Hazel (Corylus avellana) is an important understorey tree in the Caledonian Forest, providing nuts for squirrels and other rodents, and it also supports a rare lichen community.
Hazel is widely distributed throughout much of Europe, from Britain and Scandinavia eastwards to the Ural Mountains in Russia, and as far south as Spain, Italy and Greece. It also occurs in Morocco, Algeria, Turkey, Iran and the Caucasus region.
Within this large range its distribution is uneven and it typically grows as an understorey component of deciduous forest, especially with oaks (Quercus spp.), although it also occurs with conifers.
Archaeological evidence from pollen analysis has shown there was a rapid expansion in the range of hazel during the Mesolithic period (from 11,000 to 6,000 years ago). Because the large nuts are not dispersed over great distances by small mammals, this has led to speculation that Mesolithic peoples may have transported the nuts with them as a food source, and thereby aided the expansion of the tree’s range.
Distribution in Scotland
Hazel is known from all parts of Scotland, including the Outer Hebrides, Orkney and Shetland, and was formerly much more abundant, especially on some of the Hebridean islands. On the mainland, it is common in the western Highlands, and there are stands of almost pure hazel in Argyll. Elsewhere, it occurs in the more fertile, lower-lying parts of the country, usually in association with oak (Quercus spp.), ash (Fraxinus excelsior) and also birch (Betula spp.). It is rarely found on acid soils, such as those of the pinewoods of the Caledonian Forest, and is also uncommon in the Southern Uplands, but this is most likely due to the extensive deforestation of that region, rather than a natural absence.
In Glen Affric, hazel is found primarily in the Affric River gorge, between Dog Falls and Badger Falls, growing amongst birch on south-facing slopes with relatively rich soils. However, it also occurs on the north shore of Loch Beinn a’Mheadhoin and at the east end of Loch Affric, indicating that it was formerly more widespread in the glen.
Hazel is a member of the birch family of trees, Betulaceae, and can grow to a height of 10 metres, although in Scotland it is usually no more than 6 metres tall. Typically it has a number of shoots or trunks branching out at, or just above, ground level, and this growth habit has led to some people referring to it as a bush rather than a tree, because it doesn’t meet the strict definition for a tree, of having a single stem that is unbranched near the ground.
Hazel’s ability to produce multiple stems gives it a dense, spreading appearance and has led to its extensive use for coppicing. It is a short-lived tree, reaching 50-70 years in age, but if it is coppiced, either by people or naturally through damage to its trunks, it will live much longer.
The growth of successive new stems leads to the formation of a large base, or stool, which can be up to 2 metres in diameter, and in this way coppiced hazels can live for several hundred years.
The bark is smooth and shiny, and is greyish-brown in colour. It peels off in strips as the tree gets older, and the trunks are often covered in mosses, liverworts and lichens, especially in the wetter parts of its range. The twigs are covered in long stiff hairs, and the buds are smooth and ovoid in shape.
The leaves are roundish in shape, with a point at the end, and are about 10 cm. across. Leaf edges are doubly serrate, or double toothed, and the leaves are hairy, which gives them a rough texture. Hazel is deciduous, with the new leaves appearing in April each year, and turning a bright yellow before being shed in October.
Male flowers are in the form of catkins, which are pale yellow in colour and up to 5 cm. long. They open in February, when hazel and its companion deciduous trees are all leafless, so they are one of the first obvious signs of spring in the forest. The female flowers are tiny red tufts, growing out of what look like swollen buds, and are visible on the same branches as the male catkins. Pollination is by wind, and hazel is self-incompatible – successful pollination only occurs between different trees, as a single tree cannot pollinate itself.
Fertilised female flowers grow into nuts which are up to 2 cm. in size and occur in clusters of 1 to 4. Each nut is partially enclosed by a cup-shaped sheath of papery bracts, or modified leaves. The nuts ripen to a brown colour in September and October, with the nut itself enclosed by a tough woody shell. Empty nuts are an occasional occurrence.
The nuts are distributed by red squirrels (Sciurus vulgaris), woodpeckers (Dendrocopos major), and, after falling to the ground, by other small rodents. Most of the nuts are consumed by these dispersers, but some of those which are hoarded for winter, or are overlooked, germinate and grow the following spring.
In Scotland hazel normally occurs as an understorey component in deciduous forests characterised by oaks, ash or birch. However, in comparison to oak and birch it has relatively few mycorrhizal relationships with fungi – only 21 species of fungi are recorded as having this mutualistic association with hazel. Of these, one species, the fiery milk mushroom or hazel milk-cap (Lactarius pyrogalus) is largely restricted to growing with hazel.
Hazel is important, however, in providing the main habitat for an ascomycete fungus (Hypocreopsis rhododendri) which was only known from rhododendrons in North America until it was discovered on hazel in Mull in the 1970s, and then elsewhere in western Scotland. It mainly grows on standing dead stems of hazel, but also has been found on living branches. Because of its rarity in Britain, a Species Action Plan has been prepared for this fungus.
Hazel is very important for lichens, and is the best host species in the UK for Graphidion lichens – those which grow on smooth-barked trees. Several of these lichens are rare and endangered, and are the subject of Species Action Plans, including one (Arthothelium macounii) which is the only host for a parasitic fungus (Arthonia cohabitans). Another lichen (Graphis alboscripta) is also almost entirely restricted to hazel, and is only known from Scotland. Hazel is also a good host for the Lobarion group of lichens – the larger, leafy lichens, which include tree lungwort (Lobaria pulmonaria) – particularly in western Scotland.
Unlike trees such as birch, hazel has relatively few insect species associated with it. However, there are 5 species of moths which are specialist feeders on hazel, including a narrow-winged leaf miner (Parornix devoniella), whose larvae live under a folded down leaf edge, and a nepticulid moth (Phyllonorycter coryli), whose larvae form ‘blotch’ mines in the leaves. A few beetles, especially weevils, and some flies are also known to use hazel, while there is a range of insects associated with the nuts, particularly in continental Europe.
Because of its growth as a densely-branched understorey component in forests, hazel plays a significant role in increasing the vertical structure within woodland, which is important for bird diversity. Hazel leaves are eaten by roe deer (Capreolus capreolus) and red deer (Cervus elaphus), and the nuts, which are rich in fats and protein, are eaten by the wood mouse (Apodemus sylvaticus) and the red squirrel. Squirrels split the shell of the nut in two halves to get the kernel inside, whereas wood mice will gnaw a hole through the shell. In England, hazel is an important tree for the dormouse (Muscardinus avellanarius), but this species is absent from Scotland.
Until recently hazel has received less conservation attention than some other tree species, but this is changing now that its importance has been recognised, particularly in the temperate rainforests of western Scotland.