Wych elm (Ulmus glabra) is a relatively scarce deciduous tree which supports abundant insect life in the forest, but is at risk due to the spread of Dutch elm disease.
Wych elm occurs throughout much of Europe, but is mainly concentrated in the more northern regions. It ranges from north of the Arctic Circle in Norway to the mountains of northern Spain and the Peloponnese Peninsula in Greece, and from Ireland east to the Ural Mountains in Russia. It also occurs in the Hyrcanian Forest on the northern slopes of the Alborz Mountains in Iran, facing the Caspian Sea. In the southern parts of its range it grows in montane situations, at elevations up to 1,500 metres, and most commonly is found on moist sites with high humidity, such as ravines. Wych elm is now recognised as the only species of elm that is definitely native to Britain.
Distribution in Scotland
Wych elm occurs throughout Scotland, but was originally more common in the south east Borders area, where there were formerly forests of wych elm – nowadays it is mainly confined to the borders of fields there. It grows naturally as far north as Sutherland and also on Skye, and has been planted in a few places on the Orkney and Shetland Islands. It is comparatively scarce in the pinewood areas of the Caledonian Forest, with only a handful of trees in glens such as Affric and Strathfarrar, but is likely to have been more common in the past, in sheltered areas with better soils. Its Gaelic name of Liobhann is anglicised as Leven, and gives its name to the two Loch Levens – one in Kinross and the other near Ballachulish in Lochaber – thereby giving an indication of the tree’s geographic spread. The range and abundance of wych elm in Scotland has been reduced in recent decades by the spread of Dutch elm disease.
Wych elm is a large deciduous tree in the Ulmaceae family, and in good situations it can reach a height of 40 metres, although it doesn't grow that tall in the wild in Scotland (the UK Champion wych elm, at Brahan in Inverness-shire, was 25.2 metres tall in 2002). It often has multiple large branches coming off the main trunk quite low down on the tree, which give it a broad, spreading crown. The bark is initially pale grey in colour, and is smooth when young, thereby giving the tree the second part of its binomial scientific name, glabra. The bark darkens and becomes rougher and fissured as the tree ages. On a mature tree, the trunk and main branches often have burls or burrs on them. These abnormal growths are caused by the swelling of dormant buds in the tree's cambium layer, usually due to stress conditions.
The leaves grow in an alternate pattern on the branches and are variable in size, ranging from 7 – 17 cm. in length and up to 10 cm. in width. They have an asymmetrical base and their margins are double-toothed, with both large and small teeth. The leaves are bright green when they first appear, becoming duller over time, and they are hairy and rough to the touch. The new leaves appear in April, and turn a bright yellow colour before being shed in October.
Wych elm can reportedly live for 400 years, although most trees in Scotland today are much younger than that.
Wych elm is similar to hazel (Corylus avellana) in that its flowers appear in early spring, well before the new leaves open out. The flowers are dark reddish-purple in colour, 4 mm. in size, and produced in clusters of 10 to 20, spaced out along the twigs and small branches. They are hermaphroditic, meaning that the organs for both sexes occur in each flower, and pollination is by the wind.
Pollinated flowers grow to become fruits called samaras, which are ovoid in shape with a small notch at the end, and are up to 2 cm long and 1.5 cm across. A samara is a type of fruit in which the seed has a flattened wing attached to it, and in the case of wych elm the seed is in the centre of the samara, and the wing surrounds it on all sides. The samaras are initially a light, bright green colour, turning brown as they ripen, and are shed from the tree in early July. Dispersal of the seeds is by the wind, and the wing of each samara enables them to travel further than the unwinged seeds of other trees.
Unlike other elms, wych elm does not normally reproduce via suckers, only doing so exceptionally, in conditions of stress. It does however produce adventitious growth – new shoots that sprout from the base of the main trunk, and if these are not browsed they will grow to become new branches.
