Betula pendula, commonly known as silver birch or warty birch, is a species of
tree in the family Betulaceae, native to Europe and parts of Asia, though in
southern Europe it is only found at higher altitudes. Its range extends into
Siberia, China and southwest Asia in the mountains of northern Turkey, the
Caucasus and northern Iran. It has been introduced into North America, where it
is known as the European white birch, and is considered invasive in some
states in USA and in parts of Canada. The silver birch is a medium-sized
deciduous tree that owes its common name to the white peeling bark on the trunk.
The twigs are slender and often pendulous and the leaves are roughly
triangular with doubly serrate margins and turn yellow in autumn before they
fall. The flowers are catkins and the light, winged seed get widely scattered
by the wind. The silver birch is a hardy tree, a pioneer species, and one of the
first trees to appear on bare or fire-swept land. Many species of birds
and animals are found in birch woodland, the tree supports a wide range of
insects and the light shade it casts allows shrubby and other plants to grow
beneath its canopy. It is planted decoratively in parks and gardens and is
used for forest products such as joinery timber, firewood, tanning, racecourse
jumps and brooms. Various parts of the tree are used in traditional medicine
and the bark contains triterpenes which have been shown to have medicinal
properties. Description
The silver birch is a medium-sized deciduous tree, typically reaching 15 to
25 m tall), with a slender trunk usually under 40 cm diameter. The bark on the
trunk and branches is golden-brown at first, but later this turns to white as
a result of papery tissue developing on the surface and peeling off in flakes.
The bark remains smooth until the tree gets quite large, but in older trees,
the bark thickens, becoming irregular, dark and rugged. Young branches have
whitish resin warts and the twigs are slender, hairless and often pendulous.
The buds are small and sticky, and development is sympodial, that is to say
the terminal bud dies away and growth continues from a lateral bud. Some
shoots are long and bear the male catkins at the tip, while others are
short and bear female catkins. The immature male catkins are present during
the winter but the female catkins develop in the spring, soon after the
leaves unfurl. The leaves have short slender stalks and
are 3 to 7 cm long, triangular with broad, untoothed, wedge-shaped bases,
slender pointed tips and coarsely double-toothed, serrated margins. They
are sticky with resin at first but this dries as they age leaving small white
scales. The foliage is a pale to medium green and turns yellow early in the
autumn before the leaves fall. In mid-summer, the female catkins mature
and the male catkins expand and release pollen, and wind pollination takes
place. The small 1 to 2 mm winged seeds ripen in late summer on pendulous,
cylindrical catkins 2 to 4 cm long and 7 mm broad. The seeds are very numerous
and are separated by scales, and when ripe, the whole catkin disintegrates and
the seeds are spread widely by the wind. Distribution and habitat
The silver birch grows naturally from western Europe eastwards to Kazakhstan,
the Sakha Republic in Siberia, Mongolia and the Xinjiang province in China, and
southwards to the mountains of the Caucasus and northern Iran, Iraq and
Turkey. It is also native to northern Morocco and has become naturalised in
some other parts of the world. In the southern parts of its range it is mainly
found in mountainous regions. Its light seeds are easily blown by the wind and
it is a pioneer species, one of the first trees to sprout on bare land or
after a forest fire. It needs plenty of light and does best on dry, acid soils
and is found on heathland, mountainsides and clinging to crags. Its tolerance to
pollution make it suitable for planting in industrial areas and exposed sites.
It has been introduced into North America where it is known as the
European white birch, and is considered invasive in the states of Kentucky,
Maryland, Washington and Wisconsin. It is naturalised and locally invasive in
parts of Canada. Taxonomy
The closely related Betula platyphylla in northern Asia and Betula szechuanica
of central Asia are also treated as varieties of silver birch by some
botanists, as B. pendula var. platyphylla and B. pendula var.
szechuanica respectively. B. pendula is distinguished from the
related downy birch in having hairless, warty shoots, more triangular leaves
with double serration on the margins, and whiter bark often with scattered
black fissures. It is also distinguished cytologically, silver birch being
diploid, whereas downy birch is tetraploid. Hybrids between the two are
known, but are very rare, and being triploid, are sterile. The two have
differences in habitat requirements, with silver birch found mainly on dry,
sandy soils, and downy birch more common on wet, poorly drained sites such as
clay soils and peat bogs. Silver birch also demands slightly more summer warmth
than does downy birch, which is significant in the cooler parts of
Europe. Many North American texts treat the two species as conspecific, but they
are regarded as distinct species throughout Europe.
Synonyms include Betula pendula var. carelica Hämet-Ahti, B. pendula var.
laciniata Tidestr., B. pendula var. lapponica Hämet-Ahti, B. aetnensis Raf.,
B. montana V.N.Vassil, B. talassica Poljakov, B. verrucosa Ehrh., B.
verrucosa var. lapponica Lindq., and B. fontqueri Rothm. The rejected name
Betula alba L. also applied in part to B. pendula, though also to B. pubescens.
