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Fraxinus excelsior L., Ash

Account Summary

Native, common, locally abundant. European temperate.

1884; Barrington, R.M.; Co Fermanagh.

Throughout the year.

Growth form and preferred habitats

Ash is the only member of the Oleaceae, the Olive family, which penetrates this far north in Europe and it is the only wind-pollinated species in the family, all the other members being insect-pollinated (Milner 1992). Ash is very prevalent in semi-natural woods, timber plantations (chiefly for poles or staves), secondary scrub and managed hedgerows. It is also a pioneer colonist on cliffs, riverbanks, lakeshores, roadside and field hedgerows and as a weed in old quarries. It occurs on all types of soil in Fermanagh except the most acid and peaty, the permanently water-logged, or the extremely shallow or very steeply sloping and unstable ones. Wardle (1961) reckoned Ash is usually absent where the pH of the surface soil goes lower than 4.2.

Ash trees are never very tall in Fermanagh, often only around 10 m, occasionally up to 15 m in height and only rarely in more nearly optimal growing conditions are they taller than this. Despite this lack of stature, Ash still forms the dominant tree in old valley woods over limestone, eg around the Knockmore area. It definitely prefers and is most abundant in areas with calcareous rocks and near neutral soils. Having said this, young plants in particular often produce pale yellow foliage and appear to suffer from lime chlorosis when growing on basic soils.

In areas of more acid geology and soils, Ash is more frequent or almost restricted to lower slopes of valleys, hollows and stream banks, where percolating ground water is sufficiently enriched with nutrient bases to support the tree and allow it to compete. Previously, on the better loam soils in upland sheltered valleys and on lake islands, Ash would have shared the canopy of mixed deciduous woodland with Quercus petraea (Sessile Oak). Now, however, apart from the odd inaccessible scrap of such woods and on a few protected estate and NR examples, almost all of these semi-natural woodland stands have been felled and destroyed to create additional sheep pasture.

On cliffs, rocks, or on screes that are sufficiently stable to permit the growth of any woody species, the young soils present in crevices are generally able to supply the Ash tree's needs irrespective of the rock chemistry (Wardle 1961). On limestone scree, Ash seedlings can withstand limited down-slope movement better than Hazel (Corylus avellana), but the saplings and eventual trees often end up with very strongly curved trunks!

F. excelsior also competes very well on winter-wet or seasonally water-logged, often clay soils around many lake shores, as well as on better drained soils subject to intermittent flooding along riverbanks. This is because the young seedling requires only quite a shallow depth of well-drained soil on which to establish itself in wet ground. Subsequent growth in water-logged habitats is poor, but the species will respond rapidly to any improvement in drainage. It has been suggested that the poor growth of Ash on water-logged sites may paradoxically be due to periodic water shortage in the plant, since its roots are unable to penetrate deeply in such soils and suffer when even a brief drought occurs (Wardle 1961).

Fossil record and early Ash woodland colonisation

The fossil pollen record of Fraxinus excelsior is virtually restricted to earlier interglacials going back to the Pastonian, plus current Flandrian periods (Godwin 1975). As the tree is recognised as being frost-sensitive, it is not really surprising that it is totally absent from all the intervening glacial stages. The reappearance of Ash pollen and macrofossil wood in the Flandrian interglacial was, in Godwin's opinion, rather tardy. It did not appear at all in B & I until zone V, when just two records were found. Ash fossils make a vast leap forward in frequency and abundance in zone VIIa (the Atlantic period) 8,000-5,000 BP, the Holocene climatic optimum for both warmth and moisture. Temperature then is reckoned to have been warmer than today. Dates quoted for this period of climatic optimum vary in almost every reference examined, some making the period a thousand years later and shorter (ie 7,000-5,000 BP). The current author (RSF) is merely a lay reporter, not a palaeo-climatologist and, therefore, cannot comment further. The preceding period, known as the Boreal, had an overall temperature similar to that of today.

The increases that F. excelsior made in zone VIIa became extended and sustained in zones VIIb (the Sub-boreal) and VIII, the expansion in site numbers being accompanied by a substantial increase in the proportion of total tree pollen contributed by Ash. The low pollen productivity characteristic of Ash means that in effect abundance of the tree increased even more than initially appears the case through these time periods (Godwin 1975). Having said this, the fossil pollen figures imply that Ash was widespread in the Atlantic period (Holocene climatic optimum) wildwood, but it was only locally common at that time (Rackham 1980, p. 204).

