A severe dieback of ash (Fraxinus excelsior) caused by the fungal pathogen Hymenoscyphus fraxineus has spread across Europe. Ash is an arbuscular mycorrhizal (AM) species, whereas the other dominant tree species, such as beech and oak, are ectomycorrhizal (ECM) species. As ECM trees often allocate more carbon to their fungal partner than AM trees, ash dieback and the resulting increase in ECM trees may increase mycorrhizal hyphal production and/or metabolic activity, but this remains unclear. Here we compared mycorrhizal hyphal production and respiration rates using the hyphal ingrowth core method between ash-dominated, ring-barked ash-dominated (a treatment mimicking severe ash dieback), and beech- and oak-dominated forests every 50 days from March to September 2023 in Wytham Woods, outside Oxford, UK. Both hyphal production and respiration rates were lower in ring-barked ash-dominated, intermediate in ash-dominated, and higher in beech- and oak-dominated forests. Carbon allocation to the symbiotic mycorrhizal fungal community may decrease in the short term following the occurrence of ash dieback, but may increase in the long term due to the associated increase in ECM trees.