About Betula pendula Roth
Betula pendula Roth, commonly called silver birch, typically grows 15 to 25 m (49 to 82 ft) tall, reaching exceptional heights of up to 31 m (102 ft). It has a slender trunk usually less than 40 cm (1 ft 4 in) in diameter. The bark on its trunk and branches starts golden-brown, later turning white as papery surface tissue develops and peels off in flakes, similarly to the closely related paper birch (B. papyrifera). Bark remains smooth until the tree reaches a large size; in older trees, bark thickens and becomes irregular, dark, and rugged. Young branches have whitish resin warts, while twigs are slender, hairless, and often pendulous. Its buds are small and sticky, and growth follows a sympodial development pattern: the terminal bud dies, and growth continues from a lateral bud. This species is monoecious, meaning male and female catkins grow on the same tree. Some long shoots bear male catkins at their tip, while short shoots bear female catkins. Immature male catkins persist through the winter, and female catkins develop in spring, shortly after leaves unfurl. Its leaves have short, slender stalks, and measure 3 to 7 cm (1+1⁄8 to 2+3⁄4 in) long. Leaves are triangular with broad, untoothed, wedge-shaped bases, slender pointed tips, and coarsely double-toothed serrated margins. New leaves are sticky with resin, which dries as leaves age, leaving small white scales. Foliage ranges from pale to medium green, and turns yellow early in autumn before leaves fall. In midsummer, female catkins mature, male catkins expand and release pollen, and wind pollination occurs. On average, a single silver birch catkin produces 1.66 million pollen grains. The small, 1- to 2-mm winged seeds ripen in late summer on pendulous, cylindrical catkins that are 2 to 4 cm (3⁄4 to 1+5⁄8 in) long and 7 mm (1⁄4 in) broad. Seeds are very numerous and separated by scales; when ripe, the entire catkin disintegrates, and wind spreads seeds widely. Silver birch is easily confused with the similar downy birch (Betula pubescens). Downy birches are distinguished by hairy leaves and young shoots, while the same parts on silver birch are hairless. The leaf base of silver birch is usually at a right angle to its stalk, while downy birch leaf bases are rounded. While the two species have very different genetic structures, they do occasionally hybridize. Silver birch grows naturally from western Europe east to Kazakhstan, the Sakha Republic in Siberia, Mongolia, and China’s Xinjiang province, and south to the mountains of the Caucasus, northern Iran, Iraq, and Turkey. It is also native to northern Morocco, and has become naturalized in other parts of the world. In the southern portion of its native range, it is mainly found in mountainous regions. Its light seeds are easily carried by wind, and it is a pioneer species, one of the first trees to grow on bare land or after a forest fire. It requires abundant light, grows best on dry acid soils, and is found on heathland, mountainsides, and clinging to crags. Its tolerance to pollution makes it suitable for planting in industrial areas and exposed sites. It has been introduced to North America, where it is called European white birch. It is considered invasive in the U.S. states of Kentucky, Maryland, Washington, and Wisconsin, and is naturalized and locally invasive in parts of Canada. Silver birch has an open canopy that lets plenty of light reach the ground, supporting the growth of a variety of mosses, grasses, and flowering plants beneath it, which in turn attract insects. Flowering species commonly found in silver birch woods include primrose (Primula vulgaris), violet (Viola riviniana), bluebell (Hyacinthoides non-scripta), wood anemone (Anemone nemorosa), and wood sorrel (Oxalis acetosella). Small shrubs that grow on the forest floor under silver birch include blaeberry (Vaccinium myrtillus) and cowberry (Vaccinium vitis-idaea). Birds found in birch woodland include the chaffinch, tree pipit, willow warbler, nightingale, robin, woodcock, redpoll, and green woodpecker. Silver birch branches often host tangled masses of twigs called witch's brooms, caused by the fungus Taphrina betulina. Old trees are often killed by the decay fungus Fomitopsis betulina, and fallen branches rot quickly on the forest floor. This tree commonly forms a mutualistic relationship with the mycorrhizal fungus Amanita muscaria, particularly on acidic or nutrient-poor soils. Other mycorrhizal associates include Leccinum scabrum and Cantharellus cibarius. Besides mycorrhiza, soil microfauna support the tree’s growth by enhancing nutrient mobilization. The larvae of many species of butterflies, moths, and other insects feed on silver birch leaves and other parts. In Germany, almost 500 insect species have been found on silver and downy birch combined, including 106 beetles and 105 lepidopterans, with 133 insect species feeding almost exclusively on birch. Birch dieback disease can affect planted silver birch trees, though naturally regenerated trees appear less susceptible. This disease also affects B. pubescens, and in 2000 it was reported at many Scottish sites where birch had been planted in the 1990s. In the United States, silver birch wood is attacked by the bronze birch borer (Agrilus anxius), an insect pest to which it has no natural resistance. Silver birch is the national tree of Finland. Leafy, fragrant bunches of young silver birch boughs, called vihta or vasta, are used to gently beat the body during Finnish sauna bathing. Silver birch is commonly planted in parks and gardens for its attractive white bark and gracefully drooping shoots, and is sometimes grown even in warmer-than-optimal locations such as Los Angeles and Sydney. In Scandinavia and other northern European regions, it is grown for forest products including lumber and pulp, as well as for aesthetic purposes and ecosystem services. It is sometimes used as a pioneer and nurse tree in other areas. Silver birch wood is pale, with a light reddish-brown heartwood. It is used to make furniture, plywood, veneers, parquet blocks, skis, and kitchen utensils, and for turnery. It makes good firewood, but burns quickly. Bark slabs are used to make roof shingles, and bark strips are used for handicrafts such as bast shoes and small containers. Historically, bark was used for tanning. Bark can be heated to collect resin, which makes an excellent waterproof glue and is useful for starting fires. Thin peeling sheets of bark from young wood contain waxy resin and ignite easily even when wet. Dead twigs are also useful as kindling for outdoor fires. Bark removal was once so widespread that Carl Linnaeus expressed concern for the survival of silver birch woodlands. Birch brushwood is used for racecourse jumps and besom brooms. In spring, large volumes of sap rise through the trunk and can be tapped. This sap contains around 1% sugar, and can be used similarly to maple syrup: it is drunk fresh, concentrated by evaporation, or fermented into a "wine". Successful silver birch cultivation requires a climate cool enough to produce at least occasional winter snowfall. Because they are shallow-rooted, silver birch may need water during dry periods. They grow best in full sun, planted in deep, well-drained soil.