The fossil evidence strongly suggests that the expansion of Ash in the later stages of the Flandrian (called the 'Littletonian' in Ireland), primarily reflects human clearance of the pre-existing climax forest (which included Elm (Ulmus spp.)). The Ash trees' strong invasive response to clearances and deforestation ± coincided with, or shortly followed, the major decline of Elm pollen, which probably involved the effects of both plant disease and human felling operations, that opened up growing conditions favourable to the Ash.

Why it should have been Ash that profited from the clearing of primary forest is not clear. It is somewhat unlikely that Ash was deliberately favoured by early farmers, but the species is a good coloniser and often forms secondary woodland, especially on chalk and limestone soils. Since it is palatable to stock, it is also possible that it was used as an alternative fodder in hedges and wood-pasture to replace the missing Elm, the branches of which, prior to its decline, were an important source of animal food (Rackham 1980). Hay as storage fodder made from grass did not exist in agriculture until long after this prehistoric period.

Fermanagh occurrence

Definitely the most frequent and often the dominant tree in limestone districts, Ash ranks second only to Alder (Alnus glutinosa) among the most commonly recorded tree species in Fermanagh – provided we set aside the Hawthorn's claim to the ranking on account of it being a hedgerow shrub, ie defined as multi-stemmed and lacking a definite bole (Mitchell 1974). Either way, Ash, which after all can be present in shrub form too, is present in 478 Fermanagh tetrads, 90.5% of the VC total. It is thus one of our top two or three tall woody plants (we must not totally forget about the heathers!) in terms of both abundance and distribution. In Britain, by comparison, Ash is considered the fourth commonest woodland tree species (Milner 1992).

The most impressive Ash woodland in Fermanagh is at the bottom of Hanging Rock, a 50 m high limestone cliff below the grazed limestone uplands of the Marlbank plateau near Florencecourt. The Hanging Rock Nature Reserve sits between the cliff base and a narrow band of agricultural fields above Lower Lough Macnean. The Ash woodland is damp in the oceanic climate of this part of the world and it has a rich ground flora and an extensive list of lichens, both of which indicate it is ancient woodland. The limestone soil is shallow and rocky. The wood is characterised by numerous old, fallen trees with saplings growing up through them. The secondary canopy is composed of Hazel, Holly (Ilex aquifolium) and Bramble (Rubus spp.).

Seed production, dispersal and habitat colonisation

Winged seeds are produced most, but not every year, often appearing in large numbers (100,000 or more/tree), enabling Ash to disperse very effectively. Thus it frequently invades derelict sites, eg in old quarries or on waste-ground as a pioneer scrub species – it is virtually a woody weed in such sites. Ash is planted or arrives self-sown in many, possibly most hedgerows in Fermanagh, both along field boundaries and on roadsides. As a pioneer, F. excelsior seedlings are usually in competition with grasses and they are only able to establish where there are gaps in the sward (Wardle 1961). The established strategy of the species is categorised as C/SC, ie intermediate between Competitor and Stress-tolerant Competitor (Grime et al. 1988, 2007).

Seedling growth is rather slow at first, but once the individual is well established it becomes vigorous. In good growing conditions of full light or semi-shade, moisture and fertile soil young plants can achieve elongation growth of 50-100 cm or more per year from the second year onwards, emulating suckers of Elm (Ulmus spp.) and Wild Cherry (Prunus avium) (Oliver 1998). Young seedlings and newly opened buds, both floral and foliage, are susceptible to late frosts and are often killed in more exposed sites in April and May.

Stress tolerances and growth requirements

Growth and reproductive success of F. excelsior is also very dependent upon moisture relations, the shoots being very sensitive to any inadequacy of water supply due to drought or drying winds, particularly during the months of May and June when growth and reproduction are restarting. Rather surprisingly, this even applies to plants in sites that are water-logged in the winter months. While Ash rootlets are always concentrated in the top 5 cm of soil, lateral roots may be sparsely distributed to great depths, reaching down as far as the permanent water table. Nevertheless, larger plants display strong powers of vegetative recovery from the negative and exhausting effects of frost and drought (Wardle 1961). Ash plants therefore need some shade, shelter and relatively moist, but not too wet soil conditions in their early years but, thereafter, the plants are very much more those of a light-demanding species (Hadfield 1957).

