Although it is relatively scarce, bird cherry’s (Prunus padus) spectacular spring flowering display makes it one of the most flamboyant trees in the Caledonian forest.
Global distribution
Bird cherry has a wide geographic distribution in Europe and northern Asia, and has also been reported from Morocco in North Africa. Its range in Europe extends from northern Scandinavia, where it reaches the shore of the Arctic Ocean, to the northwest of Italy and the mountains of Spain and Portugal, although it is absent from the Mediterranean coastal region and the west of France. It occurs from Ireland and the UK eastwards to Croatia and Bulgaria, and then through northern Russia, the Caucasus region and the Himalayas to western Siberia.
Bird cherry has been introduced to North America, where it occurs in Alaska and parts of the northeastern USA and eastern Canada, but it is not considered to be a problematic or invasive species there.
Distribution in Scotland
Bird cherry is common and widespread in Scotland, occurring throughout the mainland, although it is scarcer in the mountainous core of the Highlands and in the far southwest. It is found on Skye and Mull, but is absent from the Outer Hebrides and Orkney and Shetland.
Bird cherry grows at elevations of up to 600 metres in the UK, but does not reach so high in Scotland’s mountains, because of the harsher climate. South of the border, it is common in northern England, Wales and parts of East Anglia, but is absent from the south of the country. Its close relative, the wild cherry or gean Prunus avium), is the only other cherry native to the UK, and has a similar distribution, but also occurs in the south of England.
The preferred habitat of bird cherry is damp, base-rich soils or on limestone, and it often occurs in wet flushed areas, frequently with alder trees (Alnus glutinosa). It also grows alongside roads and paths, and is usually seen growing amongst other native broad-leaved trees such as birch (Betula spp.) and rowan (Sorbus aucuparia).
Bird cherry is a relatively small deciduous tree in the Rosaceae family, and in Scotland it is also known as hagberry or hackberry, with its fruits being called ‘hags’. It can grow to a height of 15 metres, but in Scotland it’s usually only up to 10 metres tall. It branches readily, forming a spreading and sometimes multi-trunked shape, and has smooth grey-brown or dark grey bark.
The leaves grow in an alternate pattern on the branches, and are 6 – 10 cm. long. They are lanceolate (meaning that they are elliptical in shape, tapering to a pronounced point at the end) and are finely toothed or serrated on their margin. In the Highlands, the new leaves appear in late April and are dark green on the upper surface and slightly lighter in colour on the underside. There are two small nectar glands on the petiole, or leaf stem, at the base of the leaf, and these exude a chemical that attracts beneficial insects, in response to attacks by pests such as greenfly.
In October the leaves turn a yellow-orange colour before being shed, and the sap is drawn downwards, as the tree enters a state of dormancy for the winter. Bird cherry can live up to an estimated maximum of 200 years, but most trees in the Highlands today are younger than that.
The tree’s flowers open from greenish-yellow buds in May or early June and grow on upright spikes or racemes that are 15 cm. long. The racemes don’t always stay upright for long though, and each one contains up to 35 individual blossoms. The flowers have 5 white petals, with pale yellow stamens, are 1 cm. in size and smell of almonds. Pollination is carried out by insects such as bees, butterflies and beetles that are attracted by the fragrance of the blossoms.
After pollination, fertilised flowers develop into fruits that are spheroid in shape and 8 mm. in diameter. These ripen from red to black in August and contain a hard stone or pit, surrounded by fleshy fruit that is very bitter. The seeds are dispersed by birds that eat the fruit, and by small mammals that consume fallen fruits.
Unlike most other trees, bird cherry has relatively few fungi that grow in mycorrhizal association with it. There are however a number of inconspicuous fungi in the order Glomerales that form a vesicular-arbuscular mycorrhizal or endomycorrhizal association with bird cherry, 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.
Other fungi occurring with bird cherry include coral spot (Nectria cinnabarina), which grows on dead twigs, and saprotrophic, or dead-wood decomposing, species such as turkeytail fungus (Trametes versicolor) and candlesnuff fungus (Xylaria hypoxylon). Bird cherry is particularly susceptible to infection by honey fungus (Armillaria mellea), a parasitic species that weakens and kills its host.
In the UK, at least 30 species of insects have been recorded in association with bird cherry, including 10 different moths and 10 species of beetles. Amongst the beetles is a blossom-feeding weevil (Anthonomus rectirostris) that has been found on Dundreggan. Of the moths that occur on bird cherry, the most common is the bird-cherry ermine (Yponomeuta evonymella). The larvae or caterpillars of this species feed on the leaves inside communal silken tents that they weave, and can reach pest proportions, completing stripping a tree. However, the caterpillars are parasitised by the larvae of a predatory sarcophagid fly (Agria mamillata), and this acts as a natural control on their population in some cases.
Adults of the aspen hoverfly (Hammerschmidtia ferruginea), a rare saproxylic insect that breeds in the wood of dead aspens and is a Priority Species in the UK’s
Galls, in the form of raised pustules, are induced on the leaves of bird cherry by the larvae of a mite (Phytoptus padi), while the bird cherry-oat aphid (Rhopalosiphum padi) induces galls that cause the leaves to curl and wrinkle. This aphid over-winters on bird cherry but the adults can then become a serious pest of cereal crops such as barley and wheat in the summer. Galls are induced on the fruit of bird cherry by a fungus (Taphrina padi). These are known as pocket plum galls, and the affected fruits become elongated and distorted in shape.
The insects such as beetles and bees that pollinate bird cherry benefit from the nutrients in the flowers’ nectar, while at the same time providing the essential service of pollination to the tree. Despite their bitter taste, the fruits of bird cherry are eaten by birds, especially robins (Erithacus rubecula), starlings (Sturnus vulgaris) and members of the thrush family (Turdus spp.), and are relished as some of the earliest fruit to be available in the summer. While the birds benefit from this food, the tree also gains, by having its seeds dispersed in the birds’ droppings.
