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.
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.
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.