In the full sun conditions typical of a managed linear hedgerow setting, Ash grows very much more rapidly than it does in woodland shade. It is said to have the highest nutrient uptake of any native tree (Milner 1992, p. 10). The forester and tree identification expert, Alan Mitchell, who had a poor opinion of the species, referred to it disparagingly as, "a gross feeder", ie behaving like Stinging Nettle (Urtica dioica) with respect to nutrient demand (Mitchell 1996). On the other hand, Ash is very tolerant of poor soils and although often severely stunted by nutrient stress, it can survive and grow where few other tree species can, including in cracks and ledges on rocky crags. Occasionally it even manages to grow on, or through, old stone or brick walls, where conditions must obviously be very unfavourable yet clearly lie within the wide tolerances of the species.

The efficient mineral uptake by F. excelsior is reflected in the very rapid decay of its leaf litter, which in damp woods disappears within nine months, ie by the following June (Wardle 1961). In the field layer of woodland, Ash seedlings are very shade-tolerant and can persist in almost suspended animation for years under heavy canopy shade, effective increase in their height only becoming possible whenever they experience conditions of almost full sun. When a gap eventually appears in the woodland canopy, they are released from domination and shoot up in height, competing fiercely until one or more manages to fill the ecological space temporarily available (Wardle 1961; Rackham 2014).

Canopy opening, leaf fall and their incorporation in soil

While the familiar folk saying has it, 'Oak before Ash, splash, splash, splash. Ash before Oak, soak, soak, soak', in reality F. excelsior is almost always the last native tree to come into leaf, usually in mid- to late-May, but occasionally some trees in a cool season are still bare-branched, with their distinctive black buds intact in early June, eg in 2002 (RSF, personal observation). The delay in leafing is undoubtedly due to the vulnerability of the leaves and new shoot growth to attack by late season frost. The natural distribution of F. excelsior in W Europe is undoubtedly defined by its intolerance of winter cold, late spring frosts and any drought in May and June when the tree is most actively producing its new season's growth and beginning reproduction.

The leaf canopy of Ash casts a relatively light shade when compared with Oak (Quercus spp.), or even with Rowan (Sorbus acuparia), or other secondary canopy species of the typical mixed deciduous woodland in B & I and, as a result of this, a rich field and ground flora is able to grow beneath it (Kelly & Kirby 1982). The light degree of shading by Ash is partly due to the fact that the species has opposite leaves and the inevitable opposite branching pattern which leads to the development of a remarkably open crown (Edlin 1964).

Ash is also one of the first native trees to drop its leaves, which either turn yellow or fall green, typically in early October. Although the leaves are pinnately-lobed and are composed of up to 13 leaflets, they detach and fall in one piece. In the past the leaf litter attracted earthworms which dragged them down into their burrows leaving the stalks in view, sticking out of the ground at odd angles. The current author (RSF) has not seen evidence of this happening for many years since earthworms have become a rarity, possibly due to the arrival in NI of predatory New Zealand Flatworms, but perhaps also as a consequence of pollution associated with the excessive use of agrochemicals and farm slurry.

Flowering reproduction

Flowering generally begins when the tree is between 30 and 40 years old (Wardle 1961). The flowers are simple, reduced structures which open in mid-April just before the leaf canopy does. The densely clustered flowers may be either separate male and female or perfect (ie bisexual or hermaphrodite). The same tree can bear flowers of all three kinds, although in general the species is dioecious, having separate male and female trees and, as such, it is best described as partially dioecious (Edlin 1964; Grime et al. 1988, 2007). Although whole trees tend towards one sex or the other, the sex of individual tree is not clear cut and they can change the sex balance from year to year (Rackham 2014).

The flowers are as simple as flowers can be: each consists of two stamens and/or an ovary, without sepals or petals, and producing no nectar. They do tend to be purplish in colour and are borne in axillary panicles of more than 100. Pollination is mainly by wind, but for a few days sufficient pollen is produced to attract honey bees and this almost certainly results in some degree of cross-pollination between flowers and trees (Rackham 2014). The fruit is an elongated, winged, samara, referred to in bulk as 'Ash keys', each usually containing just a single seed and borne in dense bunches from the beginning of July onwards.

