Fact sheets on our natural history
The Oak Tree
Quercus
There are two native species of Oak growing in Britain – the Pedunculate (or English) Oak (Quercus robur- meaning strong, of the timber), and the Sessile Oak (Q. petraea - of the rocky places). These two species may be easily distinguished respectively by the presence or absence of acorn stalks. Curiously, the leaves behave in the opposite manner, with the Pedunculate Oak usually having no petioles, while the Sessile Oak has stalked leaves. A better means of distinguishing these two is found at the base of the leaves, the English Oak being lobed, while that of the Sessile Oak is un-lobed. In the Bristol area, the Pedunculate Oak is by far the most common. English Oak heartwood is of high quality in strength and durability, and has been in great demand throughout history, especially for the construction of barrels, houses and ships. Oak is very long-lived, some of our veteran trees being over 1000 years old. Most of our old Oaks have been managed by pollarding, that is, the harvesting of the timber by removal of the upper branches. This process can prolong the life of the tree by up to 50%. Good examples of ancient Oak trees may be found at Ashton Court, and on the south boundary of Towerhouse Wood. Both the National Trust and the Woodland Trust use Oak leaves in their logos.
The Greek word for the Oak - dryas (δρυας) may be found frequently in the English language – in words like Druid and Dryad, and also in the scientific names of many of our plants – like Dryopteris, Dryas and chamaedrys
The Pedunculate Oak probably supports the largest number of insect species of any British tree. It is also host to a large variety of fungi. Oak mildew (Microsphaera alphitoides) introduced in 1908 can cause the death of young trees by attacking the leaves, so reducing photosynthesis. Of the larger wood rotting Basidiomycetes, Fistulina hepatica (Beefsteak fungus, a brown rot) and Polyporus squamosus (Dryad’s Saddle, a white rot) both occur in Towerhouse Wood. (‘Brown rot’ fungi remove cellulose, leaving lignin, while ‘white rot’ fungi remove both lignin and cellulose.)
The Jay is probably mainly responsible for the dispersal of the acorns, since it has the habit of burying them in caches, failing to recover most. Acorns have had many uses. They were a source of food (pannage) for pigs that are able to tolerate the high concentrations of tannin. In both World Wars, an ersatz coffee was produced from the roasted acorns.
The tannins in Oak bark have been used for many centuries for the production of leather. More recently, other sources of imported tannins have been found, but there remains one company in Cornwall that continues this tradition, using native Oak.
Recent research has shown that the Oak tree, in common with Willows and Poplars, produces significant amounts of volatile organic compounds (VOCs), notably isoprene (CH2=C(CH3)CH=CH2), which interacts with oxygen by complex processes involving methylvinylketone and methacrolein to produce ozone (O3) and peroxyacetyl nitrate (MeCOO2NO2). Ozone is a pollutant in the lower atmosphere, toxic to plants and animals. It also contributes to the greenhouse effect, although in the upper atmosphere it is essential to prevent penetration of harmful UV light. The Ash tree by contrast with Oak produces only small amounts of VOCs, and may even tend to remove them from the atmosphere.
The Yew Tree
Taxus baccata
Few of our old churches can be without an ancient Yew tree close by, possibly planted to epitomise immortality. This tree is very slow growing and lives for a long time, some dating back several thousand years. The wood is very hard and dense, and is resistant to decay and insect attack. Some of the ancient artefacts found in peat bogs were made of Yew wood. In the past, the wood has found many uses ; in making long bows, in carving, wood turning and for chair arms. The sapwood is yellow and the heartwood is red-brown. The best bows were made from wood taken at the junction of sapwood and heartwood, and contained both. Many of the Yew bows were imported from Europe during the Middle Ages, mainly from Spain and Italy, due to the poorer quality of the wood of the English Yew. Our oldest Yew trees, those over 500 years old, usually become hollow, as may be seen in the churchyard of Portbury church, though they will endure for many years in this state. The woods around Nailsea have many Yew trees, and in some, like those in Bourton Combe, this tree becomes dominant.
Very few insects feed on the Yew, since it contains a range of toxins. However, a small fly (Taxomyia taxi) is dependent on the Yew, forming small galls (artichoke galls) at the tips of the growing shoots in some localities, e.g. Brockley Combe and Towerhouse Wood.
