Extensive Urban Green Roof Shows Consistent Annual Net Uptake of Carbon as Documented by 5 Years of Eddy-Covariance Flux Measurements

Vegetated roofs or green roofs may supply a number of different urban ecosystem services such as thermal regulation of local air temperature by evaporative cooling or climate change mitigation by sequestration of CO2. However, knowledge about the annual and seasonal variations of green roof carbon uptake due to changes in meteorological conditions and water availability remains scarce. We studied the annual variation of net ecosystem exchange (NEE) by eddy-covariance flux measurements during a 5-year period from 2014 to 2019 on a large and unirrigated green roof at the Berlin-Brandenburg airport (BER) in Berlin, Germany. The roof vegetation was dominated by Sedum species and herbs. The BER green roof showed net uptake of carbon in each of the 5 study years with an average uptake of -141.1 g C m(-2) y(-1) and an average measurement uncertainty of +/- 15.5 g C m(-2) y(-1). The NEE, however, was characterized by significant interannual variation in which the maximum annual uptake of -189 g C m(-2) y(-1) in a year with above average precipitation roughly doubles the minimum uptake of -95 g C m(-2) y(-1) in a dry year. The variation of NEE was a function of the annual amount of precipitation and the frequency of rainfall, that is, the number of dry days per year (DDY). The roof might lose its carbon uptake function in a warmer future climate with an increasing number of DDY. Sustainable irrigation of the roof (e.g., rainwater harvesting) could help to maintain the carbon uptake function of the green roof vegetation.

Automated summary

Research revealed that green roofs exhibit significant interannual variation in carbon uptake, highly influenced by precipitation and dry days. A warmer climate in the future may cause green roofs to lose their carbon uptake function, while sustainable irrigation methods could potentially help maintain this function.