Viable seed samara are blown from the tree from September onwards, but many continue to remain on the parent tree until the following spring. Dispersal distances of up to 200 m or more by winter storm winds carrying samaras have been recorded (Ridley 1930; Edlin 1964; Rackham 2014). However, in conditions of woodland shelter, obviously most keys simply drop immediately beneath the parent tree.

When released from the tree in its samara fruit, the embryo seed is immature and requires a prolonged period of dormancy in order to mature and become capable of germination. Ash seed also has a chilling requirement and needs a stratification of two winter's duration. Together these dual delays mean that germination usually takes place in the spring of the second year after shedding (Wardle 1961; Gardner 1977). During this long delay, a great many dormant seeds are eaten by mice, voles, birds and caterpillars (Milner 1992). Under natural conditions, a small number of seedlings can be expected to appear every spring and some seed may remain viable and hidden from predators for up to six years. Seedlings are also subject to similar animal predation, including by rabbits; damping off is a very common fate if the adjacent vegetation and the soil are too moist. Saplings that suffer the deleterious effects of browsing, however, often demonstrate strong powers of recovery (Wardle 1961).

Vegetative reproduction

Although the plant responds superbly well to coppice cutting by re-sprouting from the basal crown, there normally is not any really effective mode of vegetative reproduction in F. excelsior and therefore no clonal growth. However, when the basal root crown of a tree becomes decadent due to old age or unfavourable changes in habitat conditions, epicormic shoots often spring from the lower part of the tree trunk. True suckering has been reported, but it must be of rare occurrence (C.D. Pigott, in: Wardle 1961).

Toxicity and herbivory

Sheep and goats are fond of browsing the tree foliage and in past times young shoots of coppice Ash were collected and used as animal fodder (Milner 1992). The tree contains poisonous principles, however, including a glycoside called 'aesculin' which also occurs in Aesculus species. Cattle feeding on recently fallen green leaves may suffer from an impaction of the rumen, a condition formerly known in the English Midlands as 'wood evil', the symptoms of which include drowsiness, abdominal pain and constipation. The condition also results in reduced milk yields. There are no reports of human poisoning, but handling the foliage can cause dermatitis (Cooper & Johnson 1998).

A local study in England found young, fast-growing sapling Ash with pale-green leaflets were often heavily grazed by deer, hares and especially rabbits, that ate every leaf and gnawed the bark, sometimes ring-barking and killing plants. Slower-growing varieties in the same area, with more leathery, darker leaflets were less often savaged by herbivores and also seemed to be less susceptible to being burnt and blackened by late spring and early autumn frosts. These observations suggest that penalties for vigour may include both greater palatability and frost-tenderness (Oliver 1998).

Considering that Ash is a long-resident native tree, only a moderate number of invertebrates feed on it, a total of 68 species, approximately half of them macro-Lepidoptera and Heteroptera (Kennedy & Southwood 1984). Of these, 29 are specific to Ash (Rackham 2014). The database of the Biological Records Centre has a list of eleven insect and mite species for which Ash is the obligate food-plant. Presumably the relatively low number of feeding invertebrates is a direct consequence of the chemical defences of the species.

Epiphytes

On the other hand, Ash bark can be either acidic or alkaline and a total of 230 species of epiphytic lichens have been recorded on the tree, making it second only to oak in this respect (F. Rose, in: Morris & Perring 1974, p. 256, Table 3; Gilbert 2000).

Uses

Reproduction in Ash is virtually entirely by seed but the mature tree also displays remarkable powers of vegetative regrowth when cut to the ground. Ash responds well to being cut as coppice in this way, rapidly re-sprouting numerous straight poles from the cut crown that traditionally have been used as walking sticks and as staves for all kinds of building and support functions. "Throughout history Ash has generally been the second commonest recorded timber tree." (Rackham 1980). The uses go back well into pre-history, eg Ash and Hazel were much used in Neolithic trackways in the Somerset Levels (Rackham 1980).

Top quality Ash timber has great impact strength and elasticity which in past times made staves both suitable as weapons themselves, or as shafts for spears. This use is reflected in the English common name, 'Ash', which is derived from the Anglo-Saxon 'aesc', a poetic word meaning, 'a spear' (Milner 1992). The Anglo-Saxon 'aesc' could also mean, however, 'a vessel', meaning a boat, since Ash timber was also much used in boat building, although it is not suitable for the outer planking as it rots quickly in water (Prior 1879; Rackham 1980).

