Green roofs are increasingly being implemented in cities to reduce stormwater runoff. Most green roof plant species are chosen on their ability to survive hot and dry conditions and not on the basis of plant water use. However, to optimise retention, green roofs need plants with high water use to dry out substrates after rainfall but which can also cope with water limitations in dry substrates. Most succulents cannot achieve this, however the ability to balance high water use with survival has been shown in rock outcrop species. This paper aims to relate water use and survival to plant traits to improve green roof plant selection. Therefore, we determined water use under well-watered conditions (stormwater retention) and the dry-down rate under water deficit conditions (survival) of 18 dryland species across a range of life-forms under glasshouse conditions and related these strategies were related to morphological traits; stomatal density, SLA, root mass fraction (RMF) and leaf succulence.
All species had traits related to drought tolerance and demonstrated a range of physiological mechanisms for coping with water stress, including conservative and plastic water use strategies. Plants which had greater water use on a shoot-basis under well-watered conditions took the longest to dry out the substrates, indicating down-regulation in water use during water deficit. Both SLA and RMF were positively correlated with the rate of dry-down under water deficit daily water use under well-watered conditions, regardless of life-form. Previous green roof plant selection has focused on traits such as succulence which relate only to survival or growth under drought conditions. However, we show that SLA in particular, provides an easy way to select green roof plants from dryland ecosystems which can achieve high water use for stormwater management and survive drought.