It is a pity about the common names. There is nothing common about this magnificent plant except the frequency with which it is encountered throughout the British Isles. Quercus robur is also, inaccurately, called the English oak, but there is nothing quintessentially English about it. If anything it is the European oak, the climax plant species in European temperate forest ecosystems and one of the most important of our native tree species economically.
So, what does pedunculate mean? It refers to the stalk or peduncle on which the fruits, of one or a few acorns, are borne. In Quercus robur, the leaves are short-stalked, almost sessile, and the acorns long-stalked or pedunculate. These are the main features that differentiate it from the other common British native deciduous oak species Quercus petraea, in which the acorns are almost sessile and the leaves are borne on longer stalks or petioles.
Quercus robur and Quercus petraea are not only highly variable but their characters overlap, and therefore the boundaries between the species are fuzzy. There may be morphological differences even within the same plant, for example between normal foliage leaves and the Lammas leaves produced in midsummer. Despite statements in older literature that the species do not hybridise (Dengler, 1941), it is recognized that they are interfertile, and that the crosses, named Q. x rosacea, are intermediate in all characters with the parents. To make separation even more difficult, the hybrids freely backcross with the parents to form a practically continuous range of intermediate forms, so that the species limits cannot easily be specified. Many experienced botanists give up the struggle to separate them and simply treat them as an aggregate.
Darwin (1859) comments on this variability in his discussion of “Doubtful species” in The Origin of Species, writing that “a German author (sic) makes more than a dozen species out of forms which are almost universally considered to be varieties; and in this country the highest botanical authorities and practical men can be quoted to show that the sessile and pedunculated oaks are either good and distinct species or mere varieties.” He considered the species boundaries to be arbitrary, and variability to be the melting pot of characters upon which natural selection could act.
There are some differences in overall form of the trees, open-grown mature specimens of Q. petraea being more upright and straight, while those of Q. robur often broad-crowned and gnarly. Leaf lobes are rounder and fewer in Q. robur, smaller and more numerous in Q. petraea. Q. robur has short petioles and long peduncles, Q petraea longer petioles and nearly sessile acorns. The leaf bases of Q. robur typically have auricles, while those of Q. petraea are poorly defined (if present), and the leaf base may taper gradually to the petiole, and Q. robur leaves generally have intercalary veins between the lobes. These characters are nicely described and tabulated in a BSBI Plant Crib that can be downloaded from here (Rushton, 1997).
Viewed at hectad level on the distribution maps prepared by the Botanical Society of Britain and Ireland, which can be found here, the distributions of the species appear similar, but Q. petraea better tolerates damper habitats with acidic soils and may be more common than Q. robur in the north and far west of Britain. Q. petraea avoids the more alkaline, heavy clay soils on chalk and limestone in central and eastern England, and in these areas Q. robur is the dominant species.
Oaks are monoecious species, meaning that they have entirely separate male and female flowers borne on the same plant. The catkins of clustered apetalous male flowers of Q. robur have about 4-12 anthers per flower, while the female flowers, also apetalous, are borne 1 or a few at a time on stiff peduncles. They are reduced to the absolute essentials, an ovary, a short style and a stigma, surrounded by a few bract scales that become much more numerous after fertilisation and ultimately form the cupule protecting the growing acorn.
ovary, reduced style and three-lobed stigma, surrounded by a few basal bracts that increase in number after fertilisation to form the acorn cup. Image by Dimitar Nàyedonov, licensed under CC-BY-SA 4.0
The trunks of young trees can be tall, straight and narrow, with few side branches, especially if they are grown in close proximity to each other or with other trees. Trees grown in the open are shorter and wider, often developing a broad canopy of twisting and contorted branches that droop low to the ground, and they mature several decades earlier than those growing in close proximity. The oldest trees have extraordinary tenacity, surviving despite massive fungal erosion of their trunks and lightning strikes that split stems apart. Adventitious branching is a common feature on the trunks of mature oaks. Over a period of many years, large groups of adventitious shoots may develop into conspicuous swellings called burrs, the pippy, contorted wood of which is very decorative and highly sought after by makers of fine furniture such as John Makepeace and the late Tim Stead.
