Net ecosystem exchange of extensive green roofs: the role of coupled energy, carbon, and water fluxes quantified by long-term micrometeorological observations

Abstract. Vegetated roofs (i.e. green roofs, GRs) have been emerging as a promising nature-based solution in urban environments to mitigate climate change impacts, such as heat waves, urban flooding, and increased greenhouse gas emissions. Green roofs were shown to provide various ecosystem services, such as carbon sequestration from the urban atmosphere.

The present study leverages a 9-year time series of continuous, integrated flux measurements on a large, extensive GR in Berlin, Germany, using the eddy-covariance technique. We investigate the coupling between carbon, water, and energy fluxes at different temporal scales to determine their role in shaping the net ecosystem exchange (NEE) of the GR ecosystem.

The extensive GR was a moderate carbon sink with an average annual NEE of –92 g C m^–2, ranging from –154 to +8 g C m^–2 in the study period from 2015–2023. In the final two years, 2022 and 2023, the annual NEE shifted toward net respiration, coinciding with increased nocturnal CO₂ efflux and an abrupt rise in substrate organic carbon, which stemmed from external carbon sources. During the study period, the roof retained 51 % of precipitation, with a strong coupling between soil moisture, evapotranspiration, sensible heat flux, and carbon assimilation. Low substrate water content (below ≈ 0.05 m³ m^–3) reduced evaporative cooling and suppressed carbon assimilation during the summer.

The findings demonstrate the importance of integrated flux monitoring and emphasise the multifaceted environmental benefits of extensive GRs while also pointing to their structural constraints.