A genus of between twenty and thirty species of deciduous (rarely subevergreen) trees, some of the largest size, natives of the northern hemisphere in both the Old and New Worlds, but not occurring in N. America west of the Rocky Mountains. Leaves alternate, toothed, usually oblique and sometimes very unequal at the base. Flowers produced in clusters or short racemes from axillary buds, either on the naked shoots in early spring, or on the leafy ones in autumn. They are very small, consisting of a green or red-tinged perianth with four to nine (usually about five) lobes. Stamens usually equal in number to the perianth-lobes, their purplish or reddish anthers being usually the most conspicuous feature of the flower. The fruit is most characteristic, being a flat, membranous, semi-transparent disk called a samara, enclosing the single seed in a cavity at the centre or towards the apex, where it is slightly or deeply notched.
On a healthy, unlopped adult tree the greater part of the foliage is borne on short-shoots of determinate growth, and the leaves on these, in particular the upper ones (distal and subdistal leaves) characterise the species, variety or hybrid. Leaves on suckers and water-sprouts (epicormic shoots) are of a juvenile type, and often very different from the adult ones, not only in shape and size but also in indumentum, being often hairy even when the adult leaves are glabrous. Also uncharacteristic are the leaves of lammas-shoots and on ‘proliferating’ shoots.
The genus has been grouped into sections largely on the basis of flowering-time, and on characters of the perianths, inflorescence and samaras. Foliage does not serve to distinguish the sections reliably.
sect. Ulmus (Madocarpus). – Flowers short-stalked, borne in clusters before the leaves expand. Perianth with four to seven equal lobes. Samaras rarely ciliate. The most important group. Apart from the N. American U. rubra all are natives of the Old World. Those treated here are:
Europe, S.W. and C. Asia: U. carpinifolia (also in N. Africa) and U. angustifolia, U. coritana, U. plotii, included in U. carpinifolia by some authorities; U. elliptica; U. glabra; U. procera; U. pumila (also in N.E. Asia and the inner Himalaya).
Himalaya: U. villosa; U. wallichiana (for other Himalayan elms see Melville and Heybroek, op. cit. in Bibliography).
E. Asia: U. davidiana and the closely allied U. japonica; U. laciniata; U. macrocarpa; U. wilsoniana.
sect. Blepharocalyx. – Flowers on slender stalks of unequal length, opening before the leaves expand. Perianth deeply divided. Samaras ciliate.
Ulmus americana (N. America); U. laevis (W. Eurasia).
sect. Chaetoptelea. – Flowers before the leaves, on slender stalks, arranged in racemes. Samaras downy and ciliate.
Two N. American species: U. alata; U. thomasii.
sect. Microptelea. – Flowers opening in autumn. Perianth deeply divided. Leaves rather leathery, falling late. Samaras glabrous.
Ulmus parvifolia (E. Asia); U. crassifolia (N. America).
sect. Trichoptelea. – Flowers opening in autumn as in the preceding section, but borne in pendulous racemes. Samaras hairy.
Ulmus serotina (N. America).
The present revision has been prepared under the shadow of the new and lethal outbreak of the Dutch elm disease, which has killed many of the elms that ornamented the parks and gardens of this country and, far worse, destroyed or defaced some of our loveliest rural landscapes. The contents of the present treatment are substantially as they were in previous editions, though it must be sadly acknowledged that many of the elms described, including some of the largest and finest, will never be planted again, and it is only to be hoped that resistant substitutes will eventually become available. The taxonomy of the British elms of the U. carpinifolia complex is a controversial matter, but it is one that in the main concerns countryside elms rather than those planted for ornament. The taxonomic position of a cultivar is also a matter of dispute in many cases, but controversy can here usually be avoided by placing the cultivar-name directly under Ulmus (cf. P. S. Green, op. cit., in Bibliography). For example, the name of the Jersey elm – ‘Sarniensis’ – could be placed under U. minor (if that is to be taken as the earliest name for the field elm), U. carpinifolia, U. angustifolia or U. × sarniensis. This instability of nomenclature is avoided if it is called Ulmus ‘Sarniensis’. The same device can be used in presenting the names of the many probably hybrid cultivars that have in the past been placed under U. procera or U. carpinifolia, or confusingly shuttlecocked between them. See also U. × hollandica.