The plant is very popular habitat for fungi, insects and other arthropods, and can support a significant population of epiphytic lichens, ferns, mosses and liverworts.
Almost every bud or leaf of a sample branch I collected in early May was already full of galls or holes, patches of mesophyll scraped down to the veins, stitched up with silk, remodelled by wasps or eaten ragged by hungry caterpillars Tortrix viridana L. and Operophtera brumata L. When I opened the ziplock bag at home, a host of arthropods using various modes of locomotion leapt, ran, crawled, or flew to illusory freedom in my study. On opening a silk-bound curled leaf, a small plant bug ran onto my hand and drilled into it, causing a swelling that lasted for days. Clearly these guys are no vegans, they don’t seem to care where their food comes from.
By August, various types of galls are already well developed. Galls are swellings of plant tissue containing the larvae of gall wasps that secrete plant growth regulators which control the development of the oak tissues. The brown spheres of the oak apple gall begin life as translucent green shells with evenly distributed brown spots. Inside, a core of tissue is suspended from the shell by radiating silken strands. The galls contain the larva of a tiny Cynipid wasp Biorhiza pallida that injects its eggs in the young tissue of a leaf or bud.
The common spangle gall which appears on the undersides of oak leaves as raised circular red patches are formed by another Cynipid wasp, Neuroterus quercusbaccarum. Leaves affected by spangle gall typically host many tens of galls, but cause little apparent harm to the tree, although affected leaves senesce and fall several weeks earlier than healthy leaves.
The artichoke gall is formed by hypertrophy of an oak bud, under the chemical influence of growth regulators produced by a single larva of the Cynipid wasp Andricus foecundatrix. At maturity, the enlarged galls look like miniature artichokes. Within each, the larva develops inside a shiny brown hard casing, looking very much like a miniature acorn.
Left: Knopper galls caused by the Cynipid wasp Andricus quercuscalisis, affect the development of the acorns. Image by Lenny Worthington and licensed under cc-by-sa-2.0 Right: interior of knopper gall showing the wasp larva enclosed in a hard, acorn-like shell. Image by Wilhelm Zimmerling and licensed under cc-by-sa-4.0
The knopper galls produced by another species of wasp in the same genus, Andricus quercuscalicis, form by chemical modification of the growth of developing acorns. The acorn forms convoluted flutes, distorting it until it begins to look like a walnut. Knopper galls were first recorded in the British Isles in the 1960s, and the first record in Scotland was at Eglinton Country Park, North Ayrshire, in 2007. From there it appears to have spread rapidly, presumably in response to changing climate. We have found the galls this year in the oak wood at Dalkeith Country Park and on a neighbour’s oak in Inveresk. In both cases, the intensity of the infestations suggests it has the potential to cause a significant reduction in the fecundity of the affected oak tree, but adjacent oak trees may be unaffected, suggesting that individuals vary in their susceptibility. An intriguing aspect of this species is that it has a life cycle involving separate, alternating sexual and parthenogenetic stages on different oak species, the alternate host being the introduced alien Turkey oak, Quercus cerris. The sexual phase involves the formation of small conical galls on the male catkins of the Turkey oak, so we’ll be on the lookout for these in our area in the spring.
Acorns have been few and far between this year, with some trees producing none or few, although there was considerable individual variation in this, occasional specimens producing a normal crop. Perhaps the fruiting behaviour is not synchronised. Oak, like beech and plums, is noted for having fruit years with large crops separated by four to six years of rest, during which the trees replenish stocks of nitrogen, potassium and phosphorus ready for the next masting.