Ash wood was also used for the handles of more peaceful woodworking and gardening tools, and for the frames and shafts of vehicles. This latter use continued from early chariots to the Morris Traveller car, the Morgan sports-car, and even to aircraft construction.

Ash has the best burning qualities of any native tree timber, even when it is green or wet and Milner (1992, p. 101) quotes an interesting poem to the effect. There is a substantial body of folklore, superstition, divination and magic associated with Ash. It was one of the Irish 8th century, 'Nobles of the wood', and several authors go into the matter in more detail than there is room to include here (eg Grigson 1955, 1987; Milner 1992; Nelson & Walsh 1993; Vickery 1995).

Ash was also held to have herbal medicinal qualities and these and numerous other uses of the timber are described in detail by Grieve (1931).

Lifespan and senescence

Longevity of the standard (ie un-pollarded) Ash tree stretches to around 250 years at a maximum, with most trees less than 200 years (Rackham 1980; Nelson & Walsh 1993; Mitchell 1996; Thomas 2000). However, observation regularly confirms that rather stunted individuals are frequently overcome by evergreen Ivy (Hedera helix), which the light open canopy of the Ash tree is unable to shade out. These festooned trees readily fall victim to wind throw, probably when only about half their potential age or less. Old, senescent trees tend to drop major boughs as well as the perfectly normal small branch shedding from the crown that continues throughout the life of the individual specimen. These older trees then often begin to form basal epicormic shoots (ie dormant buds on the trunk producing new shoots, literally, 'upon the bark'). These vigorous shoots are almost, but not quite, suckering, invariably attached right at the base of the original tree, rather than spreading out around it underground like true suckers.

On the other hand, Ash stools and pollards, formed by regularly cutback either to ground level or trunk height of around 2 m, are capable of living indefinitely, being rejuvenated at every three or four year cut-down. Very old stools in ancient English woods can measure up to 3 m in diameter and most stools in this type of woodland are 1.5 m in diameter (Rackham 1980).

Declining tree size

In the not quite so distant past, there were some enormous historic elderly Ash trees in Ireland, including the famous, 'Great Ash of Leix', the circumference of which was measured in 1792 at one foot (30 cm) from the ground to be 40 feet 6 inches (equivalent to 12.46 m) (Nelson & Walsh 1993). Mitchell (1996) lists modern so called 'champion trees', but the largest extant Irish Ash (the Ballynatray Ash, Youghal, Co Cork) has a girth of only 8.5 m (26 ft). It is very misshapen, has split or decayed and is now represented by two rugged, hollow trucks. It does not appear to have been pollarded, which can prolong the lifespan (eg the Glen Lyon Ash in Perthshire, Scotland (Stokes & Rodger 2004)) and is estimated to be perhaps 400 years old. Two hundred years is a very good age for an Ash, while 300 years would be exceptional (Fennell 2013).

This observation of surviving old tress supports something also noticed with other species, such as Alder, that suggests, for unknown reasons, trees nowadays do not seem to achieve anything like the dimensions they previously did in historical or pre-historic times. Evidence for this arises from both old published records (which could be erroneous) and from preserved tree trunks recovered from bogs that unequivocally prove the point.

British and Irish status and occurrence

In the British Isles, F. excelsior is native and common throughout England, Wales and Ireland, although it becomes scarce or absent in more exposed coastal sites in W Ireland. In Scotland, it is regarded as native on limestone but here, as elsewhere, it is widely planted and it becomes rarer as one travels northwards (Preston et al. 2002).

Ash disease

Ash dieback first came to public notice in B & I in October 2012 when it was reckoned to be both 'new' and 'terrible' (Rackham 2014). The disease was not new; it had been making its way across Europe for the previous 20 years and should have been noted by those in charge of tree health attending International conferences on the subject. However, B & I representatives did not attend these meetings and they did not take heed of the impending arrival of the disease organism on these shores. The 'new' Ash disease is caused by a microscopic fungus called Chalara fraxinea. It invades the leaves and damages the tree by manufacturing a chemical called 'viridol' that is extremely toxic. In summer, the fungus in the leaves is in an asexual phase of its life-cycle and produces spores called conidia which form sticky masses and that spread with rain to infect the leaves of adjacent trees.

