Her fine young figure is clothed in smooth white satin.
In spring she dons upon her head a veil of pale green,
shimmering in the warm breeze.
Her luring perfume is as fresh as the April rains.
In autumn her veil is the golden-yellow of the Sun.
As the Sun fades so her veil falls gently to the ground
and she scatters new life upon the air.

The silver birch (Betula pendula) is also known as the Lady of the Woods with her slender silver-white frame, small delicate leaves and seeds and subtle but enchanting fragrance. The birch is a pioneer species across much of Britain - it is a smallish, relatively short-lived tree that reproduces and grows quickly, exploiting openings in the tree canopy before other slower growing trees like oak eventually take over. In Scotland, however, birch is a dominant species along with pine with which it forms mixed woodland, due to its ability to endure cold and thrive on poor acid highland soils. Indeed parts of the Caledonian forest are essentially original wildwood, apart from the odd grazing by deer and sheep, they are little changed from their wild state since they took root after the Ice Age.

Question: what is a pioneer species? A pioneer species is one of the first species to colonise newly exposed ground, be it a gap in a forest canopy where a tree has fallen (colonised by birch), or newly solidified volcanic rock (colonised by bacteria and lichens) or exposed desert rocks (colonised by bacteria), etc. To beat their rivals pioneer  species must have a way to arrive on the scene quickly. The birch tree does this by producing millions of very tiny seeds with wings that carry them far and wide on the air. Wherever a gap appears, if the soil is suitable, then a birch seed is almost bound to find the area and take root. Pioneers must also be fast-growing to beat competitors - in the case of trees they must reach the light before rivals drown them out. Birch achieves this by investing in a tall but slender trunk, but at the expense of longevity. Longer lived trees invest more in a thicker trunk, which means that they areslower growing and not effective pioneer species.

Some drawings of parts of silver birch trees that I made: a small branch, showing lenticels, a strip of bark, showing lenticels, a leaf, showing some of the veination, a close up of part of the leaf showing its vasculature and a birch 'seed' (actually a fruit) with its two wings.

Question: what is a lenticel? A lenticel is an opening, usually a small slit, in the bark of a tree that allows oxygen in
to the living cells underneath so that they can respire.

BIRCH

There are about 40 species of birch. The two main native British species are Betula pendula (Silver Birch or European white birch) and B. pubescens (Downy Birch or White birch).

Height – 28 m, Max. 38 m (silver birch: 25 m, downy birch: 28 m).
Longevity - 80 years, rarely > 100, occasionally up to 200 years. Birch is a pioneer of open areas and invests in a tall thin trunk of less durable wood that grows in height quickly, but is therefore more prone to windfall.

Habitat - Birch is a fast-growing pioneer on acid soils, rarely growing on chalk and is found on nutrient-poor sands and peat. Silver birch prefers dry sandy/gravelly soils -heathlands of southern England. Downy birch prefers wetter soils and a cooler climate- uplands of Scotland, Wales and Ireland. Birches are tolerant to frost, but susceptible to sulphur dioxide damage.

Bark - thin, paper-like shed in strips as the tree grows. Remarkably resistant to decay and water proof (has been used for roofing). Phellem with thick cell walls and conspicuous growth layers (B. populifolia). Type 3 lenticels, visible as horizontal Slits, up to about 5 cm long. Bark of B. pendula peels in horizontal strips. Alternating layers of thin- and thick-walled cork cells results in the shedding of thin papery sheets of bark.

Question: what is phellem? Phellem is the corky tissue produced in the bark of trees and is both waterproof and protects the vital sugary sap-conducting cells (phloem) beneath the outer layers of bark against knocks and blows and would-be grazers and insects which have a hard job getting through the corky tissue to the critical phloem underneath. Bark also gives the tree a degree of fire-resistance.

Wood - burns well -still the main winter fuel in parts of Scandinavia and central Europe. Timber is a pale cream-brown colour - a hard but perishable wood. Birches have diffuse porous wood. The sugary spring sap, flowing through the trunk, is tapped to make wine.

Leaves - appear towards end of April, unfolding from small, pointed buds arranged alternately along the thin, purplish-brown twigs. Bright green, 3 cm long, usually triangular, but varying from diamond to oval. Unevenly toothed margins, taper to a point. Borne on slender stalks, allowing them to twist and flutter in the breeze. Often much larger leaves on young trees. Turn bright yellow in autumn, falling in October. Glandular hairs and colleters on young leaf primordia (young leaf buds) produce a sticky resin that covers and permeates the entire bud. After a heavy shower, in spring, the aromatic resin washed from the unfurling leaves, and from tiny warts on the twigs, leaves a noticeable fragrance in the air. On average, a birch tree has some 200 000 leaves and a normal sized birch tree uses some 17 000 litres of water in summer.

Flowers – monoecious (single sex) catkins of both sexes are borne on the same tree. Males develop during autumn and are about 5cm long by April/May and covered by reddish brown scales, which separate to release their pollen. Female catkins develop while the leaves unfold, and are about 2-3 cm long and held upright on the twigs and are made up of overlapping green scales each shielding an ovary from which two purple stigmas protrude. Wind-pollinated.

