Fast plants have water-use and drought strategies that balance rainfall retention and drought survival on green roofs

Green roofs can improve ecosystem services in cities; however, this depends on appropriate plant selection. For stormwater management, plants should have high water use to maximize retention and also survive dry periods. Plants adapted to wetter habitats develop "fast" traits for growth, whereas plants from drier habitats develop "slow" traits to conserve water use and survive drought. Therefore, we hypothesized that (1) plants with fast traits would have greater water use, (2) plants with slow traits would have greater drought tolerance, (3) fast-slow traits would be consistent across the plant, and (4) fast plants with greater water use could avoid drought stress. We evaluated 14 green roof species in a glasshouse experiment under well-watered (WW) and water-deficit (WD) conditions to determine relationships between fast-slow traits, water use, and drought resistance. Traits measured were shoot dry mass, specific leaf area (SLA), root mass fraction (RMF), and specific root length (SRL). Daily evapotranspiration per shoot dry mass was used to describe water use. Drought resistance was represented by (1) days to stomatal closure; (2) cumulative ET before stomatal closure; and (3) degree of iso-anisohydry (difference between midday leaf water potential (psi(MD)) of WW and WD plants; Delta psi(MD)). Plants with greater water use had fast aboveground traits (greater shoot biomass and SLA). Plants with slow traits had greater drought tolerance as plants with lower shoot dry mass closed their stomata later under WD, and plants with greater root allocation were more anisohydric. Fast-slow traits were not consistent across the plant. Although SLA and SRL were positively related, SRL was not related to water use or drought resistance. Shoot dry mass was inversely related to SLA and had a stronger influence on stomatal closure. Though plants with greater water use under well-watered conditions closed their stomates earlier to avoid drought stress, they were not more isohydric (smaller increment psi(MD)) and did not necessarily use more water under WD. Fast aboveground traits can be used to select green roof plants with high water use that avoid drought stress to optimize rainfall retention without jeopardizing drought survival. This will facilitate rapid plant selection using trait information from online databases.