About Sorbus aucuparia L.
Sorbus aucuparia L. is a small tree or shrub that typically reaches 5 to 15 m (15 to 50 ft) in height. It has a wide, loose, roundish or irregular crown, and often grows multiple slender, cylindrical trunks that can reach up to 40 cm (15+1⁄2 inches) in diameter. Its branches extend outward and angle upward. Young bark is yellowish gray and glossy, and develops to gray-black with lengthwise cracks and small flaking as the tree ages. Elongated, bright ocher-colored lenticels are visible on the bark. Sorbus aucuparia rarely lives longer than 80 years, making it one of the shortest-lived trees in temperate climates. Its wood has broad reddish white sapwood and light brown to reddish brown heartwood; it is diffuse-porous, flexible, elastic, and tough, but not durable. Dried Sorbus aucuparia wood has a density of 600 to 700 kg/m3 (1,000 to 1,200 pounds per cubic yard). The plant produces wide, deep roots, is capable of root sprouting, and can regenerate after coppicing.
Sorbus aucuparia has pinnately compound leaves, with four to nine pairs of leaflets arranged along a round central stalk, plus a single terminal leaflet. Mature leaves grow up to 20 cm (8 in) long and 8 to 12 cm (3 to 4+1⁄2 in) wide. They have paired leaf-like stipules at the base of the petiole, and are arranged alternately along branches, which distinguishes them from the opposite, unstipulated leaves of ash (Fraxinus excelsior). Leaflets are elongated-lanceolate, 2 to 6 cm (3⁄4 to 2+1⁄4 in) long and 1 to 2.5 cm (1⁄2 to 1 in) wide, with sharply serrated edges. All but the outermost leaflets are either stalkless or have very short stems and sit close to the central vein. Newly sprouted leaflets are covered in gray-silvery hairs, but become mostly hairless after unfurling. The upper surface of leaflets is dark green, while the underside is grayish green and felted. Crushed young leaflets have a marzipan scent, and their base is asymmetrical. Foliage emerges in May and turns yellow in autumn, or dark red when grown in dry locations.
Sorbus aucuparia buds are usually longer than 1 cm (3⁄8 in), and are covered in flossy to felted hairs that fade over time; the hairs cover dark brown to black bud scales. Terminal buds are oval and pointed, and larger than the narrow, oval, pointed axillary buds, which sit close to the twig and often curve toward it. This species is monoecious: it reaches reproductive maturity at 10 years old, and produces abundant fruit almost every year. It flowers from May to June, and occasionally blooms again in September, producing many large, upright, bulging yellowish white corymbs that hold around 250 flowers each. Individual flowers are 8 to 10 mm in diameter, with five small, triangular yellowish green sepals that may be hairless or hairy. They have five yellowish white round or oval petals, up to 25 stamens fused to the corolla to form a hypanthium, and an ovary with two to five styles; the style is fused to the receptacle. Flowers have an unpleasant trimethylamine odor, and their nectar is high in fructose and glucose.
The fruit of Sorbus aucuparia are round pomes 8 to 10 mm (5⁄16 to 3⁄8 inch) in diameter, that ripen between August and October. Unripe fruit are green, and typically turn orange or scarlet when ripe. The sepals remain on ripe fruit as a black five-pointed star. A single corymb holds 80 to 100 pomes. Each pome contains a star-shaped ovary with two to five locules, each holding one or two flat, narrow, pointed reddish seeds. Fruit flesh contains carotenoids, citric acid, malic acid, parasorbic acid, pectin, provitamin A, sorbitol, tannin, and vitamin C, while seeds contain glycoside. On average, fruit persist for 100.6 days, contain 2.5 seeds each, are 73.0% water by weight, and their dry weight is 8.9% carbohydrates and 3.1% lipids. The species has a chromosome number of 2n=34.
