Performance of Two Simulated Green Roofs in the Mediterranean Area

The green roofs sector is a relatively recent phenomenon in Mediterranean countries, meaning that as yet there is no complete understanding of which plants are suited to this particular environment. Such plants would need to be adaptable to the green roof itself as well as to the drought and intense lighting typical of a Mediterranean summer. Two simulated green roofs were planted with a variety of species and life forms and subjected to two minimal irrigation treatments and one treatment without irrigation. Mainly subshrub species were planted in one simulation, while the other contained a prevalence of groundcover species. The study looked at performance in terms of species, life forms, and simulation. We analysed flowering, mortality, frequency of planted species and of invasive species, and biomass. We took periodic measurements of cover and of the Shannon-Wiener vegetal diversity index. The groundcover species obtained a higher degree of cover, but suffered more from seasonal stresses. Both green roofs saw an increase in the abundance of a few species, leading to a simplification of the original design. Diversity and the ability to host colonising species were influenced mainly by differences in vegetational structure. Of the subshrub species, Centranthus ruber and Helichrysum stoechas performed best, while Frankenia laevis and Thymus serpyllum came top among groundcover species. In order to withstand severe drought stress, a minimal amount of irrigation was found to be necessary. Our study demonstrates that it is possible to obtain good results from a Mediterranean green roof by using an appropriate combination of vegetal species with different structures and development.