Sedum-dominated green-roofs in a semi-arid region increase CO₂ concentrations during the dry season

Green roofs are expected to absorb and store carbon in plants and soils and thereby reduce the high CO₂ concentration levels in big cities. Sedum species, which are succulent perennials, are commonly used in extensive green roofs due to their shallow root system and ability to withstand long water deficiencies. Here we examined CO₂ fixation and emission rates for Mediterranean Sedum sediforme on green-roof experimental plots. During late winter to early spring, we monitored CO₂ concentrations inside transparent tents placed over 1 m² plots and followed gas exchange at the leaf level using a portable gas-exchange system. We found high rates of CO₂ emission at daytime, which is when CO₂ concentration in the city is the highest. Both plot- and leaf-scale measurements showed that these CO₂ emissions were not fully compensated by the nighttime uptake. We conclude that although carbon sequestration may only be a secondary benefit of green roofs, for improving this ecosystem service, other plant species than Sedum should also be considered for use in green roofs, especially in Mediterranean and other semi-arid climates.