Bancroft, Helen. – ‘Notes on the Status and Nomenclature of British Elms’, Gard. Chron., Vol. 96 (1934), pp. 122, 139, 208, 244, 298, 334, 372.
Clapham, A. R., Tutin, T. G., and Warburg, E. F. – Flora of the British Isles, ed. 1 (1952), pp. 715-24.
Elwes, H. J., and Henry, A. – Trees of Great Britain and Ireland, Vol. VII (1913), pp. 1847-1929.
Fontaine, F. J. – Dendroflora No. 5 (1968), pp. 37-55.
Green, P. S. – ‘Registration of Cultivar Names in Ulmus’, Arnoldia, Vol. 24 (1964), pp. 41-80.
Hadfield, Miles. – British Trees (1957), pp. 226-47.
Hillier and Sons. – Hillier’s Manual of Trees and Shrubs, Ed. 3 (1973). pp. 398-402.
Jackson, A. B. – ‘The British Elms’, New Flora and Sylva, Vol. 2 (1930), pp. 219-29.
Jobling, J., and Mitchell, A. F. – Field Recognition of British Elms. Forestry Commission Booklet 42 (1974).
Meikle, R. D. – British Trees and Shrubs (1958), pp. 148-55.
Melville, R. – Ulmus, in Stace, ed., Hybridisation and the Flora of the British Isles (1975), pp. 292-9.
Other studies by Dr Melville are cited in the text.
Melville, R., and Heybroek, H. M. – ‘The Elms of the Himalaya’, Kew Bulletin, Vol. 26 (1971), pp. 5-28.
Mitchell, A. F. – A Field Guide to the Trees of Great Britain and Northern Europe (1974), pp. 247-54 and pl. 22.
Moss, C. E. – ‘British Elms’, Gard. Chron., Vol. 51 (1912), pp. 199, 216, 234.
– – The Cambridge British Flora (1914), Vol. II, pp. 88-96.
Richens, R. H. – ‘Studies on Ulmus’. A series of regional studies: Part I (E. Anglia), Watsonia, Vol. 3 (1958), pp. 138-53; Part II (S. Cambs), Forestry, Vol. 31 (1959), pp. 132-46; Part III (Herts), ibid., Vol. 32 (1959), pp. 138-54; Part IV (Hunts), ibid., Vol. 34 (1961), pp. 47-64; Part V (Beds), ibid., Vol. 34 (1961), pp. 181-200; Part VI (Fenland), ibid., Vol. 38 (1965), pp. 225-35; Part VII (Essex), ibid., Vol. 40 (1967), pp. 185-206.
– – ‘Variation, Cytogenetics and Breeding of the European Field Elm’, Annales Forestales (Anali za Sumarstvo), Vol. 7 (1976), pp. 107-41. An invaluable survey, with an extensive bibliography.
Ross-Craig, Stella. – Drawings of British Plants, Part XXVII (1970), Ulmaceae, plates 1-6.
Schneider, C. – Plantae Wilsonianae, Vol. III (1916), pp. 238-65.
Wyman, D. – Trees for American Gardens, Ed. 2 (1965), pp. 461-73.
Wilkinson, G. – Epitaph for the Elm. 1978.
DUTCH ELM DISEASE
One of the most important factors influencing the elms in Britain in the 20th century has been Dutch elm disease. This disease spreads rapidly through elm populations causing widespread wilting and death of trees. It was initially found in Western Europe soon after the first World War and was given its name following the invaluable early research carried out in Holland. It was first positively identified in Britain at Totteridge in Hertfordshire in 1927 but was almost certainly present some years earlier. The disease rapidly reached epidemic proportions in southern England and during the 1930s caused widespread deaths of most species of elm growing in the region. By 1936 the epidemic had reached its peak and thereafter the disease declined both in numbers of trees affected and in the severity of the symptoms on individual trees. During the subsequent two decades local flare-ups occurred but the disease came to be regarded as an endemic problem of no great consequence.
However in the late 1960s fresh serious outbreaks of Dutch elm disease were reported from various localities in southern England. It soon became clear that a second epidemic had started on a greater scale than in the earlier outbreak. Surveys of disease incidence were instituted in 1971 in southern Britain and by 1977 about 50 per cent of the total elm population of 23 million elms present at the beginning of the survey period had been killed.
