The impact of plants on ecosystem functioning is mediated by mycorrhizas, intimate plant-fungal associations formed by most plants and a diverse subset of soil fungi. Mycorrhizal fungi play a key role in terrestrial ecosystems by enhancing plant access to soil nutrients and water but also as regulators of the carbon cycle. However, these belowground fungi are affected by environmental and anthropogenic changes, with potential consequences on the roles they develop in our ecosystems. In this seminar, I will talk about the main drivers and threats of ectomycorrhizal fungal communities in forests, the impact of changes in these communities in forest functioning and the research we are doing to measure, understand and predict the contribution of different mycorrhizal fungi to carbon sequestration in soils for habitat ecosystem assessment.