How to Replicate the Functions and Biodiversity of a Threatened Tree Species? The Case of Fraxinus excelsior in Britain

The suitability of alternative tree species to replace species that are either threatened by pests/disease or at risk from climate change is commonly assessed by their ability to grow in a predicted future climate, their resistance to disease and their production potential. The ecological implications of a change in tree species are seldom considered. Here, we develop and test 3 methods to assess the ecological suitability of alternative trees. We use as our case study the systematic search for an alternative tree species to Fraxinus excelsior (currently declining throughout Europe due to Hymenoscyphus fraxineus). Those trees assessed as most similar to F. excelsior in selected ecosystem functions (decomposition, leaf litter and soil chemistry) (Method A) were least similar when assessed by the number of ash-associated species that also use them (Method B) and vice versa. Method C simultaneously assessed ecosystem functions and species use, allowing trade-offs between supporting ecosystem function and species use to be identified. Using Method C to develop hypothetical scenarios of different tree species mixtures showed that prioritising ecosystem function and then increasing the mixture of tree species to support the greatest number of ash-associated species possible, results in a mixture of trees more ecologically similar to F. excelsior than by simply mixing tree species together to support the greatest number of ash-associated species. We conclude that establishing alternative tree species results in changes in both ecosystem function and species supported and have developed a general method to assess suitability that simultaneously integrates both ecosystem function and the 'number of species supported'.