Morphological differences between Silver and Downy Birch
Oliver Sheridan, Teagasc Forestry Research Officer, tells us more about Silver Birch (Betula pendula Roth) and Downy Birch (Betula pubescens Ehrh.) - two birch tree species that occur naturally throughout most of Europe.
Silver Birch is one of the most important broadleaved species in Northern and Eastern Europe. It is also an important commercial tree species in Russia and Belarus. Not only is it an important commercial tree, large numbers of species feed on or live together with birch including mycorrhiza forming fungi, herbivores, wood decaying fungi and saproxylic insects.
Both birches are native to Ireland and occur naturally throughout the country. In Ireland, there has been an interest in birch for many years - mainly for ecological reasons - but in more recent years, there has been more of a focus towards their economic benefits. Birch produces a light coloured timber, which can be used for high quality pulp, sawlog and veneer. The general appearance of Silver Birch and Downy Birch is very similar; they are both white stemmed and can reach a height of 24–30m.
Silver Birch grows best on well-aerated, fertile soils such as light mineral soils and sandy soils. They dislike compacted, water-logged, infertile soils and on these sites growth is poor. Downy Birch can tolerate a wider range of soils than silver birch. They are tolerant of wet or waterlogged conditions, heavy clays and wet peatlands, but will also produce good growth on fertile mineral soils. Differences exist also in the morphology of the leaves, twigs, bark, seeds and catkin scales. They differ in chromosome numbers also, i.e. Betula pendula is a diploid species with 2n = 28 chromosomes in its vegetative cells, while Betula pubescens is a tetraploid with 2n = 56. The two species can co-exist together and are capable of hybridizing, but hybrids are considered to be rare due to a biochemical incompatibility mechanism between the two species.
But morphological differences between Silver Birch and Downy Birch can sometimes be difficult to distinguish, particularly in the case of B. pubescens having similar and highly variable morphological characteristics. The two species have different yield potential and, because they need different management regimes, it is imperative that they can be correctly identified to assist in the management of the plantations and maximise quality timber production. The most extensive growth and yield research on birch has been carried out in the Nordic countries. The total volume yield in managed silver birch plantations up to the age of 60 years varies between 360 and 560 m3ha-1. The volume yield for Downy Birch would be somewhat less because it is not as widely planted as Silver Birch. Methods to distinguish between the two species, other than relying on morphological differences, are often used; the most reliable being chromosome counting, but this method can be very time consuming and often trying to get suitable material for testing can prove difficult. Fortunately, however, the two species in some cases display distinct differences in the morphology of their leaves, twigs and bark. There are also differences between their seeds and catkin scales, but these are less obvious to the naked eye.
The leaves of Silver Birch are broadly ovate (triangular) to almost rhombic in shape. They have a coarsely double serrate margin, with acute apex and a slender petiole.
The leaves of Downy Birch are broadly ovate to round triangular in shape. The margin is coarsely serrate, but not double serrate. The leaf apex is less acute and the base tapered with a thick petiole. Overall leaf shape is somewhat cordate (heart shaped). Young leaves are often covered with fine hairs.
Young twigs of Silver Birch are generally pendulous, slender, glabrous (free from hair or down) and covered with numerous resinous glands and lenticels. Lenticels are raised pores in the stem of woody plants that allow gas exchange between the atmosphere and the internal tissues.
Twigs of Downy Birch are pubescent without resinous glands and little or no lenticels present. Shoots on young trees are never pendulous.
The bark of the Silver Birch is silvery white and peeling when young, becoming thick, deeply fissured and black at the base of the trunk with age. The bark of downy birch tends to be smooth, peeling off in thin papery strips, dull white to pale brown and not becoming coarse or fissured.