Growth and survival capacity of native plant species in vegetated roofs

Plants have great influence on the ecosystem services and benefits of green roofs, among which improved rainwater management, reduced urban heat island (UHI) effects, increased urban wildlife habitat and biodiversity. A 24-months study was carried out in the Agripolis Campus (northeastern Italy) to test different solutions of extensive green roofs. Specifically, one of the purposes was to analyze growth and survival capacity of plants by evaluating the percentage of vegetation cover. Thirty-six GRs microcosms were set up by the factorial combination of two storage/drainage layers (a preformed in recycled high-density polyethylene, empty PL, or filled with expanded clay, PC), three substrates (volcanic lapillus, VU, mix with expanded clay, EC, recycled materials, RE) and two plant mixtures (mix of Sedum species only, S, and mixture of Sedum and perennial herbaceous species, M), obtaining 12 systems replicated three times. The species selected were Sedum acre, S. album, S. reflexum, S. sexangulare, S. spurium, Bromus erectus, Potentilla pusilla, Euphorbia cyparissias, Dianthus sylvestris, Thymus serpyllum and Anthemis tinctoria. A photo survey was carried out to evaluate the trend of growth and survival capacity in each of the 12 systems. Images analysis was performed to obtain a percentage value of plant coverage for each microcosm. Although supplemental irrigation was scheduled to guarantee plant survival, the death of some plants was observed among perennial species (e.g., A. tinctoria, E. cyparissias), particularly in RE substrate. Among main factors, plant mixture and substrate affected plant cover. Mixture S had values on average of 12.8 and 6.8% higher than those of mixture M, in the first and second year, respectively. When EC substrate was used, vegetation cover was about doubled of RE, and with VU it further increased by about a half. These two main factors interacted in influencing the vegetation cover, with mixture S that showed higher values than mixture M only in VU and EC substrates.