Carbon exchange of two large extensive green roofs

Urban green roofs (GRs) reduce greenhouse gas emissions from buildings and thus counteract climate change. Green roofs enhance energy efficiency due to thermal insulation and promote carbon uptake by sequestering CO2 from the atmosphere via plant photosynthesis.

Recent studies have shown that the micrometeorological eddy covariance (EC) technique can be applied to large GRs to track carbon uptake over longer time periods. However, even GRs with similar designs may exhibit significant differences in carbon uptake, making the transferability of results challenging.

This study examines carbon fluxes measured on two extensive green roofs located in Germany over an annual cycle using EC. The two GRs, built in 2012 and 2019, respectively, showed similar mean diurnal courses of carbon flux, but respiration fluxes at night were 75 % larger on average on the older green roof. Our analysis suggests that this is due to an increase in substrate organic carbon content observed at the older GR. Higher respiration led to a net release of CO2 (+ 20.2 gC m-2 a-1), while the younger GR showed a net uptake (- 147.9 gC m-2 a-1) comparable in magnitude to previous years from the older site. Seasonality was similar but larger differences between the sites were evident on shorter time scales (i.e. individual months).

We conclude that the carbon exchange of similarly constructed extensive GRs will be comparable under similar meteorological conditions, but general transferability between roofs is challenging, e.g. due to substrate aging and vegetation development over time.