The wood of Q. robur is highly valued not so much for its appearance but for its durability, hardness and strength, and no other European hardwood has such a wide range of traditional uses. It was the timber of choice for ship building by the Royal Navy until iron and steel began to have an impact as a a structural material as the Industrial Revolution got under way in mid- 19th century. For this purpose the contorted branches of open-grown oaks were especially valuable because they could be riven, split without sawing, to form curved structural members in which the grain was continuous from end to end. The Mary Rose, Henry VIII’s largest, state-of-the-art signature warship sank in 1545 after 33 years of service, probably having been destabilised following refitting with an ambitiously large tonnage of heavy iron guns. Construction of the ship is said to have consumed the wood of 600 mature oak trees.
In the middle ages, the frames and roof trusses of timber framed buildings, and roof timbers of larger, stone buildings such as the roof and spire of Notre Dame cathedral in Paris, which was constructed in 13th century were typically made of oak. The hammerbeam oak roof of the Great Hall at Edinburgh Castle still stands and was referenced in the recent reconstruction of the roof of the Great Hall of Stirling Castle.
Oak wood and bark contain gallic acid, trihydroxy benzoic acid, a phenolic acid or tannin also found in tea leaves. It corrodes iron nails. Consequently, traditional construction methods made use of wooden pegs instead of metal fastenings.
Other uses of oak wood that continue to this day include the making of barrels for storage and ageing of wine and spirits whisky, bourbon (they were made by coopers, and hence the surname Cooper). Smoke from smouldering wood chips is used to flavour salmon, pork and other meats and cheeses. Both trunks and finer branches are used to make charcoal, which until coke replaced it during the 19th century was essential fuel for iron smelting and steel making.
Galls and bark are rich in tannins used in leather tanning, but the cork used to seal wine bottles comes from the much thicker bark of the cork oak Quercus suber, and not from common oak.
Robin Hood is said to have sheltered beneath the Major Oak in Sherwood Forest. It is up to 1000 years old and has a huge girth of 10 metres, but even that is overtaken by the Bowthorpe oak in Lincolnshire, the 12.5 metre trunk of which earned it a place in the Guinness Book of Records.
In Scotland there are also many remnants of ancient oak woodland, in Dalkeith Country Park near Edinburgh, and the Cadzow oaks in an area of wood pasture on the outskirts of Hamilton, some specimens of which date back to mid-15th century (Dougall & Dickson 1997). Coppicing of oaks was practiced in Scotland at least from medieval times, and Dickson and Dickson comment that a few of the coppice stools in Mugdoc Wood north of Glasgow are massive enough to have been coppiced since that time.
References
Darwin, C. (1859) On the origin of species by means of natural selection or the preservation of favoured races in the struggle for life. Chapter II. Variation under nature. John Murray, London.
Dengler, A. (1941) Berichte uber Kreuzungsversuche zwischen Trauben – und Stieleiche. (Quercus sessiliflora Smith u. Q. pedunculata Her. Bzw Robur L.)*. Mitteilungen H, Goring-Academie Deutsche Forstwissenschaft, 1, 87-109. *Translates as: Reports on crossing attempts between grape oak (sessile oak) and pedunculate oak.
Dickson, C. and Dickson, J.H. (2000) Plants & people in ancient Scotland. Tempus publishing, Stroud. P273. ISBN 0 7524 1905 6
Dougall, M. and Dickson, J. (1997) Old managed oaks in the Glasgow area. Pp 75-84. In: Smout, T.C. (ed.) (1997) Scottish woodland history. Scottish Cultural Press. Dalkeith. ISBN 1 898218 53 6
Rushton, BS, (1997) Plant Crib: Quercus 1. Quercus petraea / Q. robur / Q. rosacea. Ed. by TCG Rich & AC Jermy. Botanical Society of the British Isles in association with National Museums of Wales. Produced by HBR Cleal. https://bsbi.org/wp-content/uploads/dlm_uploads/Quercus_Crib.pdf
© Chris Jeffree
Botany in Scotland 