Birch: Graceful Lady of the Woods

Few trees match the style of the birch. The narrow trunk, silvery bark and slender branches ending in a mass of drooping twigs have the outline of a fountain that produces a cascade of delicate leaves. No wonder it has become known as the Lady of the Woods.

There are three native species of birch in the British Isles but only two, the silver birch and the downy birch, are common. They thrive particularly well on acid soils, but rarely grow on chalk – the silver birch grows best on dry sandy or gravelly soils such as those of the heathlands of southern England, but the downy birch prefers wetter soils and a cooler climate and so is most common in the uplands of Scotland, Wales and Ireland. However, their ranges overlap considerably and they are often found growing in the same areas, exploiting local variations in the wetness or drainage of the soil.

Differences in detail

In general form the silver and downy birch are very similar and can be described together, although they differ in a number of details.

The silver birch, as its name suggests. Has much whiter bark, whereas the bark of the downy birch is more variable, ranging from silver-grey to brownish in colour. The only way to distinguish the two species from a distance is by the colour of the bark. A characteristic of old silver birches is the patches of rough, knobbly black bark replacing the white bark at the base of the trunk. In downy birches the bark remains more or less smooth right down to the ground.

Secondly, unlike downy birches, silver birches typically have hanging or ‘weeping’ twigs – a feature that has been exaggerated by selective breeding to produce ornamental varieties that grace many parks and gardens. And the twigs of the silver birch are smooth, whereas those of the downy birch are covered with short fine hairs.

Leaves, flowers and fruits

The foliage appears towards the end of April, the leaves unfolding from small, pointed buds that are arranged alternately along the thin, purplish-brown twigs. The bright green leaves, about 3cm (1 in) long, are usually triangular in outline but the shape may vary from diamond to oval. Their margins are rather unevenly toothed and they taper to a sharp point. They are borne on slender stalks which allow them to twist and flutter in the breeze. The leaves of young birches are often much larger than those on full grown trees because they grow much more vigorously. After a heavy shower in late spring the air in a dense stand of birch has a delicate fragrance from the aromatic resin washed from the unfurling leaves and from tiny warts on the twigs. In autumn the leaves turn bright yellow, before falling in October to leave the trees bare, but still beautiful, their trunks gleaming white in the pale wintry sunlight.

Birch flowers are catkins and both sexes are borne on the same tree. The males develop during autumn and by April or May have turned into a dangling catkin about 5cm (2in) long, covered with reddish brown scales which separate to release their pollen. The female catkins appear with the unfolding leaves. They are held erect on the twigs, 2-3cm (lin) long, and are made up of overlapping green scales, each shielding an ovary from which two purple stigmas protrude to catch pollen grains wafted on the wind.

After fertilisation the female catkins expand into club-shaped cone-like structures that slowly disintegrate in autumn to release the tiny winged fruits. These are so light they may be carried for considerable distances by strong winds.

Birch woods are a favourite haunt of redpolls and siskins (both our resident birds and winter visitors from Scandinavia). They feed on the seeds scattered on the ground.

Hardy pioneer

The birch is an extremely hardy tree and has established itself in some northern-most forests at the frontier of the treeless tundra. The pollen record shows that birches and pines were the first trees to advance into the British Isles when the climate warmed after the last Ice Age. Both species formed vast forests but were confined to the better-drained uplands.

Today the birch is found in most woodlands on the poorer soils, from English oak woods to Scottish pine woods. It also forms pure birch woods – you can see good examples on the sandy heaths of southern England and the steep hillsides of Scotland.

The birch is an aggressive colonizer of forest clearings, ungrazed heaths, and areas which have suffered recent burning. Its tiny winged seeds spread in profusion and germinate quickly to invade new areas. The pioneer trees form dense thickets of fast growing saplings that develop into pure birch stands up to 25m (80ft) tall at maturity.

Fungi associates

Birch trees have a remarkable relationship with the fly agaric fungus which sends up its striking red toadstools in autumn. The fungus is attached to the tree’s roots and both benefit from this association: the fungus speeds the entry of soil nutrients into the root system of the tree in return for sugars manufactured by the tree.

You may also find the bracket fungus clamped on to the trunks of older trees. It is a parasite, feeding on the tree’s sap but giving nothing in return. Indeed, this fungal assault so weakens the tree that it soon perishes. And the fungus continues to feed on the dead wood. Although the birches’ death is often hastened by fungal attack, birches do not live for a long time anyway rarely exceeding 80 years. Dead or dying trees are favourite nesting sites for woodpeckers whose bills can easily tunnel holes into the wood softened by decay.

Witches broom

Many birch trees appear to have large birds’ nests among their branches. In fact these untidy tangles of twigs are galls, a growth deformity caused when the buds are attacked by either a fungus or a tiny mite. A dense mass of twigs sprouts at the point of attack to form the so-called witches broom. (Actually birch twigs are cut and bundled to make the sort of broom that witches are supposed to fly around on at night. They are also used for steeple-chase fences to fill out the jumps.)

Bark and timber

The thin paper-like bark is shed in strips as the tree grows, and is replaced anew from underneath. Birch bark is remarkably resistant to decay – the bark on a fallen log remains for months after the wood inside has rotted to a soft pulp. It is also waterproof, and has been used in parts of Europe for rooting. Birch bark makes excellent kindling for camp fires and the wood itself burns very well. In parts of Scandinavia and Central Europe birch wood is still the main winter fuel.

Birch timber is a pale creamy brown colour and has been put to a wide variety of uses: for instance, the making of carts and packing cases, and smaller items such as floor tiles. Cotton reels, spools and bobbins for the textile trade. But although it is a hard wood, it does not last long out of doors.

In spring there is a copious flow of sugary sap through the trunk and in some country districts this is collected by tapping the trees: the liquid is fermented into wine.

Dwarf birch

There is a third species, the dwarf birch (Betalu nana), an arctic-alpine shrub less than lm (3ft) tall which grows at a few sites in the Scottish Highlands. It also has the distinction of being the only woody species to grow in the frozen wastes of Greenland – the world’s northern limit for woody plants. Here it takes the form of a low wiry bush, spreading close to the ground, its growth stunted by the icy blasts of Arctic gales. Its leaves are smaller and rounder in shape than those of the other two species. A few specimens occur in Northumberland and Durham, but it is generally a rare species.

14. November 2011 by Dave Pinkney
Categories: Featured Articles, Woodlands | Tags: , , , , , , , , , , | Comments Off on Birch: Graceful Lady of the Woods

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