Biological Flora of the British Isles: Fraxinus excelsior
Summary This account presents information on all aspects of the biology of Fraxinus excelsior L. (Ash) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation. Fraxinus excelsior is a large forest tree, native throughout the main islands of Britain and much of mainland Europe. Seedlings are shade tolerant, but adults are not so it tends to be an intermediate successional species, invading gaps in mixed stands rather than forming extensive pure stands. Ash grows on a wide range of soils but is commonest on nutrient rich soils with a high base status and pH 4.2, and is at its best on dry calcareous screes and fertile alluvial soils. Fraxinus excelsior is trioecious or subdioecious with male, hermaphrodite and female flowers and trees. Seed production is prolific with periodic higher producing mast years. Seeds are primarily wind dispersed, but they can float and be moved considerable distances along waterways. Germination is delayed by dormancy until usually the second spring after being shed. Ash is tolerant of drought, but intolerant of spring frosts and so is predicted to fare well under current climate change scenarios, and indeed has recently been expanding in range in Europe. However, ash health and survival is currently seriously compromised by ash dieback caused by the fungus Hymenoscyphus fraxineus (Chalara fraxinea) that has the potential to kill all but a very few resistant trees. Moreover, the emerald ash borer beetle Agrilus planipennis, a serious pest of ash species in N. America, has reached Europe (though not yet the British Isles) and poses an equally if not more serious long term threat to ash.