Collapsing foundations: The ecology of the British oak, implications of its decline and mitigation options

Oak (Quercus spp.) is declining globally due to a variety of pests, pathogens and climate change. Assessments of the impact of losing keystone species such as oak, should include the impact on associated biodiversity and ecosystem functioning, and consider mitigation options. Here, we assess the potential ecological implications of a decline in Quercus petraea/robur within the UK. We collated a database of 2300 species associated with Q. petraea/robur of which 326 were found to be obligate associates (only found on Q. petraea/robur). One potential mitigating measure for lessening the impact of oak decline on associated biodiversity would be establishing alternative tree species. However, of 30 alternative tree species assessed, none supported a high proportion of the oak-associated species (maximum 28% by Fraxinus excelsior ash, which is currently declining due to Hymenoscyphus fraxineus, a fungus). However, the functioning of oak (leaf litter/soil chemistry and decomposition) was potentially replicable owing to its similarities with other tree species. The impact on the four main oak woodland communities within the UK, of a theoretical 50% decline in oak on ecosystem functioning and associated species was explored for five scenarios, that differed in the selection of replacement tree species. The most resilient woodland communities (in all the aspects assessed) were those capable of supporting the greatest diversity of tree species and when the currently occurring tree species replaced oak. The greatest change was predicted where F. excelsior was lost in addition to a decline in oak, and if only one species, particularly Acer pseudoplantanus sycamore, filled the canopy gaps.