Piptoporus betulinus which grows on birch trunks.
People are often intrigued by the discovery of galls, peculiarly deformed growths found on many trees. Most are caused by gall wasps, a group of insects most of which are tiny and hence easily overlooked. They lay their eggs on tree buds, leaves or flowers (according to species), and galls form from plant tissue in response to chemicals produced by the gall wasp larvae, which live inside the gall. A gall’s appearance is unique to each gall wasp species. Many species have two entirely separate generations, each one producing distinct galls.
Pedunculate Oak hosts a particularly large quota of gall wasp species, not to mention some striking galls. Among the more familiar are the following: Marble Galls, spherical and woody galls, up to 25mm across, caused by Andricus kollari; Oak Apples, spherical but knobbly galls, caused by Biorhiza pallida (the second generation form galls on oak roots); Artichoke Galls, where buds become swollen and artichoke-like, caused by Andricus fecundator; Spangle Galls, silk-button or flat button galls on the underside of leaves, caused by wasps of the genus Neuroterus; Cherry Galls, caused by members of the genus Cynips; Knopper Galls, distorted outgrowths on acorns, caused by Andricus quercuscalicis.
Although insects are the main causal agents of galls on trees in the British Isles, they are not alone in this ability. Many people will be familiar with the twiggy masses known as ‘witches’ brooms’ that form on birch trees. These are caused by the fungus Taphrina betulina. A closely related species, T. pruni, causes Pocket Plum galls on Blackthorn, where ripening sloes become distorted to resemble miniature runner beans.
Marble Galls.
Oak Apples.
Artichoke Gall.
Derick Bonsall
Knopper Gall.
Nail galls on Lime, caused by the mite Eriophyes tiliae.
Wasp of genus Cynips, causal host of Cherry Galls in oak.
Witches’ brooms are most evident on leafless birches during the winter months. A large specimen can measure 40cm across or more.
Cherry Galls.
MOSSES, LIVERWORTS AND LICHENS
From a climatic perspective, trees help moderate temperature extremes and increase humidity within woodland. Unsurprisingly, this has a beneficial effect upon mosses, liverworts and lichens, many of which are prone to desiccation. Native woodland floors are often carpeted with these primitive plants, and fallen logs and rotting timber are soon cloaked in their fresh green growth.
In addition to their climatic effect, trees have the added benefit, ecologically, of creating a third dimension in any woodland. Bryophytes take full advantage of this added woodland complexity and, under certain circumstances, trunks and branches can be festooned with a lush growth of mosses, liverworts and lichens. When growing in such places, these plants are referred to as epiphytes – they grow on the surface of the tree, are not parasitic, and cause no damage to the host.
In western Britain, where annual rainfall is high, tree trunks are often completely cloaked in mosses and liverworts.
White Fork Moss Leucobryum glaucum forms sizeable clumps on undisturbed woodland floors, particularly under Beech.
Visit almost any native woodland in Britain and you will find a profusion of mosses, liverworts and lichens growing both on the woodland floor and as epiphytes. However, for truly spectacular displays of epiphytic bryophytes you will need to visit woodlands in the west of the country, particularly those that grow on west-facing, rain-soaked hills or on the coast. In places it can be almost impossible to discern bare bark, with trunks, branches and twigs simply dripping with epiphytes. They cause no harm to the trees, other than the added weight perhaps, and gain their nutrition from nutrient run-off and from rain. Of course, the same is true of that other familiar epiphyte Ivy (a flowering plant), which is often maligned but hugely important in wildlife terms.
In a few cases, hedgerows that we see today are the linear remnants of ancient forests, all that is left of woodlands cleared for early agriculture from the Iron Age onwards. However, most hedgerows have been deliberately planted by man and are often hundreds of years old. For many people, they form the visual backbone of the British and Irish countryside.
Some hedgerows were planted to mark parish boundaries and most of these were in existence by the time of the Norman Conquest. However, many more were planted during the process of land enclosure, mainly between the 16th and 18th centuries. Before this time, much of the landscape was open land, used mainly for rough grazing with a little piecemeal agriculture. Enclosure, land clearance and hedge planting helped define and demarcate land ownership and in many cases hedges were maintained as stockproof barriers.
Ancient hedgerow.
When first created, hedgerows would probably have been planted with easily available, locally sourced shrubs and trees noted for their branching habit and, in some cases, spines. No surprise, then, that we find shrubs such as Blackthorn, Hawthorn and willow species predominating. Various elm species were popular hedgerow shrubs and trees in many parts, particularly East Anglia, while Beech prevailed in parts of the West Country, notably on Exmoor.
Over time, of course, natural additions to most hedgerows have occurred, seeds being transported by the wind or by roosting birds, depending on the method of seed dispersal of the tree or shrub in question. Given time, a hedgerow takes on the species composition of neighbouring woodland and much of the wildlife associated with woodland edges too. Research has found that there is a reasonably well-defined correlation between the number of woody species in a hedgerow and its age. The exact formula for calculating a hedgerow’s age is complicated, but a rough guide gives a reasonable degree of accuracy: count a 30m stretch of hedgerow and if 10 woody species are present, then it is likely to be around 1,000 years old; each woody species represents approximately 100 years.
Most hedgerows are man-made, and people control their fate to this day. Regular management – proper laying rather than indiscriminate flailing – is required to keep a hedge in good order. But, sadly, that seldom happens and,