Potential impacts of the loss ofFraxinus excelsior(Oleaceae) due to ash dieback on woodland vegetation in Great Britain
New Journal of Botany
Nineteen fungi and 13 lichens, where more than 50% of records (from the above databases) were on ash. • Six bryophytes and 24 invertebrates where all the available literature stated that the species were rarely found on tree species other than ash. 15. Assessments based on the number of species which are 'obligate' on, or 'highly associated with', ash identified bryophytes, fungi, invertebrates and lichens as the groups considered most at risk from ash dieback. (Confidence: High). 16. Using a
... mbination of the conservation importance of the species and their level of association with ash, we classified the species that use ash trees into Red, Amber, Yellow and Green codings, indicating level of risk with respect to the likely impact of ash dieback. This gave 69 Red-coded species, 169 Amber-coded species, 383 Yellowcoded species and 330 Green-coded species. (Confidence: Medium). Alternative tree species 19. Twenty-two tree species were assessed for their suitability as replacements for ash: field maple, Norway maple, sycamore, alder, silver birch, downy birch, hornbeam, sweet chestnut, hazel, hawthorn, beech, aspen, wild cherry, bird cherry, Douglas fir, sessile oak, pedunculate oak, goat willow, grey willow, whitebeam, yew, and small-leaved lime. These species were chosen to cover a range of management objectives and as being likely to regenerate naturally or be planted by woodland managers because of their suitability to establish and grow on site types that support ash. Douglas fir and sweet chestnut were included on the list as examples of tree species that are currently suggested for planting for climate proofing (coping with possible climate change) and have production potential on sites currently occupied by ash. The inclusion of a tree species in the assessment does not necessarily mean that this species is being promoted as a replacement for ash if the objective is to manage for ash-associated biodiversity. 20. Ash-associated species were assessed as to whether they also used any of the 22 alternative tree species (above). Oak supported 69% of the ash-associated species but no single tree species out of those 22 would make a good overall alternative to ash. Birch, beech and oak are used by many bird species that use ash. Field maple, sycamore, alder, hazel, hawthorn, oak, aspen and the willow species are used by many bryophytes. Oaks, hazel and sycamore are important as potential substitute hosts for ash-associated lichens. Trees such as sweet chestnut, Douglas fir or yew are used by very few of the species that use ash, and were identified as the least suitable out of the 22 alternative tree species. Similarity indices between the alternative tree species and ash, based on the level of use made of the tree species by the ash-associated species showed that oak, alder, beech and aspen were most similar to ash. A mixture of tree species rather than a single tree species will support a greater variety of ash-associated species. However, it must be noted that for many ash-associated species, data on the use of these 22 alternative tree species is lacking. (Data lacking for some species: Confidence: Low). 21. A trait based analysis was also done to assess the similarity of these 22 tree species to ash. Nineteen traits were collated for ash and the 22 alternative tree species: bark acidity, mycorrhizal association, diaspore type, duration of flowering, floral rewards, fruit type, leaf form, leaf persistence, pollen vector, height, Ellenberg light, Ellenberg moisture, Ellenberg reaction, Ellenberg nitrogen, leaf dry matter content, leaf size, seed mass, release height, and specific leaf area. For single traits there were some matches between ash and the alternative tree species, but multi-variate analysis of all traits showed that none of the 22 tree species were very similar to ash overall. Alder and aspen were identified as the trees most similar to ash, with similarity indices of 0.7. Sweet chestnut and Douglas fir were the most dissimilar (similarity indices of 0.5). (Assessment based on published trait data and standard statistical methods. Confidence: Medium). 22. The two assessments of similarity of alternative tree species, i.e. use or traits (as above) were compared. The assessment of which tree alternative is most similar to ash depended on the method used to assess similarity. However, aspen was ranked relatively similar by both types of methods, and Douglas fir was ranked very dissimilar to ash by both methods. Likely changes in woodland communities 23. Ash is dominant in eight NVC communities: W8a, W8b, W8c, W8d, W8e, W8g, W9a and W12a. (Confidence: High). For each community, predictions based on expert opinion and ecological information were made about the changes in vegetation following the loss of ash. 24. Regional differences in how the tree and ground flora woodland community may respond following the loss of ash were assessed. This assumed natural regeneration was unhindered and sites were not modified beyond that caused by the dying of ash trees.