Investigation of the carbon dioxide exchange over a living roof in San Francisco

Urban surfaces have been shown to be significant sources of atmospheric CO₂ up to 10 kg m⁻² a⁻¹ compared to natural ecosystems that tend to be sinks of about 0.1 to 1 kg m⁻² a⁻¹. Living roofs have the potential to help offset urban CO₂ emissions through sequestration of atmospheric CO₂ into the living biomass and soil, but their net ecosystem exchange (NEE) is largely unknown to date. This paper presents results from one year of eddy covariance derived CO₂ fluxes over a one-hectare living roof on the California Academy of Sciences Building in San Francisco, CA. The roof CO2 fluxes corresponded closely with ecosystem functioning, producing a sink peaking at ~-0.2 mg C m⁻² s⁻¹ during daylight hours and a weak source of 0.1-0.2 mg C m⁻² s⁻¹ during the night. Monthly average daily NEE showed significant seasonal patterns, becoming a weaker daily source or net sink in spring, with daily values reaching -1.3 g C m⁻²d⁻¹ in May. Like natural ecosystems, the roof was a daily CO₂ sink during the growth period, but a much higher daily source during the pre-growth and dry periods (-1.1, 3.7, and 4.3 g C m⁻²d⁻¹). During all months of the year, a distinct asymmetric diurnal cycle was observed, suggesting that CO₂ sources from the building below may have caused the unnatural pattern. Even though the living roof was a net annual source of CO₂ of 1235 g C m⁻²a⁻¹, it may have had a mitigating effect and helped offset anthropogenic CO₂ emissions to a certain degree.