Goat willow (Salix caprea) is a widespread but relatively uncommon deciduous tree which supports a large range of species in the Caledonian Forest.
Goat willow occurs throughout all of Europe, from Britain and Ireland eastwards to Russia, and from northern Scandinavia to the Mediterranean. It is also widely distributed in northern Asia, with a range that stretches from western Russia to the eastern side of Lake Baikal in Siberia, and includes the Korean peninsula and Japan, while isolated populations occur in parts of China and western Mongolia. Goat willow also occurs in Georgia and Armenia in the Caucasus, in parts of Turkey and in northern Iran. It has been introduced to eastern North America, where it has become naturalised.
Distribution in Scotland
Goat willow is distributed widely throughout Scotland, occurring in all parts of the mainland, from Dumfries and Galloway to Caithness and Sutherland, and on most of the Western Isles, including Islay, Mull, Rum, Skye and Lewis. It has also been recorded on Orkney. It does not occur in dense groups or stands, but is normally found as scattered individuals in woodlands, and often near rivers and lochs. As a relatively fast growing pioneer species it also occurs on open ground and can tolerate some wetness in the soil.
Goat willow is a broadleaved deciduous tree in the Salicaceae family that is also sometimes known as great sallow, or pussy willow. It is relatively small in size, typically growing to a maximum height of 12 metres, and also occurs as a multi-stemmed shrub. The bark is smooth, grey or greyish-brown, and has diamond-shaped lenticels on it, through which the tree exchanges gases with the atmosphere. Over time, the lenticels develop into vertical fissures and old goat willows have a characteristic pattern of these on their trunks.
The twigs are greenish in colour, relatively thick and are initially hairy, but becoming smooth as they develop more. The buds are brown and chunky. The leaves of the goat willow are broader than those on most other willow species, and are oval in shape, with the tip angled slightly to one side. They are up to 12 cm. long and 6 cm. in width, and are arranged alternately on the stems. The upper surface is green and shiny, while the underside is pubescent or downy and a whitish-pale green colour. The leaves turn a dull yellow-green in autumn before being shed, and the tree is leafless until the new ones appear in April.
Goat willow is a dioecious species, so individual trees are either male or female (in contrast to most trees, such as birch or oak, for example, where male and female flowers occur on the same tree). The flowers appear in late March or April, before the new leaves emerge. The male flowers are oval and initially covered in fine grey hairs, thereby being said to resemble the paws of a cat, and this has given rise to the tree’s common name of pussy willow. They turn bright yellow as they open out to form catkins, with stamens tipped with pollen. Female catkins are green and are larger than the males, being up to 7 cm. long. They are erect and taper slightly towards their top, with conical ovaries protruding out around them.
Goat willow is somewhat unusual in that pollination is achieved by both the wind and flying insects, with bumblebees (Bombus spp.) being the main insect pollinators (most trees rely on one or other method of pollination, rather than a combination of both). Fertilised female catkins ripen to produce a mass of tiny seeds, each about 0.2 mm. in size, and which are surrounded with white fluffy hair to aid their dispersal by the wind. The seeds only remain viable for a few days, so there is a short window of opportunity for them to reach a good site for germination and growth. Once established, however, a young tree will grow quite quickly and can live for up to 300 years.
Like most other species of willow, the goat willow is able to propagate itself by vegetative means – if a branch comes into contact with the ground, it will take root. However, it does not propagate easily from cuttings. Hybridisation between goat willow and other species is a common occurrence, especially with grey willow (Salix cinerea) and eared willow (Salix aurita). The offspring trees have intermediate features between both parents, and are usually fertile themselves. This contributes to the difficulty that is often experienced in identifying willows to the level of species.
In common with most other tree species, goat willow grows in mycorrhizal association with a range of fungi. This is a symbiotic partnership under which both the tree and the fungi benefit through an exchange of nutrients, which takes place when the fungal hyphae wrap around or interpenetrate the roots of the tree. Fungi are unable to photosynthesise, because they have no chlorophyll, whereas goat willow can, and the tree passes carbohydrates and sugars that it produces using the sun’s energy to the fungi. Through their mycelium, or network of hyphae, fungi are able to access minerals and other nutrients in the soil and they are passed on to the tree.
Fungi that have a mycorrhizal partnership with goat willow include a webcap fungus (Cortinarius saturninus), the willow milkcap (Lactarius aspideus) and the willow brittlegill (Russula laccata). Other fungi have a parasitic relationship on the tree, including the willow tarspot fungus (Rhytisma salicinum), which forms blackened sections on the leaves, and a rust fungus (Melampsora caprearum) that commonly occurs as clusters of orange-yellow blobs on the undersides of the leaves. The rust fungus in turn is fed upon by the larvae of several midge species (Mycodiplosis spp.) that match the colour of the rust. Saprotrophic fungi – those that feed on decaying or dead wood – that occur on goat willow include the blushing bracket (Daedaleopsis confragosa), the willow bracket (Phellinus igniarius), the willow shield (Pluteus salicinus) and the bleeding broadleaf crust fungus (Stereum rugosum).
The bark of the goat willow is less acidic than other trees such as birch (Betula spp.), and it therefore provides the habitat for a wider range of lichens. In the Highlands, particularly in the west, these include tree lungwort (Lobaria pulmonaria), plum fruited felt lichen (Degelia plumbea) and red-eyed shingle lichen (Pannaria rubiginosa).
Willows in general are the host for large numbers of insects, being rivaled only by oak (Quercus spp.) in that regard, and goat willow is no exception. There are possibly hundreds of moth species whose larvae feed on goat willow, although most of them feed on other trees as well. Notable species associated with goat willow in Scotland include the poplar hawk-moth (Laothoe populi), puss moth (Cerura vinula), sallow kitten (Furcula furcula) and the pebble prominent (Notodonta ziczac). The larva of a micro-moth (Phyllonorycter dubitella) makes a mine in the leaves of goat willow. Larvae of the sallow moth (Cirrhia icteritia) and the pink-barred sallow (Xanthia togata) both feed on the catkins of goat willow when they are young, before moving on to other plants when they are larger.
Because goat willow flowers before its leaves appear, it provides an important source of nectar and pollen early in the spring for a range of pollinators, including bumblebees and hoverflies.
In Britain, 24 species of aphids have been recorded on goat willow, including common species such as the small willow aphid (Aphis farinosa) and the pale sallow leaf aphid (Chaitophorus capreae). They feed by sucking the sap of the tree, as does the willow scale insect (Chionaspis salicis).
Galls are induced on the leaves of goat willow by a number of invertebrates, including a mite (Iteomyia capreae), two sawflies (Pontania bridgmanii and Eupontania pedunculi) and a midge (Rhabdophaga rosaria). The latter causes the tree to form a terminal rosette or camellia gall, which is a distinctive, dense cluster of leaves at the tip of a branch or twig that persists after the leaves have fallen in winter. This gall is often used as a residence by the larva of another midge (Perrisia iteophila), but that plays no part in inducing the gall.
The bark of willows, including goat willow, is a favoured winter food of the European beaver (Castor fiber) and that, together with the insectivorous birds attracted by the invertebrates the tree supports, makes goat willow an important tree in the Caledonian Forest for a wide range of other organisms.