Floral formulae:

           Male: K(0-6) C 0 A 2-18 G 0
           Female: K 0-6 C 0 A 0 / G(2)

The ovaries of the female flowers are inferior. Fruit- fertilised female catkins expand into club-shaped cone-like structures that slowly disintegrate in autumn to release tiny (about 2 mm), easily dispersed winged samara fruit. The fruit and seeds are tiny and there are some 5.9 million seeds per kg in silver birch, and 8.45 million per kg in dwarf birch! The high numbers of seed that can be produced per unit of resources and the ease with which birch fruit disperse enables the rapid reproduction of
birch trees and allows them to exploit open spaces as pioneers.

Silver birch has whiter bark than downy birch, which ranges from silver-grey to brownish. On old silver birch trees, knobbly black bark replaces the white bark at the base of the trunk. Silver birches have hanging or weeping twigs, especially in ornamental varieties. The twigs of silver birch are smooth, those of downy birch are covered with short fine hairs.

The birch is a hardy tree occurring at higher altitudes than any other tree. Downy birch is the first tree to colonise areas on the edge of the Northern Ice Cap.

Yellow birch (B. alleghaniensis) –NE USA and SE Canada.
River or red birch (B. nigra) –S/E USA, occurs most commonly in wet or swampy soils in the southern USA.
Canoe or paper birch (B. papyrifera) -used by American Indians to build canoes.
B. albo-sinensis is found in the mountains of western China.
B. ermanii occurs in NE Asia and Japan.
Cherry birch (B. lenta) – eastern North America.
Japanese cherry birch (B. grossa) - Mountains in Japan.
Monarch birch (B. maximowicziana) – central and northern Japan.
Himalayan birch (B. utilis) -Mountains in China, Himalayas.

Dwarf birch (B. nana) -a third species native to Britain. An arctic-alpine shrub less than 1m tall - grows in the Scottish Highlands. The only woody plant to grow in the frozen wastes of Greenland, growing here as a small wiry bush growing close to the ground. Its leaves are smaller and rounder.

Birchwoods

Birchwoods occur naturally on Scottish hillsides, on poor acid soils. Scottish birch woods are often mixed with Scot's pine or rowan. Dwarf birch is a Scottish upland moorland shrub, whilst the downy birch is dominant in British uplands and highlands and wet soils in southern England. The silver birch is found on the drier soils of southern England, except chalk which is not suitable for the birch which avoids limey soils.

Birchwoods are characterised by well-spaced trees that allow the light to easily reach the floor, enabling a variety of shrub and herb layer plants to grow underneath, such as Bilberry (Vaccinium myrtilis), Crowberry (Empetrum nigrum), heather, Wavy Hair-grass (Deschampsia flexuosa), bracken and sometimes juniper (Juniperus) and also (Yellow) Tormentil (Potentilla erecta), Wood Sorrel (Oxalis acetosella), (White) Heath Bedstraw (Galium saxatile), Lesser Twayblade (Neottia cordata) and Heath Milkwort (Polygala serpyllifolia). Mosses that are found in birchwoods (and in nearby pine and sessile oak woods) include: Rhytidiadelphus loreus, Pleurozium schreberi and Plagiothecium undulatum.

Fungi that form mycorhizae with birch trees include the famous Fly Agaric (Amanita muscaria), the Ugly Milk-cap (Lactarius turpis), the Coconut-scented Milk-cap (Lactarius glyciosmus) and the Brown Birch Bolete (Leccinum scabrum - external link).

Question: what are mycorhizae (singular mycorhiza)? Mycorhizae are fungi that grow in association with plant roots. The fungus forms a massive network (called a mycelium) of fine threads in the soil which are excellent at absorbing minerals from the soil. The fungus is intimately connecting to its host tree and transports minerals to the tree in exchange for sugars that the tree makes by using carbon dioxide from the air and light by the process of photosynthesis. Thus both partners benefit, in fact most trees grow very badly, if at all, without their partner fungus. The fungus and the tree benefit each other and live in what we call a state of mutual symbiosis. This is just one of many examples that demonstrate that the greatest leaps in evolution came about by cooperation not competition! It also illustrates how everything is connected in nature - organisms are intimately connected with one another and with their environments.

Insects: many moth species are supported by birch trees, e.g. the mottled umber defoliates birches (and oak trees). The birch shieldbug, parent shieldbug and the birch sawfly are also characteristic.

Mammals and birds: none are specific to birchwoods in Britain, but the red deer, roe deer, wild cats, stoats and pine martens occur in mixed birch/pinewoods. The chaffinch, willow warbler, tree pipit, robin, redstart, various tits and redwings are also found here. Birch seeds are eaten by finches, woodpeckers, jays and squirrels.

Above and below: Silver Birch, Betula pendula.Click all images for full size.

Below:The Birch Polypore (Piptoporus betulinus or Polyporus betulinus) or Razor Strop Fungus a bracket fungus that grows only on Birch trees. It can be found throughout the year and the brackets used to be dried and used to shrpen razors. This fungus grows only on dead birch trunks and is annual, although the brackets persist.

Back to Nature Tech

Article updated: 21/3/2014, 25/3/2020, 3/11/2020

The Silver Birch - Betula pendula