Betula Pendula - Description

Description

It is a medium-sized deciduous tree, typically reaching 15–25 metres (49–82 ft) tall (exceptionally up to 39 metres (128 ft)), with a slender trunk usually under 40 centimetres (16 in) diameter, but exceptionally to 1 metre (3.3 ft) diameter, and a crown of arched branches with drooping branchlets. The bark is white, often with black diamond-shaped marks or larger patches, particularly at the base. The shoots are rough with small warts, and hairless, and the leaves 3–7 centimetres (1.2–2.8 in) long, triangular with a broad base and pointed tip, and coarsely double-toothed serrated margins. The flowers are wind-pollinated catkins, produced before the leaves in early spring, the small 1-2mm winged seeds ripening in late summer on pendulous, cylindrical catkins 2–4 centimetres (0.79–1.6 in) long and 7 mm broad.

It is distinguished from the related downy birch (B. pubescens, the other common European birch) in having hairless, warty shoots (hairy and without warts in downy birch), more triangular leaves with double serration on the margins (more ovoid and with single serrations in downy birch), and whiter bark often with scattered black fissures (greyer, less fissured, in downy birch). It is also distinguished cytologically, silver birch being diploid (with two sets of chromosomes), whereas downy birch is tetraploid (four sets of chromosomes). 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 (and cause confusion by combining the downy birch's alternative vernacular name 'white birch', with the scientific name B. pendula of the other species), but they are regarded as distinct species throughout Europe.

It 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. Old trees are often killed by the decay fungus Piptoporus betulinus, and the branches often have witch's brooms caused by the fungus Taphrina betulina.