silver birch


Silver birch is distributed throughout almost all of Europe and in Asia Minor. Downy birch also occurs throughout much of Europe and in north Asia, and is one of the very few native trees in Iceland.  Both species of birch are fast-growing pioneer trees which readily colonise open ground. Silver birch is the faster growing of the two, and also the taller, reaching a height of up to 30 metres, whereas downy birch seldom exceeds 21 metres. As pioneer species, they are short-lived, with typical lifespans being between 60 and 90 years old, although some individuals can live up to 150 years. The trees are slender, with their trunks not normally exceeding a diameter of 40 cm. at breast height. In young trees the bark is reddish-brown, but this changes to white as they mature. The white bark is most prominent on silver birch, where it is interspersed with conspicuous black patches. By contrast, the bark of downy birch is more greyish-white, with horizontal grooves on it. On old silver birches the bark can become corklike and deeply fissured, with parts covered by large colonies of the yellow foliose lichen, Candelaria concolor.

Birches are deciduous, and before their new leaves appear in spring the twigs and buds exhibit a characteristic reddish-purple colour, which is especially apparent after rain. The new leaves emerge in April and are bright green at first, with the colour darkening to a duller green after a week or two. The colour changes to yellow or brown in autumn, with the colours becoming more intense after sharp frosts. Silver birch leaves tend to turn a brighter yellow than those of downy birch, which are usually dull or brownish. The leaves are dropped at the end of October or early November, although this can be earlier, and the appearance of the new leaves in spring later, at higher elevations, where the climate is harsher.

The two species can be distinguished by their leaves, with those of downy birch being rounder in shape than silver birch, and having a single row of teeth on the leaf margin, in contrast to the double row of teeth on silver birch leaves. They can also be identified by their twigs, which in silver birch have small white warts, whereas those on downy birch are covered in small hairs or 'down'. In general, silver birch has an overall drooping, pendulous shape to its branches, which gives rise to its specific name, whilst the branches on downy birch tend to be more upright in their growth form. However, intermediate forms exist between the two species, with various combinations of these characteristics, and this can make the identification of individual trees difficult.

Birches are monoecious, meaning that each individual tree has both male and female flowers. The trees can begin flowering when they are as young as 5 - 10 years old, and the flowers appear in the spring at the same time as the new leaves. The male flowers are drooping catkins, up to 3 cm. in length, whilst the female flowers are upright and 1.2 - 2 cm. in height. Pollination is by wind, and the female flowers ripen to form hanging catkins up to 3 cm. long in late summer or early autumn. The catkins contain hundreds of tiny seeds, each a two-thousandth of a gram in weight and having 2 transparent wings, which facilitate their dispersal by the wind. A large tree can produce up to 1 million seeds in a year, but only a few of these will germinate and grow into mature trees. The majority of seedling trees become established within 100 metres of their parent, but some seeds can travel long distances in the wind.

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