Arbuscular mycorrhiza in the urban jungle: <i>Glomeromycotina</i> communities of the dominant city tree across Amsterdam

Societal Impact Statement Trees in cities provide a great number of benefits to people and nature, but they are challenged by harsh conditions. Trees rely on helpful fungi in their roots to get essential nutrients from the soil, but we do not know which of these fungi are resistant to city landscapes. By studying these fungi, we can learn how their communities are affected by cities and which of them survive best there. Understanding this can help us develop effective ways to make use of these microbes or to improve city conditions for the benefit of urban trees and their fungi and, therefore, us. Summary Urban trees are important green features in cities. Arbuscular mycorrhizal fungi (AMF) in roots may alleviate urban environmental pressures affecting urban trees. These pressures also appear detrimental for AMF, reducing root colonization rates, spore production and spore diversity. However, a limited number of molecular studies on urban AMF contrast these findings, but the matter remains unresolved. Therefore, we investigated the hitherto understudied AMF communities in the roots of urban trees, their diversity, spatial and neighbouring‐tree effects and interaction networks. We did this by metabarcoding fungi from tree roots across an urbanization gradient in Amsterdam (the Netherlands), focussing on Dutch elm ( Ulmus x hollandica ). Samples were collected from three urbanization classes, differentiated by the degree of soil sealing and management: an urban forest, park and street. In the urban forest and park, root samples were collected in a grid, allowing the construction of interaction networks and assessment of neighbouring‐tree root effects, whereas the street trees stood in solitary pits. Using the ITS2 barcode, we detect distinct, diverse and heterogenous AMF communities. The (phylogenetic) diversity of AMF increased with urbanization. Contrary to our expectations, we found no evidence that AMF communities in street trees were more spatially homogenous. Moreover, neighbouring root diversity did not appear to affect AMF diversity. Our findings suggest a strong response of AMF communities to urbanization. An urban‐induced change in mycorrhizal partners, rather than a loss of partners and interaction complexity, demonstrates the high adaptability of the arbuscular mycorrhizal symbiosis to urban stressors, which has important management implications.