Rowan (Sorbus aucuparia) is a fast-growing pioneer tree in the Caledonian Forest, characterised by its brilliant red berries at the end of summer.
Rowan occurs widely throughout Europe, in western Asia in Russia and the Caucasus region, and in north Africa in the mountains of Morocco. Throughout its range it occurs in a variety of habitats, but it commonly grows in mountains and has been recorded at elevations of up to 2,000 metres in France. Together with downy birch (Betula pubescens) and some willows (Salix spp.), it grows at the most northerly limit of trees in Europe, at latitude 70° north, in Finnmark in Norway.
Five subspecies of rowan are recognised by scientists, and three of these have restricted distributions – one is limited to Bulgaria, another to northeastern Russia and a third to southern Italy, Sicily and Corsica. The other two subspecies are more widely distributed throughout the tree’s range in Europe.
Distribution in Scotland
Rowan grows in most parts of Britain, but is more common in the north and west, and is found throughout Scotland. It grows at a higher altitude than any other tree in the country and occurs at elevations of almost 1,000 metres in parts of the Highlands.
This attribute, together with the similarity of its leaves to those of the ash tree (Fraxinus excelsior), gives rise to its alternative common name of mountain ash. At higher elevations it survives as small saplings which are frequently stunted in form.
In Scotland today, rowans are often found growing in inaccessible locations, such as cliffs, steep stream-sides and on top of large boulders. However, these are not the preferred locations for the species, but rather are the only places where it has been able to grow out of reach of herbivores such as red deer (Cervus elaphus) and sheep.
Rowan is a fast-growing, short-lived pioneer tree in the rose family, Rosaceae. It is typically a small tree, reaching a maximum height of 10-15 metres, or exceptionally, 20 metres. It is slender in form, although mature trees can be quite substantial – an old rowan at Carnach Mor on the West Affric Estate has a trunk which is over 40 cm. in diameter. Multi-stemmed forms are quite common, as a result of browsing by mammals and the subsequent production of basal shoots. The greyish-brown bark is smooth and shiny when wet, with dark raised dots or lenticels scattered across it. The branches are typically upward-pointing and terminate in ovoid, purplish buds, which are often covered in grey hairs.
Rowan leaves are compound and pinnate in form, meaning that each leaf is made up of matched pairs of leaflets on either side of a stem or rachis, with a terminal leaflet at the end. Leaves are up to 20 cm. in length, and are comprised of 9-15 leaflets, which are serrated with small teeth. Rowan is a deciduous tree, with the new leaves appearing in April, and they turn a bright orange-red colour in autumn before being shed.
The flowers blossom after the leaves have appeared, usually in May or early June, and are creamy-white in colour. Individual flowers are about 1 cm. in diameter and they grow in dense clusters or corymbs, each containing up to 250 flowers, and measuring 8-15 cm. across. The strong, sweet scent attracts pollinating insects, including many species of flies, bees and beetles.
The fertilised flowers grow into berries which are 8 mm. in diameter and these ripen to a bright red colour in August or early September. The berries are rich in ascorbic acid (vitamin C), and contain up to 8 small seeds, although 2 seeds per fruit is most common. They are eaten primarily by birds, who disperse the seeds in their droppings.
Seed production begins when the tree is about 15 years old, and in mild climates, rowan will fruit each year. However, in harsher environments such as Glen Affric, fruiting is irregular, and mast seed production, when all the trees produce a heavy crop, occurs every few years, with very little fruiting taking place in between. The tough coat of the rowan seed requires cold weather to break down, and germination usually occurs in the first or second spring after the berries have been produced. Seedlings and saplings are shade-tolerant, and are often found under the branches of large Scots pines (Pinus sylvestris), where they have grown in the droppings of birds which perched on the branches above.
For similar reasons, rowans also germinate in the forks of the trunks of pines and other trees, but in most cases there is insufficient organic matter for these seedlings to grow to more than a metre or two in size. Exceptions to this do occur, and near Glac Daraich in Glen Affric there is a good example of a rowan which became established about two metres up an alder (Alnus glutinosa) and grew into a mature tree, partly embracing the alder, after its roots reached the soil below.
Rowan is an integral part of the Caledonian Forest, where it grows in association with Scots pines, and it also occurs in oak (Quercus spp.) and birch ( Betula spp. ) woodlands. It rarely forms single-species stands of its own.
A variety of mycorrhizal fungi are associated with rowan, including an arbuscular mycorrhizal fungus (Glomus intradices) and an unusual ectomycorrhizal fungus (Cenoccum geophilum). In the symbiotic relationships formed by these fungi and the tree, both the partners benefit through an exchange of nutrients which each organism cannot access directly itself.
A rust fungus ( Gymnosporangium cornutum ) which infects juniper (Juniperus communis) spends its aecial or spore phase on rowan, where it produces galls which take the form of yellowish pustules on the upper surface of the leaves.
Rowan is a good host tree for lichens, and is the second best in the UK for Graphidion lichens (those which grow on smooth-barked trees) after hazel (Corylus avellana). Tree lungwort (Lobaria pulmonaria) is common on rowan trunks in areas with a wet climate or constant humidity, while a recently identified lichen (Arthothelium dictyosporum) which is endemic to Scotland occurs mainly on rowan.
The foliage is palatable and highly attractive to browsing animals, and red deer will eat it in preference to most other tree species in the Caledonian Forest. Mountain hares (Lepus timidus) eat the leaves, and deer also feed on the bark and stems.
In contrast to many other broadleaved trees, the leaves are not palatable to phytophagous (ie plant-eating) insects, so comparatively few of them are associated with rowan.
However, the larvae of several species of leaf-mining moths (Stigmella spp.) make mines in rowan leaves, and the caterpillars of the Welsh wave moth (Venusia cambrica) feed on the leaves. Larvae of the apple fruit moth (Argyresthia conjugella) are frequently found in the berries. A snail (Helix aspersa) has been shown to feed on the leaves, whilst a beetle (Byturus fumatus) feeds on the flowers, often completely eating the stamens.
The berries are eaten by a variety of birds in the forest, including chaffinches (Fringilla coelebs) and siskins (Carduelis spinus), while in towns and rural areas blackbirds (Turdus merula) are the main seed dispersers. Fieldfares (Turdus pilaris) and redwings (Turdus iliacus) time their migrations from Scandinavia to the UK to coincide with the availability of rowan berries, and in good fruit years, flocks of them will descend on the trees as they pass to the south. On their way, they will disperse the seeds, thereby enabling a new generation of young rowans to grow and take their place as a distinctive and beautiful feature of the Caledonian Forest.