Abstract
About 90% of all land plants form mycorrhiza to facilitate the acquisition of essential nutrients such as phosphorus, nitrogen, and sometimes carbon. Based on the morphology of the interaction and the identity of the interacting plants and fungi, four major mycorrhizal types have been distinguished: arbuscular mycorrhiza (AM), ectomycorrhizal (EcM), ericoid mycorrhiza, and orchid mycorrhiza. Although most plants are assumed to form only one type of mycorrhiza, some species simultaneously form associations with two mycorrhizal types within a single root system. However, the dual-mycorrhizal status of many species is under discussion and in some plant species the simultaneous association with two mycorrhizal types varies in space or time or depends on the ecological context. Here, we assessed the mycorrhizal communities associating with common hawthorn (Crataegus monogyna), a small tree that commonly associates with AM fungi, and investigated the potential factors that underlie variation in mycorrhizal community composition. Histological staining of C. monogyna roots showed the presence of a Hartig net and hyphal sheaths in and around the roots, demonstrating the capacity of C. monogyna to form EcM. Meta-barcoding of soil and root samples of C. monogyna collected in AM-dominated grassland vegetation and in mixed AM + EcM forest vegetation showed a much higher number of EcM sequences and OTUs in root and soil samples from mixed AM + EcM vegetation than in samples from pure AM vegetation. We conclude that C. monogyna is able to form both AM and EcM, but that the extent to which it does depends on the environmental context, i.e., the mycorrhizal type of the surrounding vegetation.