Drought tolerance in different vegetation types for extensive green roofs: Effects of watering and diversity

The harsh and stressful growing environment of extensive green roofs means that only a limited range of plant species is routinely used. However, ecological theory suggests that highly diverse or species-rich vegetation might be more resistant and resilient to severe environmental stress. This paper describes an experiment investigating the influence of vegetation diversity on green roof plant survival following an imposed drought. Twelve species were selected from the three major taxonomic and functional plant groups that are commonly used for extensive green roofs (forbs, sedums and grasses). Four species were chosen from each group and planted in combinations of increasing diversity and complexity: monocultures, four-species mixtures and twelve-species mixtures. Three watering regimes were imposed: wet, moderate and dry (watering every 1 week, 2 weeks and 3 weeks, respectively). It was concluded that a diverse plant mix was more advantageous than a monoculture in terms of greater survivability and higher visual rating under dry conditions. It was shown that combinations of species differing in functional diversity and complexity achieved this more effectively compared to plants of the same taxonomic group that compete for resources when grown together. Drought tolerance in sedums was superior to that in forbs and grasses. Little difference in drought tolerance was observed between forbs and grasses, which must be watered before reaching a permanent wilting point. Plants that exhibited over 50% leaf desiccation were unable to survive to the end of the study.