Measuring the effect of plant-community composition on carbon fixation on green roofs.

Green roofs provide many ecosystem services such as regulation of building temperatures, reducing urban heat-island effects and draining rainwater. In addition, they are expected to reduce the high levels of CO 2 concentrations in big cities. Previous CO 2 fixation studies on green roofs were done by taking long-time-period samples using expensive equipment and with limited replication. To plan green roofs for optimal CO 2 reduction, a simple method to quantify CO 2 fixation rate in relation to plant species-composition is required. The method we tested is direct measurement of CO 2 concentrations with a portable air-quality meter, which allows a large number of samples. Here we focus on differences in the photosynthetic effect between plots containing the local Mediterranean succulent, Sedum sediforme and plots containing various annuals. In a factorial design (presence or absence of Sedum crossed with presence or absence of annuals), we tested the effect of sedum and annual treatments on CO 2 concentrations. To compare our results with a commonly used method, and to evaluate the role of the different species, we examined the photosynthetic activity at the single plant level under these treatments by using a portable gas-exchange measuring system. We found that our method can detect the effect of different green roof plots and can be used as a simple and reliable tool for green-roof planers. We found that annuals reduced CO 2 concentrations, but only in the absence of Sedum . Sedum alone had no effect on CO 2 concentrations. This emphasizes the importance of integrating plots with annual plants in Sedum -based green roofs. [ABSTRACT FROM AUTHOR]