Birch trees with silver-grey bark that are a common sight in Nailsea, include the slender Silver Birch. This tree and many of its close relatives are able to survive in very cold climates. The Birch is the national tree of Finland and its habitat extends to Greenland and the Arctic. Birch belongs to the Betulaceae, the family that includes the hazel and the alder.
Birch is one of the ‘pioneer’ species that establishes itself on bare or poor soil that then becomes sufficiently enriched to allow the growth of other plants. Birch does not cast deep shade on the ground beneath it. Male and female flowers on distinguishable catkins are borne on the same tree. They start to open in April and catkins eventually produce seeds that are dispersed by the wind. Birch seeds have two small wings and differ from their counterparts on the ash and maple trees, which have one larger wing. Birch is considered to have been among the first species of tree to grow after the last ice age receded.
In pre-Christian times, Silver Birch was regarded as holy and the practice of birching people with its twigs probably originated with the aim of driving out evil spirits.
The wood does not differentiate into sapwood and heartwood; its present commercial significance is as a source of plywood and of pulp for paper. Throughout history, in the Northern Hemisphere at least, Birch bark’s stability and imperviousness to water has provided a variety of significant uses – as a writing surface (parchment), and in roofing, leggings and canoes. Birch tar served as the glue for fixing flint arrowheads to the shafts of arrows, and the sugary sap of Birch trees was fermented into wine.
The white bark is due to betulin, a compound that has anti-microbial properties. Betulinic acid, also found in Birch, will selectively kill certain cancer cells, raising the prospect of Birch trees being an inexpensive source.of this drug. Like willow, Birch also contains methyl-salicylate with a chemical structure similar to aspirin. This is the active component of oil of wintergreen also found in other plants, but which can only be applied externally. Methyl-salicylate vapour is thought to be formed when they are threatened with disease. As with many other trees, Birch roots are associated with beneficial fungi (mycorrhiza).
Birches can host a variety of galls (abnormal tissue outgrowths) of which the most prominent is witches broom, an aggregate of small twigs caused by a fungus (or sometimes by an insect). Less obvious are galls (in leaves and female catkins) caused by certain species of midges, mites and moths. These can become host to communities of other insects whose larvae live on the gall tissue or on larvae of the creatures that initiated its development.
Aging Birch bark can be attacked by a fungus (Piptoporus betulinus), also known as the razor-strop fungus.. The trunks of older forest Birches are often rotten inside, a condition called ‘brittleheart’ then becoming more prone to fungal attack.
Downy Birch (Betula pubescens) is the other species of Birch native to Britain. It tends to grow on wetter upland sites. Its branches are less pendulous than those of the Silver Birch (Betula pendula); the serrations on the edges of its leaves are more regular and the leaves are a rather different shape. Its twigs have a coating of downy hair, unlike those of Silver Birch. Cell nuclei of the Downy Birch, being tetraploid, have double the number of chromosomes found in Silver Birch (diploid). Some Birch trees planted in Nailsea that have a very bright bark are likely to be the Betula jacquemontii, which originated in the Himalayas (see photograph above).
The trunks of Birch trees commonly display thin horizontal pores in the bark called lenticels, which allow entry of oxygen. When older, the bark of Silver Birch becomes disfigured with dark scars. Birch bark, in association with a ‘tinder fungus’ that can grow on Birch, is a useful source of fire kindling in cold and wet outdoor conditions that threaten human survival. One of the mycorrhizal fungal associates in the roots is also sometimes seen as the bright red hallucinogenic fly agaric (Amanita muscaria).
(ii) Resistance to cold
In North America the trend is for deciduous trees in the more northern forests to have paler trunks. The hardiness in such trees may stem from the reflective properties of the whitish bark that can protect the surface of the trunk. Birch bark is very water-resistant, and this might also contribute to the tree’s capacity for survival.
However not all species of Birch have light colored bark and if trees with darker trunks still prove to be very hardy, this raises the question of other aids to survival being at work. Animals that must remain alive in frozen environments possess biochemical mechanisms protecting their cells from damage. These include anti-freeze chemicals, among which can be sugars of low molecular weight. For example Birch sap has a high sugar content (glucose and fructose), and the sugar (sucrose) of the Sugar Maple of North America may function similarly. Some frogs in freezing conditions can generate protective amounts of glucose from reserves of polysaccharide and frozen potatoes become sweet. Birch and Sugar Maple can store starch in roots and cells associated with the nutrient conducting system.
The ancient Chinese used mucilage from birch leaves in making paper, and it is worth asking if this substance serves to protect the living leaf and trunk Like anti-freeze proteins elsewhere (and stabilizing gums in ice cream), it might help prevent damaging ice crystals from forming inside the leaf cells exposed to temperatures that fluctuate above and below freezing.
(iii) Chemicals of medicinal use
There is evidence that betulinic acid causes cancer cells to self-destruct and that normal cells are affected less. Betulin is responsible for the whitish colour of Birch bark and is present in concentrations that can reach 25%. Both compounds belong to the terpenoid group of chemicals whose members consist of isoprene molecules linked together – each isoprene unit containing 5 carbon atoms. Betulin and betulinic acid have a steroid-like ring structure. Anti-cancer compounds (taxols) of a different chemical nature have been isolated from yew trees.
Willow’s counterpart of the methyl-salicylic acid found in Birch is salicin (salicylic acid with a sugar molecule attached). Aspirin is acetyl-salicylic acid, synthesized in the laboratory. It has been suggested that salicylic acid is a plant growth hormone, supporting observations that the life of cut flowers can be prolonged by adding aspirin to their water
We shall be glad to hear of any observations that you may have on Birch trees.
Grant Burleigh amd Terry Smith 22th February 2006