Like many other fungi, the organism has an alternation of generations and in autumn it turns into its sexual phase, a white cup-fungus fruit body about 3 mm across, called Hymenoscyphus pseudoalbidus or H. fraxineus that appears on the midrib of the ash leaves. This midrib or rachis falls separately from the leaflets that it normally bears. When it reaches the woodland floor, the cups of the fungus puff out little clouds of 'ascospores'; it is a type of fungus called an 'Ascomycete'. A single midrib may bear 20 cup-fungi each releasing 1,500 per hour for up to two weeks, ie several million spores from each Ash leaf! The spores are extremely light and float on the slightest breeze. They can travel to start infections on distant trees and when new leaves appear in the spring, the fungus reverts to the infectious Chalara phase, penetrates the tissues and continues the cycle within the tree.

Experience in continental Europe, being replicated in the UK in 2012, indicates that 'Chalara Ash dieback', as it is now referred to, can kill young and coppiced ash trees quite quickly. However, older trees are able to resist it for some time until prolonged exposure, or another pest or pathogen such as Armillaria (Honey Fungus), attacking them in their weakened state, eventually causes them to succumb (Rackham 2014). Needs a reference?

Infected trees can be distinguished from a distance by the thinning leaves on the crown and side branches, and bare, dead or dying, defoliated uppermost branches. However, shoot death and dieback can have other causes apart from Chalara attack and fruit hanging on the tree in autumn and winter can be mistaken for the blackened, dead leaves characteristic of the disease. Closer inspection shows four characteristics of the Chalara dieback: wilted, rigid, blackish-brown leaves from July to September; a blackish strip of cankerous bark (lesions) spreading up and down the twig; long, narrow dark cankers (lesions) where twigs join a larger branch or trunk with a dead twig in the middle; long, re-growth (epicormic) shoots springing from the branches, leading to bunched foliage, a phenomenon known as 'proliferation', giving a 'witches'-broom' appearance to the tree foliage (Rackham 2014; https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/ash-dieback-hymenoscyphus-fraxineus/, accessed 12 August 2022).

Chalara does not always kill infected trees. There is some genetically based resistance to the disease. Studies in Finland and Czechoslovakia suggest that some infected trees can recover, while others get worse from year to year and eventually die (Rackham 2014, p. 123). The dying process may take more than 20 years, depending on the age of the tree when it is infected. Ash disease was first noticed in or near Latvia around 1990 and has apparently spread across Europe from there. H. pseudoalbidus could be an introduction to Europe from E Asia, possibly from China or Japan, or it could be a newly arisen mutation or hybridisation of an existing fungus (Rackham 2014). Does that need a reference or is it covered by Rackham 2014?

There is currently no cure for Chalara Ash dieback, and no clear method for stopping its spread. Therefore, the aim of management, as outlined in the National Chalara Management Plan, should be to slow the spread, minimise the impact of the disease and preserve as many Chalara-tolerant ash trees as possible.

Trees in areas with high levels of public access need to be monitored carefully for risks to public safety, and some felling or pruning of dead or dying trees is advisable if risk assessments show they are a hazard. When assessing trees' health, it is important to look for signs of lesions (cankers) or Honey Fungus (Armillaria) near the base of the trunks since these can weaken the trunks and make trees more prone to fall.

European and world occurrence

F. excelsior is indigenous and widespread in W & C Europe from the Cantabrian coast to beyond 63ºN up the coastal fringe of Norway. From 61ºN on the Baltic coast, it stretches SE through C Russia (excepting the steppes), to Turkey, the Caucasus and parts of the Mediterranean – although there it mingles with other Fraxinus species (especially F. angustifolia) and the distribution of the species becomes unclear (Wardle 1961; Hultén & Fries 1986, Map 1489).

Names

The genus name 'Fraxinus' is the ancient classical Latin name of F. excelsior as used by Virgil, Ovid and others of the period (Gilbert-Carter 1964). The Latin specific epithet 'excelsior' means 'very tall' (Gledhill 1985). As with Beech (Fagus sylvatica), Ash contains relatively little variation, but it has produced a number of abnormal forms, some of which are regarded as of horticultural merit.

Threats

The dieback disease has been killing many young trees in recent years and there is little evidence of it being brought under control, or of any real hope of doing so. Some trees will prove resistant, however.