Draw a tree and you will likely start with a nice straight trunk. This is what most foresters want; a lack of knots in the outer wood is also desirable. Knots form where the tree has grown out round the base of a twig or branch, so a good timber tree is generally one where there are no twigs or branches on the lower few metres of the trunk. Foresters often refer to trees having a ‘clean stem’. However, with trees, as with humans, for one reason or another growth does not always proceed in an orderly way.
(a) A good clean ash stem contrasted with (b) a most disorderly one in Wytham – cause unknown.
In oak, like many other trees, there are dormant buds just under the bark – epicormics – whose growth may be triggered by damage to the main crown, as when it is pollarded. The shoots produced from around the top of the trunk then form a new crop of branches, allowing the tree to continue growing. Heavy defoliation of the canopy such as by moth caterpillars may also trigger new shoots to form on the trunk. Epicormic growth is becoming common on ash trees suffering from dieback.
Buds under the bark of the main stem may also be stimulated to grow when the conditions around the tree suddenly change: for example trees that have grown up in a densely-packed stand may respond to heavy thinning, which increases the light and warmth reaching the tree-trunk, by a burst of epicormics. Foresters, concerned with wood production, therefore tend to thin oak stands only lightly and aim to maintain deep-crowned trees to reduce the risk of such epicormic shoots being produced. On the continent a dense understorey of hornbeam is commonly used to keep the lower part of the oak stem clean (Savill, 2013). If epicormic shoots do start to form they may be pruned off quickly, to stop a large knot developing at their base.
(a) New growth on this recently pollarded oak, arising from epicormic buds; (b) epicormic growth on dying ash; (c) epicormic growth following thinning in an oak stand
A proliferation of growth from buds under the bark can lead to the formation of burrs on oak trunks. Commonly these take the form of roughly hemi-spherical swellings. If a section is cut through a burr the usual pattern of concentric regular annual rings is missing. Instead the grain is much wilder, swirling in different directions, with the numerous buds showing up as circles, giving rise to the expressions ‘pippy oak’ or ‘cat’s paw’. This attractively-figured grain makes pippy oak valuable for veneers and in wood-carving and there are stories of promising-looking burrs being ‘poached’.
Burrs on oak
Burr development may be even more extensive, covering large areas, sometimes almost the whole of the lower trunk. Burrs may be particularly prominent on the old, open-grown oaks in parkland situations, partly because such trees have often been regularly pollarded in the past, or may have had cattle nibbling at the bark, both of which can promote epicormic bud growth; partly just because old trees will have had more time for the burrs to grow.
Burrs add to the character of oak trees as with the Elephant Tree at Windsor which heads this account. Burr growth can though cause problems if you are trying to measure and compare the girth or diameter of old oaks, and hence establish their age. Normally tree girths and diameter are measured at 1.3 or 1.5 m off the ground surface, but if there is a burr at this height it will distort those measurements. Where possible, the nearest point below the burr should be used to record the tree’s size. If that is not measurable then a point above the gall should be sought, as close to the standard 1.3 m height as possible.
Examples of heavily burred oaks; now where should you measure their diameter!
Burrs on oaks are triggered by the tree itself, as are the thickets of epicormic shoots seen around the bases of common limes. Other growths on trees may be a response to viral, bacterial, or fungal activity. The witches brooms on birch for example are associated with the fungus Taphrina betulina. They are therefore similar to the galls formed on trees by insects and mites, such as oak apples and knopper galls.
Exuberant epicormics at the base of a common lime and witches brooms in the canopy of birch.
The best time to appreciate these strange structures is in the winter when there are no leaves on the trees and the ground flora is subdued – even in plantations trees may not be so uniform as at first appears to be the case.