Plant cover and floristic composition effect on thermal behaviour of extensive green roofs

In the last few years an increasing attention has been paid to efficient energy construction systems in the building sector. Although in this contest extensive green roofs are reported to be very effective and sustainable systems, the fact that the main agents of this systems are living organisms have generated doubts, especially in locations where the development of plants and vegetation can be greatly affected by climate. This study aims to investigate the thermal performances of a 2000 m2 particular proprietary extensive green roof system, located on the city of Lleida (Spain), classified as Dry Mediterranean Continental climate. First, plant cover and floristic composition analysis were carried out to evaluate the dynamic of the plant layer over the surface. Then, according to the result of the botanic analysis, summer and winter study in terms of spatial and temporal factors were conducted focussing on the substrate layer, evapotranspiration effect and comparing the different behaviour of the system in low (10%) and high (80%) plant cover conditions. In this extensive green roof, the results showed temporal and spatial changes in floristic composition, with a stable cover of Sedum sp between 20 and 40 %, and a peak of colonizing species in spring and early summer. The increase in vegetation cover appears to have few effects on the above nearby roof environment because of the low moisture level in the substrate layer so that the cooling effect provided by the evapotranspiration does not take place. In addition, the increased presence of vegetation canopy may induce a limitation in substrate night cooling whereas serves as good shield for solar radiation during the day. Finally, the study also reveals the importance of the spatial factor in extensive green roofs, which can lead to not negligible variations on the thermal performance, as well as the floristic composition.