Influence of green roof substrate composition on the establishment of native herbaceous species in a dry climate

Green roofs provide ecological benefits to the urban environment and the selection of suitable vegetation and growing substrates are keys for achieving green roof functionality and reducing management costs. For plants, roofs are relatively stressful and drought-prone environments and the use of species from different functional groups can confer resilience to the vegetation. The aim of the study was to evaluate paper mill sludge as a growing substrate for drought-tolerant herbaceous vegetation. Paper mill sludge pellets (P), compost (C) and pumice (V) were mixed in order to obtain three substrates with similar organic content: VC, VP, VPC. The experimental trial was set up in 12 1×1 m2 boxes, filled with the 3 different substrates and planted with 30 native herbaceous species, different in functionality (forbs, graminoids, CAM, N-fixers) and in life forms (hemicryptophytes, geophytes, terophytes and chamaephytes). Measured substrate parameters were: pH, total nitrogen (Ntot), organic carbon (Corg), cation exchange capacity (CEC), electrical conductivity (EC), and substrate moisture. The vegetation properties (total canopy cover, plant composition, species richness) were recorded during the growing season. The substrates showed differences in Ntot: in VC it was higher as well as the substrate moisture resulting in a higher development of plant biomass compared to other treatments. In May, the vegetation in VC and VPC dried out, whereas in VP the low amount of biomass was stable. The presence of compost led to the dominance of CAM species, resulting in lower plant richness. The substrates containing paper mill sludge pellets resulted in limited development of biomass, conferring to the vegetation a higher drought-tolerance and reduced competition.