–4475, https://doi.org/10.5194/bg-20-4455-2023 , 2023.

Minttu Havu, Liisa Kulmala, Hei Shing Lee, Olli Saranko, Jesse Soininen, Joyson Ahongshangbam, Leena Järvi, CO2 uptake of urban vegetation in a warming Nordic city, Urban Forestry & Urban Greening, Volume 94, 2024, 128261, ISSN 1618-8667, https://doi.org/10.1016/j.ufug.2024.128261 .

Jasek-Kamińska, A., Szostak, R., Chmura, Ł., Bartyzel, J. & Zimnoch, M. (2024) Carbon dioxide and evapotranspiration fluxes in an urban area of Krakow, Poland. Quarterly Journal of the Royal Meteorological Society, 150(765), 5498–5517. Available from: https://doi.org/10.1002/qj.4884 .

Stagakis, S., Brunner, D., Li, J., Backman, L., Karvonen, A., Constantin, L., Järvi, L., Havu, M., Chen, J., Emberger, S., and Kulmala, L.: Intercomparison of biogenic CO2 flux models in four urban parks in the city of Zurich, Biogeosciences, 22, 2133–2161, https://doi.org/10.5194/bg-22-2133-2025 , 2025.

Soininen, J., Kohonen, KM., Rantala, P. et al. Carbon uptake of an urban green space inferred from carbonyl sulfide fluxes. npj Clim Atmos Sci 8, 90 (2025). https://doi.org/10.1038/s41612-025-00958-5 

Laura, B., Depuydt, J., Herig-Coimbra, P.-H., Fortineau, A., Feron, A., Stella, P., Buysse, P., Kalalian, C., Nief, G., Ramonet, M., and Loubet, B.: Eddy covariance measurements of CO2 fluxes along an urban-to-rural gradient in the Paris area, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18076, https://doi.org/10.5194/egusphere-egu25-18076 , 2025.

Stagakis, S., Loubet, B., Bignotti, L., Li, J., Chen, J., Mauder, M., Lan, C., Iserbyt, A., Jacobs, E., Gielen, B., 2025. Soil microclimate data from 24 months, 6 sites per city, tower bioclimate data / phenocam observations per city, 12 months of calibrated sap-flow data from 6 sites per city and manual measurements of soil respiration, leaf area index, leaf photosynthesis. ICOS ERIC -- Carbon Portal. https://doi.org/10.18160/F8VY-SDCU 

Stagakis, S., Loubet, B., Bignotti, L., Li, J., Chen, J., Mauder, M., Lan, C., Iserbyt, A., Gielen, B., 2025. Biospheric flux observations in Zurich, Paris and Munich. ICOS ERIC -- Carbon Portal. https://doi.org/10.18160/9965-P9J5 

 

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PAUL, Pilot Applications in Urban Landscapes - Towards integrated city observatories for greenhouse gases (ICOS Cities), has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No 101037319.