The origin of the scientific name for the genus Taxus is debatable. Some believe that it is derived from the Latin 'texere' to weave (hence the word textile), from the arrangement of the leaves, which are said to be distichous, that is - in two rows. Others suggest that it is derived from the Greek word toxon (toxin) for poison. Yet others associate it with the word for archery, toxophily.
The Yew is dioecious, having separate male and female trees, only the female tree having the conspicuous soft, bright red 'berries'; the species name 'baccata' meaning 'having berries'. The correct name for these is ‘aril, the soft part of which is edible. However the seed that it contains is said to be extremely poisonous and should never be eaten. Many birds appear to be unaffected by this toxin, probably because they do not crush the seed, which is in a thick case. Although deer appear to be immune to the cardio-active taxines, cattle may be killed within a few minutes if they eat the leaves, and this may have been another good reason.for growing Yew in churchyards where it would have been surrounded by a stock proof fence or wall. As little as 50 g is lethal to humans, slowing the heart rate to 30 beats per minute. Leaves that have been cut and are slightly wilted are thought to be particularly toxic.
Nettles Urtica dioica
Early childhood memories of Nettle stings are a strong reminder to avoid contact with that plant, since the effect is painful and lasts for several hours. Although it is not known to be fatal in humans, even in severe cases, it is reported that dogs have been killed by being badly stung. It is likely that the Nettle sting evolved as a deterrent to herbivores, and certainly the Nettle is a very successful agricultural weed, growing especially well in pastures frequented and manured by animals, and in the South West it is probably the most widespread native plant.
At one time it was thought that formic acid is the active factor in the stinging hairs, but it is now known that the concentration of this is very small in the hairs. The three organic chemicals mainly responsible are – histamine, serotonin (5-hydroxytryptamine) and acetylcholine. Histamine is a well-known cause of irritation, and the other two components act as synergists for this effect. The single celled hairs are made of silica (below), which easily fragment on puncturing the skin, releasing the toxic fluid.
The only other stinging plant in the UK is the Annual Nettle (Urtica urens). Other plants, including close relatives of the Nettle (Mucuna, Laportea) from the tropics are known to have even more powerful stings. The name Urtica is derived from the Latin ‘urere’ to burn, while the name Nettle may be related to the word ‘needle’. There may be a good reason for the use of Dock
Many animals are not deterred by the stinging hairs, most notably the larvae of several of our common butterflies (Tortoiseshell, Red Admiral, Peacock and Comma) that depend on the nettle as their food source. Animals differ in their sensitivity to the stings. Rats appear to be immune, while guinea pigs and other domesticated animals are badly affected.
Richard Mabey recommends cooked Nettle leaves in his book ‘Food for Free’. Having tried these myself I find them rather bland, but not unlike Spinach. They are best harvested when young, not beyond the middle of June. After this time apparently they become laxative and rather fibrous.
The latest research suggests that Nettle roots may provide a cure for benign prostatic hyperplasia - a common affliction in males in later life. I imagine that Godfrey in ‘Dad’s Army’ shows the classical symptoms of this affliction, and he has my sympathy! The Royal Society of Medicine also reports that Nettle stings may be used to reduce the pain of osteoarthritis. This may be related to the neurotransmitters present in the sting.
In common with the close relative, Hemp, Nettles have a useful fibre, which has been used extensively for the production of cloth. The properties of this fibre are not greatly inferior to those of flax. In the First World War the Germans used Nettle fibre to produce army shirts, each shirt needing about 40 kg of Nettles. There is now a revival of this source, and there are proposals to start the commercial cultivation of the Nettle plant for this purpose. The shoots are cut in September or October, kept in a damp atmosphere for 3-4 weeks and the fibre is then easily separated.
Nettles were also used during the Second World War in the production of the green chlorophyll to colour camouflage drapes. Nettles are a good source of this pigment since enzymes causing its oxidation are virtually absent in this plant.
The specific name of the Nettle ‘dioica’ implies that this plant is dioecious, having the sexes on two separate plants. Some floras suggest that it is possible to sex Nettles by observing the gross structure of the ‘catkins’ at the internodes. From personal experience this is not a good indicator, and it is necessary to use a hand lens to view the floral structure more closely. The male flowers are almost spherical when unopened. Opening occurs suddenly with the appearance of four very pale anthers. The female flower is smaller, somewhat pear-shaped, with a brownish stigma at the tip.