Carbon sequestration in different urban vegetation types in Southern Finland

Abstract. Many cities seek carbon neutrality and are therefore interested in the sequestration potential of urban vegetation. However, the heterogeneous nature of urban vegetation and environmental conditions limits comprehensive measurement efforts, setting expectations for carbon cycle modelling. In this study, we examined the performance of three models – the Jena Scheme for Biosphere–Atmosphere Coupling in Hamburg (JSBACH), the Lund–Potsdam–Jena General Ecosystem Simulator (LPJ-GUESS), and the Surface Urban Energy and Water Balance Scheme (SUEWS) – in estimating carbon sequestration rates in both irrigated and non-irrigated lawns, park trees (Tilia cordata), and urban forests (Betula pendula) in Helsinki, Finland. The test data included observations of various environmental parameters and component fluxes such as soil moisture and temperature, sap flow, leaf area index, photosynthesis, soil respiration, and net ecosystem exchange. Our analysis revealed that these models effectively simulated seasonal and annual variations, as well as the impacts of weather events on carbon fluxes and related factors. However, the validation of the absolute level of modelled fluxes proved difficult due to differences in the scale of the observations and models, particularly for mature trees, and due to the fact that net ecosystem exchange measurements in urban areas include some anthropogenic emissions. Irrigation emerged as a key factor often improving carbon sequestration, while tree-covered areas demonstrated greater carbon sequestration rates compared to lawns on an annual scale. Notably, all models demonstrated similar mean net ecosystem exchange over the urban vegetation sector studied on an annual scale over the study period. However, compared to JSBACH, LPJ-GUESS exhibited higher carbon sequestration rates in tree-covered areas but lower rates in grassland-type areas. All models indicated notable year-to-year differences in annual sequestration rates, but since the same factors, such as temperature and soil moisture, affect processes both assimilating and releasing carbon, connecting the years of high or low carbon sequestration to single meteorological means failed. Overall, this research emphasizes the importance of integrating diverse vegetation types and the impacts of irrigation into urban carbon modelling efforts to inform sustainable urban planning and climate change mitigation strategies.