Ecology The silver birch has an open canopy
which allows plenty of light to reach the ground. This allows a variety of
mosses, grasses and flowering plants to grow beneath which in turn attract
insects. Flowering plants often found in birch woods include primrose, violet,
bluebell, wood anemone and wood sorrel. Small shrubs that grow on the forest
floor include blaeberry and cowberry. Birds found in birch woodland include
the chaffinch, tree pipit, willow warbler, nightingale, robin, woodcock,
redpoll and green woodpecker. The branches of the silver birch often
have tangled masses of twigs known as witch's brooms growing among them,
caused by the fungus Taphrina betulina. Old trees are often killed by the decay
fungus Piptoporus betulinus and fallen branches rot rapidly on the forest
floor. This tree commonly grows with the mycorrhizal fungus Amanita muscaria in a
mutualistic relationship. This applies particularly to acidic or nutrient-poor
soils. Other mycorrhizal associates include Leccinum scabrum and
Cantharellus cibarius. It has been shown that, as well as mycorrhiza, the
presence of microfauna in the soil assists the growth of the tree, as it
enhances the mobilization of nutrients. The larvae of a large number of species
of butterflies, moths and other insects feed on the leaves and other parts of
the silver birch. In Germany, almost 500 species of insect have been found on
silver and downy birch including 106 beetles and 105 lepidopterans, with 133
insect species feeding almost exclusively on birch. In the United
States, the wood is attacked by the bronze birch borer, an insect pest to
which it has no natural resistance. Uses
Silver birch is often planted in parks and gardens, grown for its white bark
and gracefully drooping shoots, sometimes even in warmer-than-optimum
places such as Los Angeles and Sydney. In Scandinavia and other regions of
northern Europe, it is grown for forest products such as lumber and pulp, as
well as for aesthetic purposes and ecosystem services. It is sometimes used
as a pioneer and nurse tree elsewhere. Silver birch wood is pale in colour with
no distinct heartwood and is used in making furniture, plywood, veneers,
parquet blocks, skis, kitchen utensils and in turnery. It makes a good firewood
that produces a good heat when burnt but is quickly consumed by the flames. Slabs
of bark are used for making roof shingles and wooden footwear.
Historically, the bark was used for tanning. Bark can be heated and the
resin collected; the resin is an excellent waterproof glue and useful for
starting fires. The thin sheets of bark that peel off young wood contain a waxy
resin and are easy to ignite even when wet. The dead twigs are also useful as
kindling for outdoor fires. Birch brushwood is used for racecourse
jumps and besom brooms. In the spring, large quantities of sap rise up the
trunk and this can be tapped. It contains around 1% sugars and can be
used in a similar way to maple syrup, being drunk fresh, concentrated by
evaporation or fermented into a "wine". In Sweden, the bark of birch trees was
ground up and used to make bark bread, a form of famine food. The removal of bark
was at one time so widespread that Carl Linnaeus expressed his concern for the
survival of the woodlands. Silver birch is used in traditional
medicine as a diuretic and is reputed to be useful in the treatment of high blood
pressure, high cholesterol, obesity, gout, kidney stones, nephritis,
cystitis, digestive disturbances and respiratory diseases. For these
purposes, a decoction of the bark or leaves is generally used. Externally
silver birch is used to promote healing, relieve pain and treat inflammations and
infections of the skin such as eczema and psoriasis.
The outer part of the bark contains up to 20% betulin. The main components in
the essential oil of the buds are α-copaene, germacrene D and δ-cadinene.
Also present in the bark are other triterpene substances which have been
shown to have anti-inflammatory, antiviral and anti-cancer properties.
The silver birch is Finland's national tree. Leafy, fragrant boughs of silver
birch are used to gently beat oneself in the Finnish sauna culture.
Cultivation Successful birch cultivation requires a
climate cool enough for at least the occasional winter snowfall. As they are
shallow-rooted, they may require water during dry periods. They grow best in
full sun planted in deep, well-drained soil.
= Cultivars= 'Carelica' is called "curly birch" in
Finland; the wood is hard and decorative and is used in wood-carving.
'Laciniata' agm has deeply incised leaves and weeping branches.
'Purpurea' has dark purple leaves. 'Tristis' agm has an erect trunk with
weeping branchlets. 'Youngii' has dense, twiggy weeping
growth with no central leader and requires being grafted onto a standard
stem of normal silver birch. The cultivars marked agm above have
gained the Royal Horticultural Society's Award of Garden Merit.
See also List of Lepidoptera that feed on birches
References