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 plantgrowth and therefore are plantedmostly with drought-resistant species, including non- native plantspecies. As standard EGR substrates often lack potentially mutualistic soil microorganisms, it has been stated that inoculationwith arbuscular mycorrhizal fungi(AMF) might increase plantperformance and drought resistance. Aiming to support native biodiversity on EGRs, we tested whether AMF inoculationinto standard green roofsubstrate can enhance plantperformance and drought resistance of regionally occurring native dry grassland species. The results of a pot experiment with 11 native plantspecies growing with and without AMF inoculationshowed considerable differences in fitness-relevant planttraits. Over 88 days of moderate drought conditions, inoculated plantsproduced 2.5 times more above-ground biomass than control plants. In addition, the number of inflorescences on inoculated plantswas significantly higher in 5 out of 7 flowering species. Under severe drought stress created by stopping the water supply, however, inoculated plantswilted on average 2.38 days earlier than control plants. Although the underlying mechanisms of the observed results remain unresolved, AMF inoculationmight help to enhance an earlier and higher seed set, facilitating the establishment of a soil seed bank, which is necessary for a self- sustaining plantpopulation in drought-governed habitats such as EGRs.