Ash (Fraxinus excelsior)

This large and graceful deciduous tree occurs throughout Scotland, but is now at risk due to the spread of an imported fungal disease.

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Ash | Trees for Life

 

Global distribution

Ash, or European ash as it is sometimes known, occurs naturally throughout much of Europe and western parts of Asia. Its range extends from southern Scandinavia to northern Spain, Portugal and Greece, and from Ireland eastwards to Ukraine, western Russia and the Caspian Sea. It also occurs in the Caucasus Mountains and across northern Turkey to the north slopes of the Alborz Mountains in Iran. Outside of its native range, ash has been planted in parts of Canada, the eastern USA and New Zealand.

Distribution in Scotland

Ash occurs throughout Scotland, from Caithness to the Borders and Dumfries and Galloway. Its range includes many of the larger western islands such as Skye and Mull, and it has been planted on Orkney, Shetland and Harris. Ash is generally found at lower elevations, and does not grow on acid soils, so it is scarce or absent from significant areas in the northwest, and from the more mountainous parts of the country, where hard winter frosts prevent its growth. The highest elevation it has been recorded at is 450 metres, near Braemar. Although it occurs on a range of different soils, ash grows best on base-rich soils. Rassal Ashwood, situated on a limestone outcrop near Kishorn in Wester Ross, is the most northerly ash-dominated woodland in the UK. Although it is widely distributed, ash is not very abundant, and the total area covered by the tree in Scotland today is estimated at 5,000 hectares.

Ash is a large deciduous tree in the Oleaceae, or olive family. In good conditions it can reach a height of 30 metres in Scotland, although it more usually grows to 15-18 metres. The tallest ash in the UK has been measured at 38 metres. Ash bark is a light greyish-brown colour when young, with a smooth surface that is often covered with crustose lichens. As a tree ages, the bark becomes thicker, with vertical fissures forming in it. 

A distinctive feature of ash are its black buds, which stand out in contrast to the paler twigs, making the tree easy to identify in winter. Ash is one of the last trees to gain its new leaves in the spring. The leaves occur in opposing pairs, and each individual leaf is compound, consisting of 9 -13 leaflets, with an overall length of 20 – 25 cm. The leaflets are arranged opposite each other, with a terminal one at the end of the leaf. The leaflets are slightly toothed on their margins and are dark green on their upper surface; the undersides are a paler yellow-green. The leaves are amongst the first to fall in autumn, and turn a relatively inconspicuous yellowish green before doing so.

The flowers appear on the tree before the new leaves in spring. They are small and dark purple in colour, occurring in dense clusters, with the female flowers being slightly longer than the males. Unusually, ash can be either monoecious (meaning that both sexes occur on an individual tree) or dioecious, where any one tree has either all male or all female flowers. Some trees also alternate their flowering, bearing only male flowers in one year and females the next.

Pollination is by the wind, and fertilised female flowers produce a fruit called a samara, consisting of a seed and an attached single wing, which is 2.5 – 4.0 cm. in length. The samaras are also known as ash keys, and hang in dense bunches, which can persist on the branches after the leaves have fallen. Seed dispersal is mainly by wind, aided by the wing on each samara, but also takes place via water, as the seeds can survive immersion for several weeks. A large mature tree can produce up to 100,000 seeds in a good year, and individual trees can live for about 250 years.

In contrast to many tree species, ash does not form ectomycorrhizal associations with fungi, where the fungal hyphae surround the roots of a tree without penetrating them, exchanging nutrients, and producing the familiar mushrooms that fruit near the tree. However, ash does form arbuscular mycorrhizal symbiotic relationships with fungi, in which the fungal hyphae penetrate the tree roots, enabling a mutually-beneficial exchange of nutrients to take place, but these fungi do not produce visible above-ground fruiting bodies.

The shaggy bracket fungus (Inonotus hispidus) is parasitic on ash trees, causing white rot in the heartwood, and leading to the loss of branches and major limbs. Dead ash wood is the habitat for a distinctive saprotrophic fungus called King Alfred’s cakes (Daldinia concentrica), which has black, ball-shaped fruiting bodies.

Ash is a very important tree for lichens, and in the UK 536 different lichens (27.5% of the British lichen flora) have been recorded growing on it, including a number of rare and endangered species. With the relatively basic PH of its bark, ash is also a good host for bryophytes (mosses and liverworts), particularly where it grows in humid conditions, such as in gorges or near the west coast.

Compared to other trees such as oak (Quercus spp.), ash has comparatively few invertebrate species associated with it. The larvae of the ash bud moth (Prays fraxinella) make mines in the leaves and then feed on the buds at a later stage of their life cycle. Mines are also made in the leaves by the larvae of two micro-moths (Caloptilia cuculipennella and Caloptilia syringella), and by those of a small fly (Aulagromyza heringi).

The leaves are fed upon by a number of caterpillars, including those of the dusky thorn (Ennomos fuscantaria), ash pug (Eupithecia innotata f. fraxinata), centre-barred sallow (Atethmia centrago) and tawny pinion (Lithophane semibrunnea) moths. Larvae of the ash bark beetle (Hylesinus varius) bore into the wood of ash trees, creating characteristic galleries under the bark.

Galls are induced on ash by a number of different invertebrates. A plant louse (Psyllopsis fraxini) induces ash leaflet roll galls, in which the edges of the leaflets are rolled downwards and are often reddish-purple in colour. A midge (Dasineura fraxini) induces galls on the midribs of ash leaflets, while another midge (Dasineura acrophila) induces ‘pea-pod’ galls, where the leaflets become rolled up and resemble the pods of a pea plant. Irregularly-shaped woody galls, known as cauliflower galls, are induced on the keys and twigs of ash by a mite (Aceria fraxinovorus). These are brown at first, turning black as they age, and can persist on the trees for up to two years.

The bullfinch (Pyrrhula pyrrhula) often relies on ash seeds as its staple food in winter, and a variety of birds will utilise ash trees as nesting sites, including the redstart (Phoenicurus phoenicurus), nuthatch (Sitta europaea) and barn owl (Tyto alba).

In recent years, a fungal disease commonly known as ‘chalara ash dieback’ has seriously affected ash trees in mainland Europe and has now spread to the UK, most likely via imported ash seedlings. The species responsible for the disease has been identified as Hymenoscyphus pseudoalbidus, and this has superseded the name of Chalara fraxinea that was originally given to the asexual stage of the fungus. Symptoms of the disease include cankers, wilting of the leaves and the death of the crown of the tree. At present there is no known treatment or cure, although some trees appear to have resistance to the disease. As a result, the future for ash has been looking quite bleak, but in the UK efforts are currently being directed towards reducing the spread of the disease and developing resistance to it.

 

Alan Watson Featherstone

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