Plant water use related to leaf traits and CSR strategies of 10 common European green roof species

The vegetation layer contributes to multiple functions of green roofs including their hydrological function as plants remove water from substrates between rainfall events through evapotranspiration, restoring the green roofs storage capacity for rainfall retention. While individual traits have been related to water use strategies of green roof plants, these traits are inconsistent, suggesting the importance of trait combinations which may be reflected in CSR (competitor, stress tolerator, ruderal) strategies. Therefore, relating plant water use to leaf traits and CSR strategies could help facilitate green roof plant selection into new geographical regions where green roof technology is developing. For example, in high latitude northern European regions with long daylight during the growing season. Growth (shoot biomass, relative growth rate and leaf area), leaf traits (leaf dry matter content, specific leaf area and succulence) and CSR strategies were determined of 10 common European green roof plants and related to their water use under well-watered (WW) and water-deficit (WD) conditions. All three succulent species included in the experiment showed mostly stress tolerant traits and their water loss was less than the bare unplanted substrate, likely due to mulching of the substrate surface. Plants with greater water use under WW conditions had more ruderal and competitive strategies, and greater leaf area and shoot biomass, than species with lower WW water use. However, the four species with the highest water use under WW conditions were able to downregulate their water use under WD, indicating that they could both retain rainfall and survive periods of water limitations. This study indicates that, for optimal stormwater retention, green roof plant selection in high latitude regions like northern Europe, should focus on selecting non-succulent plants with predominantly competitive or ruderal strategies to make the most of the long daylight during the short growing season.