Trait-based green roof plant selection: Water use and drought response of nine common spontaneous plants

Abstract

Plant health and cover are critical components of stormwater mitigation on green roofs. Green roof plants can be selected from analogous habitats (e.g. rocky outcrops) and/or by using plant traits associated with survival (e.g. succulence). Plants selected for stormwater mitigation should have high transpiration to replenish substrate storage capacity following rainfall, which is likely associated with ‘fast’ traits such as faster growth rate or higher leaf area. However, ‘slow’ traits such as slower growth rate or smaller biomass can help plants survive dry periods. Without routine maintenance, green roof plant communities are often replaced by ‘spontaneous’ vegetation. As spontaneous species can survive green roof conditions, they may contribute to stormwater mitigation and understanding how their traits relate to drought and water use strategies may help improve green roof plant selection. Therefore, we conducted an eight-week glasshouse experiment to determine the water use and drought response of nine spontaneous species commonly found on green roofs under well-watered (WW) and water-deficit (WD) conditions. Higher transpiration under WW conditions was related to ‘fast’ traits including greater biomass, leaf area and relative growth rate. There was a trade-off between water use under WW conditions and drought resistance under WD. While ‘fast’ species reduced transpiration by 57–72 % and biomass by up to 50 % under WD, this was insufficient to maintain leaf water status (leaf RWC < 90 %) and avoid drought stress. Whereas, ‘slow’ species with less biomass used less water under both WW and WD conditions, maintained their leaf water status and resisted drought stress. Diverse water use and drought responses of the nine spontaneous species reflects fluctuating conditions on green roofs that likely favour different strategies over time. While spontaneous species may not be desirable, they are likely to provide similar stormwater mitigation to planted species where good plant cover is achieved.