Carbon sequestration in different urban vegetation types in Southern Finland

Abstract. Many cities seek carbon neutrality and are therefore interested in the sinks of urban vegetation. However, the heterogeneous nature of urban vegetation and environmental conditions limit comprehensive measurement efforts setting expectations for carbon cycle modelling. In this study, we examined the performance of three models – JSBACH, LPJ-GUESS, and 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, momentary 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, validating absolute flux levels proved challenging due to observational constraints, particularly concerning mature trees and that in urban areas net ecosystem exchange measurements include some anthropogenic emissions. Irrigation emerged as a key factor often improving carbon sequestration while tree-covered areas demonstrated greater carbon sequestration rates compared with lawns on an annual scale. Notably, all models demonstrated similar mean net ecosystem exchange across a studied urban vegetation area 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 types. 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 key meteorological means failed. Overall, this research emphasizes the importance of integrating diverse vegetation types and impacts of irrigation into urban carbon modelling efforts to inform sustainable urban planning and climate change mitigation strategies.