AMF inoculation of green roof substrate improves plant performance but reduces drought resistance of native dry grassland species

Abstract

Standard extensive green roofs (EGRs) with their shallow substrate layers represent extreme sites for plant growth and therefore are planted mostly with drought-resistant species, including non-native plant species. As standard EGR substrates often lack potentially mutualistic soil microorganisms, it has been stated that inoculation with arbuscular mycorrhizal fungi (AMF) might increase plant performance and drought resistance. Aiming to support native biodiversity on EGRs, we tested whether AMF inoculation into standard green roof substrate can enhance plant performance and drought resistance of regionally occurring native dry grassland species.The results of a pot experiment with 11 native plant species growing with and without AMF inoculation showed considerable differences in fitness-relevant plant traits. Over 88 days of moderate drought conditions, inoculated plants produced 2.5 times more above-ground biomass than control plants. In addition, the number of inflorescences on inoculated plants was significantly higher in 5 out of 7 flowering species. Under severe drought stress created by stopping the water supply, however, inoculated plants wilted on average 2.38 days earlier than control plants.Although the underlying mechanisms of the observed results remain unresolved, AMF inoculation might help to enhance an earlier and higher seed set, facilitating the establishment of a soil seed bank, which is necessary for a self-sustaining plant population in drought-governed habitats such as EGRs.