Challenges in assessing the ecological impacts of tree diseases and mitigation measures: the case of Hymenoscyphus fraxineus and Fraxinus excelsior

Forests worldwide are currently threatened by a number of non-native tree diseases. Widespread death of a tree species will have ecological impacts on species that in some way depend on that tree species to complete their life-cycle. One measure to mitigate these impacts is to establish alternative tree species to replace the threatened tree species. These alternative tree species should be as similar as possible to the threatened tree species in terms of species supported, tree traits and the environmental conditions under which the tree will grow. This study assesses the availability and quality of data to assess the ecological impact of Hymenoscyphus fraxineus on Fraxinus excelsior and the suitability of 48 alternative trees to replace F. excelsior in the UK. To make this assessment data were collected on 1) species use (whether the 955 ash-associated species will use the alternative tree species), 2) traits (bark pH, deciduous, floral reward, fruit type, height, leaf dry matter content, leaf shape, length of flowering time, mycorrhizal association, pollen vector and specific leaf area) and 3) site requirements (occurrence within northern/southern and upland/lowland Britain, detailed climatic and soil nutrient requirements). For all three assessment methods there was lower confidence in the suitability of non-native tree species to replace F. excelsior due to lack of data. Different alternative tree species were ranked most suitable depending on the methods used. We conclude that no one species is suited to all the site types associated with F. excelsior, nor will any one tree species support a high percentage of the ash-associated species while also matching many of the traits of F. excelsior. Our work provides broad guidance on the suitability of the 48 alternatives but site specific information is required to refine this selection at each site. The study highlights a lack of information to make a full assessment of the suitability of many species, particularly non-native species and calls for the collation of biological records so that rapid assessments of the potential ecological impacts of the loss of any given tree species and the suitability of their alternative tree species can be made.