Studies On Plant Selection And Ecological Services Of Green Roofs

Green roofs are an important supplement for urban green land system. They have been proved able to lessen and mitigate many urban ecological and environmental problems. Green roofs have two types:extensive and intensive. As extensive green roofs would typically use thin artificial substrate, and are often planned as needing little maintenance, they will be more competitive in establishment cost and common used than intensive green roofs. However, so far very few studies on extensive green roofs have been conducted in China. By conducting pot culture experiments, and control and site experiments using simulated green roof and conventional roof plots, this study has probed the questions of how to mix substrate and how to select plants for extensive green roofs, and investigated two ecological services provides by extensive green roofs:retaining rainwater and providing food and medicine. The studies mainly include:1) Substrate mixing. We summarized the principles and requirements for mixing substrate for extensive green roofs and provided two recipes of substrate using construction solid waste as the main ingredient. Meanwhile, by investigating the germination rate of Lolium perenne Linn.(LP) and Cynodon dactylon (Linn) Pers.(CD), we developed a new type of substrate, which has higher content of fine particle matter and good erosion resistance, and the additive for green roof substrate. The recipe of substrate is clay:distillers’grains:expended clay:cement=2:0.36:6:0.18. Sedum sarmentosum (SS) shown good alkali endurance and can survive in substrate with a cement-clay rate of23%. Our newly invented additive for green roofs can reduce the pH value of the substrate, increase germination rate of the two pot-cultured species.2) Plants selection according to their drought resistance. The method of using climate chambers to simulate high temperature and drought stress greatly improved the efficiency of drought tolerance screening thus shall be employed as a basis for formulating a standardized method for drought resistance evaluation of Sedums and Portulacas. Leaf anatomical characteristics and morphological variations were closely associated with water metabolism, which can service as basic reference indexes for fast evaluation of drought resistance of the three species of Portulaca pilosa (PP), Sedum lineare (SL) and Sedum mexicanum (SM). The differences of chlorophyll and praline content among the three species reflected their drought sensitivity after drought stress and thus provided reference indexes for fast evaluation of drought tolerance. SOD and POD activities were not accordant with their drought resistance and thus were not appropriate indexes for drought resistance evaluation. The results of rewatering experiments showed that the drought tolerance of the three species decreased in following order:PP> SL> SM. The three species can be used in the extensive green roofs in the cold-wintered and hot-summered areas in China.3) Rainwater retention of green roofs. The unit hydrograph (UH), produced by the methods of S-curve linearization treatment and curve fitting by using the data obtained from controlled experiments, has a sooth recession limb, is able to predict accurately runoff peak flows and volumes from green roofs for any rain input. The green roof retained176.4mm more rainwater than the conventional roof, which was23%of annual precipitation in2009. Rainfall volume and the substrate moisture content play import effects on rainwater retention. The green roof had stronger retention effect on lighter rainfalls than heavier ones, can retain rainwater more efficiently if the substrate was dry. Though the extensive green roof provided very limited retention for heavy rainstorms, it can prolong runoff durations, and lower the peak flow of runoff, even when the substrate was wet due to previous rainfalls.4) Edible and medicine plants provided from green roofs. The three popular edible and medicinal species of plants Portulaca oleracea (PO), SL, and SS growing on recycled crushed brick substrate of simulated extensive green roofs presented the net productivities of less than500g/m2. SL had significantly more biomass than the other two species, and PO had the smallest biomass. The aboveground parts of the three species presented more biomass than the underground parts.By investigating Cr, Ni, Cu, Zn, As, Cd, Pb, and Hg concentrations in SL, SS, and PO and in substrate in April and October in2010,2011, and analyzed. We found that, most heavy metal concentrations in plants (aboveground parts) in April were significantly lower than in October within a year, and significantly increased year on year. The difference of heavy metal concentrations between the three species were influenced by samplings. Heavy metal concentrations in aboveground parts were all significantly lower than in roots, and significantly lower than in substrate except for Zn. The three species could not be used as food a year after they were sown, only PO collected in April can be harvested as medicine, mainly due to Pb concentrations exceeding the standard. Cd concentration in substrate increased and Ni decreased during the experiment, while the others kept unchanged. The substrate was polluted according to the standard due to its high Cd concentration mainly coming from recycled brick.This study had great significance in recycling urban construction solid waste and thus lessening the pressure of garbage besieging cities, speeding up utilizing domestic plant resources, improving urban hydro-environmental quality, and enhancing the benefits of green roofs and so on. This study would promote the development of green roof industry in China in the future.