When the disease was first identified in Europe there was considerable argument as to its cause; however it soon became clear that a fungus, now called Ceratocystis ulmi (Buism.) C. Moreau, was in fact responsible. It took several years before further research demonstrated that the elm bark beetles Scolytus scolytus (Fab.) and Scolytus multistriatus (Marsh.) were the vectors which transmitted the disease from dead or dying elms to healthy trees. These bark beetles emerge from the bark of diseased elms carrying spores of C. ulmi on their bodies and often feed in the crotches of twigs on healthy elms. The feeding wounds can thus become infected with the disease and it subsequently spreads through the vascular system of the tree. The characteristic symptoms of the disease, including wilting, yellowing or browning of leaves and dieback of parts or the whole of the crown then appear. At the same time, dark brown streaks often develop in the outermost annual ring of the stem and these provide a useful diagnostic feature which can be seen when the bark is stripped away or the stem cut across.
The disease cycle is completed when the mature bark beetles invade the bark of diseased elms in order to breed. Egg-laying female beetles, together with the larvae which subsequently develop, tunnel out characteristic insect galleries within the bark. The disease can also be transmitted from one tree to its neighbour through connecting roots and this method of spread is particularly important in hedgerow trees.
Research undertaken during the course of the second epidemic in the 1970s has revealed much information about the origin of this outbreak and of the variability of the causal fungus. It was discovered that a very pathogenic, aggressive strain was responsible for the new outbreaks whereas a different, non-aggressive strain occurred in locations where infection had survived from the earlier epidemic. Detailed studies have demonstrated that the aggressive strain was probably introduced into Britain on elm logs imported from Canada and that this strain was responsible for the second and more devastating epidemic.
Much research has been directed towards finding methods of controlling the disease including trials with insecticides to kill the bark beetles and with fungicides to protect the trees from fungal invasion. None of these materials have so far proved particularly effective and the only proven method of slowing down the spread of the disease has been by sanitation felling. Experience both in the United States and in Britain has shown that an intensive programme of felling diseased trees and subsequent destruction of the bark in which the bark beetles breed can keep the disease at a relatively low level. Such sanitation felling programmes must however be initiated at an early stage in the epidemic for the task can soon become too great for the available resources.
In the longer term it may be possible to select or breed varieties of elm which are resistant to the disease and which are also suitable for planting as landscape or urban features. Resistant elms have been recognized, particularly among the Asiatic species, and breeding programmes are now in progress in Holland and USA. However, it may be some years before elm hybrids or cultivars, resistant to Dutch elm disease in its aggressive form, are available for widescale commercial planting.
Contributed by Dr D. A. Burdekin, Forest Research Station, Forestry Commission, Alice Holt.
From the Supplement (Vol. V)
A notable addition to the bibliography (page 636) is:
Richens, R. H. – Elm. Cambridge, 1983. With 152 text illustrations, many of them reproductions of paintings and engravings. The fruit of some thirty years of research, this work deals with the elms of Britain in all their aspects, with chapters on botany, history from the earliest times, vernacular names, artistic and literary associations etc., and also surveys the elms of England county by county. It is sad that this fine work should appear at such an inauspicious time.
Dutch Elm Disease. Dr D. A. Burdekin has provided the following amendments to bring his contribution up-to-date:
Page 637 para 2. The last sentence should read: Surveys of disease incidence were instituted in 1971 in southern Britain and by 1980 over 90 per cent of the total elm population of 23 million elms present at the beginning of the survey had been killed.
Page 638 para 3. The second sentence should now be replaced by: None of the insecticides has proved particularly effective, though one fungicide (thiabenzadole) injected into individual trees has shown considerable promise for the protection of, and to some extent cure of disease in, valuable specimens. Nevertheless, the only proven way of slowing down the spread of the disease on an area basis has been by sanitation felling.
Page 638 para 4. The last sentence should read: Although one or two disease-resistant clones (such as ‘Sapporo Gold’) are available, it may be some years before elm hybrids and cultivars, resistant to Dutch elm disease in its aggressive form, are planted on a wide scale.