Wych elm is similar to bird cherry (Prunus padus) in that, unlike most other trees, it has virtually no fungi that produce fruiting bodies in the form of mushrooms growing in mycorrhizal association with it. The April pinkgill fungus (Entoloma aprile) is one of the few that have been recorded with elms. There are however a number of inconspicuous fungi that form a vesicular-arbuscular mycorrhizal or endomycorrhizal association with wych elm, in which the fungal hyphae penetrate the cell walls of the tree's roots, so that an exchange of nutrients takes place. This symbiotic relationship provides benefits to both organisms, with the tree gaining nutrients that the fungus accesses in the soil, while the fungus in turn receives sugars and carbohydrates that the tree produces through the process of photosynthesis.
About 40 saprotrophic or dead wood feeding species of fungi have been recorded on wych elm, including chicken of the woods (Laetiporus sulphureus) and candlesnuff fungus (Xylaria hypoxylon). The most notorious fungi associated with wych elm are the two closely-related pathogenic species, Ophiostoma ulmi (formerly Ceratocystis ulmi) and Ophiostoma novo-ulmi , which cause Dutch elm disease. These fungi are spread by two beetles, the European elm bark beetle (Scolytus multistriatus) and the large elm bark beetle ( Scolytus scolytus ), and are fatal to elms. The initial symptoms are dieback in the crown, which spreads to the entire tree, and most elms in England died in the 1970s and 1980s as a result. There are some indications that wych elm is slightly less susceptible to the disease than English elm (Ulmus procera), and as the former is the common elm in Scotland, the effects have been less extreme north of the border. However, the disease is still spreading, particularly on the east coast, and wych elms in Nairn are now infected. The prognosis for wych elms further west and in remoter parts of the Highlands is uncertain, but it is likely the disease will spread further, possibly exacerbated by warmer temperatures due to human-induced climate change.
Wych elm supports a large variety of insects, and a report in Ireland listed 82 species of invertebrates that feed on the tree. It is a good all round tree for moths, supporting a range of common species that feed on various broadleaved trees. These include the Hebrew character (Orthosia gothica), common quaker (Orthosia cerasi) and species with twig- mimicking caterpillars, such as the scalloped hazel (Odontopera bidentata) and the peppered moth (Biston betularia). Two moths that specialise in feeding on wych elm are the dusky-lemon sallow ( Xanthia gilvago ), whose larvae feed on the ripening seeds in late spring, and the clouded magpie (Abraxas sylvata), but in Scotland that is confined to the southwest of the country. Caterpillars of the brick moth (Agrochola circellaris) also feed on wych elm seeds, but it is a polyphagous species, meaning that it feeds on more than one species of tree. The white-letter hairstreak butterfly (Satyrium w-album) feeds exclusively on elms, including wych elm, but in the UK it is confined to England and Wales, and does not occur in Scotland.
Other species recorded feeding on wych elm include the common leaf weevil (Phyllobius pyri) and the elm tree leafhopper (Ribautiana ulmi).
The larvae of several micro-moths in the genera Coleophora , Phyllonorycter and Stigmella make mines in the leaves of wych elm, as do the larvae of a sawfly (Fenusa ulmi). A number of different species induce galls in the leaves of wych elm, including a midge (Physemocecis ulmi) that causes blister galls, an aphid (Tetraneura ulmi) which causes fig-like galls to develop on the upper surface of the leaves and a fungus (Taphrina ulmi).
Another aphid (Schizoneura ulmi) is responsible for leaf curl galls, which develop on the leaves of the adventitious shoots that grow at the base of the trunk – the galls apparently do not occur on leaves higher up on the tree. This aphid is preyed upon by the elm gall bug (Anthocoris gallarum-ulmi) and a rare hoverfly (Pipiza luteitarsis). The hoverfly in turn is parasitised by an ichneumon wasp (Campocraspedon caudatus), which is rarer still. This indicates the complexity of the ecological relationships associated with wych elm, and what may be lost if Dutch elm disease continues to spread.