Birches, including Silver Birch (Betula pendula) and Downy Birch (Betula pubescens) are the most common native trees in Scotland, are a vital part of the Caledonian Forest, both as pioneer species in the pinewoods and through forming extensive stands of their own.
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.
Distribution in Scotland
Both species of birch are widespread in Scotland, with silver birch occurring principally on well-drained, drier soils and downy birch preferring wetter locations. In many parts of the country, this results in a longitudinal transition, with silver birch in the east giving way to downy birch in the west.
Birches occur within other forest types, such as pine (Pinus sylvestris) and oak woods (Quercus spp.), and as largely monospecific stands, or birchwoods, in which they are the predominant trees. Birches are the most common native tree in Scotland. However, prior to the deforestation of most of the country by humans, when much larger areas of Scots pine and oak forests flourished, it is likely that birch was proportionately less abundant than it is today.
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.
As pioneer species, one of the important functions which birch trees fulfil in ecosystems is that of improving soils. They are deep-rooted, and their roots draw up nutrients into their branches and leaves, which the trees use for their growth. Some of these nutrients are returned to the surface of the soil each year when the leaves fall in the autumn, thereby becoming available for other organisms in the forest community. An indication of the scale and significance of this nutrient cycling can be drawn from the estimate that birch trees will produce between 3 and 4 tonnes of leaf litter per hectare per year. In an undisturbed forest ecosystem, birches would be replaced by slower growing species such as oak and Scots pine, but in Scotland today this successional process has been interrupted in most places by human exploitation of the land.
The roots of birch trees have mycorrhizal associations with various species of fungi. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions. One of the best known fungi associated with birch trees is the fly agaric (Amanita muscaria), while two edible boletes (Leccinum scabrum, L. versipelle) and the chanterelle (Cantharellus cibarius) also form mycorrhizal relationships with them. The names of some other fungi reflect a similar association with birches – Russula betularum andCortinarius betuletorum – while the polypores Lenzites betulina and Piptoporus betulinus grow on the wood of dead birches. Another polypore, the tinder fungus (Fomes fomentarius) also grows on dead birches, with its hard, wood-like fruiting bodies reaching a large size and persisting for many years. These polypores all help to break down the tough cellulose of the wood, and by doing so make the nutrients in it available for other organisms. Not all fungi have mutually beneficial relationships with trees, however, and the witches' broom fungus (Taphrina betulina) is parasitic on birch trees, causing an abnormally dense growth of small twigs, which radiate from a point on a branch.
A number of different flowers are associated with birchwoods, including primroses (Primula vulgaris) and violets (Viola riviniana) which flower in early spring, before the trees' new leaves limit the light reaching the forest floor. Other common flowers in birchwoods include bluebells (Hyacinthoides non-scripta), wood anemones (Anemone nemorosa) and wood sorrel (Oxalis acetosella). Birch trees also provide the shade for typical understorey plants of the Caledonian Forest, such as blaeberry (Vaccinium myrtillus) and cowberries (Vaccinium vitis-idaea), to flourish on the forest floor beneath them.
Birches support a large community of insects and other invertebrates, with 334 species known to feed on them – more than any other trees native to Scotland, except for oaks and willows. These include the caterpillars of the pebble hook-tip moth (Drepana falcataria) and the Kentish glory moth (Endromis versicolora). The invertebrates in turn are food for various bird species, whilst other birds such as the siskin (Carduelis spinus) feed on the seeds in autumn. Red deer (Cervus elaphus) eat the leaves of young birches, and their grazing is the main limiting factor which has prevented the return of the birches, and other Scottish native trees, to more of their original habitat in the Highlands.
Now, however, with all the regeneration measures initiated for the Caledonian Forest since the mid-1980s, birchwoods are once again expanding. This means that not only are these trees recovering more of their former territory, but also that all the species which depend on them have an opportunity to flourish once more in greater numbers.