There are five recognized subspecies of Sorbus aucuparia: Sorbus aucuparia subsp. aucuparia, found across most of the species' range and less common in the South; Sorbus aucuparia subsp. fenenkiana (Georgiev & Stoj.), which has thin, sparsely hairy leaflets and depressed-globose fruit, and is restricted to Bulgaria; Sorbus aucuparia subsp. glabrata (Wimm. & Grab.), which has less hair, found in Northern Europe and Central European mountains; Sorbus aucuparia subsp. praemorsa (Guss.), which has hairy leaflets and ovoid fruit, found in Southern Italy, Sicily, and Corsica; and Sorbus aucuparia subsp. sibirica (Hedl.), which is nearly hairless, found in North Eastern Russia.
Sorbus aucuparia is native to almost all of Europe and the Caucasus, extending north to Northern Russia and Siberia. It is not native to Southern Spain, Southern Greece, Sardinia, the Balearic Islands, the Azores, and the Faroe Islands. It was introduced to North America as an ornamental species, and has naturalized very successfully from Washington to Alaska, across Canada and in the northeastern United States. It grows from plains up to the tree line, where it is the only deciduous tree species growing among krummholz. In the Alps it grows at elevations up to 2,000 m (6,600 ft). It occurs north of the boreal forest at the arctic tree line, and in Norway it grows as far north as the 71st parallel north.
Sorbus aucuparia is an undemanding species that can tolerate shade, frost, winter dryness, and a short growing season. It is resistant to air pollution, wind, and snow pressure. It most often grows on moderately dry to moderately damp, acidic, low-nutrient, sandy, loose soil, and also occurs frequently in stony soil, clay, sandy soil, or wet peat. It grows best on fresh, loose, fertile soil with average humidity, and cannot tolerate saline soil or waterlogging. It is found in all types of open woodland, grows as a pioneer species on fallen dead trees, in clearcut areas, along forest edges, and beside roads. Its seeds germinate easily, so it can also establish on inaccessible rock, ruins, branch forks, and hollow trees. Exceptionally large specimens have been recorded: the tallest known individual in the United Kingdom grows in the Chiltern Hills of South East England, reaching 28 m (92 ft) tall with a 56 cm (22 in) trunk diameter; an unusually large specimen near Wendisch Waren, Mecklenburg-Vorpommern, Germany, is over 15 m (49 ft) tall, around 100 years old, with a 70 cm (28 in) trunk diameter; the tallest known specimen in Ireland is 18 m (59 ft) tall at Glenstal Abbey, County Limerick.
Sorbus aucuparia is pollinated by bees and flies. Its seeds pass intact through the digestive tracts of birds, which disperse the species through their droppings. Around 60 bird species and several mammal species eat its fruit, with thrushes and other songbirds particularly favoring it; other consumers include cloven-hoofed game, red fox, European badger, dormouse, squirrel, and migratory winter birds such as Bohemian waxwing, spotted nutcracker, and redwing. Cloven-hoofed game also heavily browse its foliage and bark. The plant's roots form symbioses with arbuscular mycorrhizal fungi, and less commonly with ectomycorrhizal fungi. It is usually eventually outcompeted and replaced by larger forest trees. In Central Europe, it often grows in association with red elderberry, goat willow, Eurasian aspen, and silver birch. It is highly flammable and does not tend to accumulate plant litter.
Other Sorbus species easily hybridize with Sorbus aucuparia, which can lead to hybrid speciation. Known hybrids include Sorbus × hybrida, a small tree with oval serrated leaves and two to three pairs of leaflets that is a hybrid with Sorbus × intermedia, and S. thuringiaca, a medium-size tree with elongated leaves and one to three pairs of leaflets that are sometimes fused along the central vein, which is a hybrid with S. aria. The main pests of Sorbus aucuparia are the apple fruit moth Argyresthia conjugella and the mountain-ash sawfly Hoplocampa alpina. The rust fungus Gymnosporangium cornutum forms leaf galls on the plant. While mature leaves are not palatable to insects, they are used by insect larvae including the moth Venusia cambrica, the case-bearer moth Coleophora anatipennella, and leaf miner moths of the genus Stigmella. The snail Cornu aspersum feeds on its leaves, and the plant can develop fire blight.
