Ectomycorrhizal fungi (EMF) mediate nutrient exchange between host plants and soil, playing an important role in the biogeochemical cycle of different elements. Currently, there is a limited amount of knowledge on the functional diversity of EMF in association with native and non-native tree species. The aim of this project is to gain understanding of the relationships between mycorrhizal fungal assemblages and tree nutrition in pure and mixed forest stands consisting of European beech (Fagus sylvatica L.), Norway spruce (Picea abies), and Douglas fir (Pseudotsuga menziesii). This project focuses on describing the fungal community structure, evaluating tree nitrogen uptake and retention in the ecosystem, and investigating the underlying mechanisms of fungal-root interactions in the context of water and nutrient availability.