Biological Flora of the British Isles:Fraxinus excelsior

Abstract

Summary1. 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. 2. 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. 3. 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. 4. 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.

Genetic Structure in the Northern Range Margins of Common Ash, Fraxinus excelsior L.

“…In south-eastern Europe, pollen diagrams indicate a late glacial presence of common ash in the Balkan peninsula [80–82]. Pollen diagrams from the northern Alps and the Carpathians document rapid early Holocene population expansions suggesting that the full glacial distribution of common ash was large and may have included regions as far north as the Carpathian Basin [83].…”

“…When using 0.5% of terrestrial pollen as an indication of regional presence of Fraxinus , it can be deduced that the taxon was present in Poland and the Baltic States around 9 cal kyr BP (pollen threshold 2–4%), and in southern Sweden 8 cal kyr. BP (0.5–1% pollen threshold) [81, 82]. In this period a mainland connection existed between southern Sweden and northern Germany/Denmark following the Ancylus-regression of the Baltic Sea [84].…”

“…The Swedish populations have in addition low allelic richness, which may suggest founder events. Denser pollen records [81, 82] and more genetic data would however be necessary to trace the different migration routes.…”

“…The slightly elevated private allelic richness in the population may also have contributed to its genetic partition, although it is still lower than the mean value plus standard deviation (0.130; SD 0.120) for all the populations. Interestingly, Fraxinus pollen (0.5–1%) was detected in the Trøndelag region in the early Holocene (at 7.5 and 6.0 cal kyr BP [81, 82]). Pollen records from the central Scandes Mountains, a region close to Trøndelag, suggested an early Holocene forest with Corylus avellana , Alnus glutinosa and Ulmus glabra as common constituents, along with rare occurrences of Tilia cordata [86].…”

Disentangling the pollen signal from fen systems: Modern and Holocene studies from southern and eastern England

“…Few fossil samples plot within the AU group (Alnus-Fraxinus woodland). In fen carr, Fraxinus excelsior is often associated with drier areas (Thomas, 2016) and margins and it is therefore unlikely to have been abundant at the sites investigated on Romney Marsh or Fenland during phases of widespread peat formation. In Fenland, some Holocene samples with high arboreal pollen frequencies (notably from Redmere, the Welney 3 rd peat and Meadowgate Lane) plot away from the woodland groups.…”