Oak, English (Quercus robur)

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English oak is arguably the best known and loved of British native trees. It is the most common tree species in the UK, especially in southern and central British deciduous woods.

Common name: English oak

Scientific name: Quercus robur
Family: Fagaceae

UK provenance: native

Interesting Fact: acorns are not produced until the tree is at least 40 years old. Peak acorn fecundity usually occurs around 80 – 120 years.

What does oak look like?

Overview: English oak is a large deciduous tree up to 20-40m tall. In England, the English oak has assumed the status of a national emblem. As common oaks mature they form a broad and spreading crown with sturdy branches beneath. Their open canopy enables light to penetrate through to the woodland floor, allowing bluebells and primroses to grow below. Their smooth and silvery brown bark becomes rugged and deeply fissured with age. Oak tree growth is particularly rapid in youth but gradually slows at around 120 years. Oaks even shorten with age in order to extend their lifespan.

Leaves: around 10cm long with 4-5 deep lobes with smooth edges. Leaf-burst occurs mid-May and the leaves have almost no stem and grow in bunches. 

Flowers: are long yellow hanging catkins which distribute pollen into the air.

Fruits: its fruit, commonly known as acorns, are 2–2.5cm long, borne on lengthy stalks and held tightly by cupules (the cup-shaped base of the acorn). As it ripens, the green acorn takes on a more autumnal, browner colour, loosens from the cupule and falls to the canopy below.

Most acorns will never get the chance to germinate, they are rich food source, eaten by many wild creatures including jays, mice and squirrels. Acorns need to germinate and root quickly to prevent drying out or becoming victims of the harvest. Following successful germination, a new sapling will appear the following spring.

Look out for: it has distinctive lobed leaves with short leaf stalks (petioles). Leaf lobes are rounded.

Could be confused with: sessile oak (Quercus petraea). English oak has acorns on stalks (or peduncles) whereas sessile oak does not.

Identified in winter by: rounded buds are in clusters. Each bud has more than three scales.

Where to find oak

It is native to the northern hemisphere existing in cool regions right through to tropical climates. 

Value to wildlife

Oak forests provide a habitat rich in biodiversity; they support more life forms than any other native trees. They host hundreds of species of insect, supplying many British birds with an important food source. In autumn mammals such as badgers and deer take advantage of the falling acorns.

Flower and leaf buds of English oak and sessile oak are the foodplants of the caterpillars of purple hairstreak butterflies.

The soft leaves of English oaks breakdown with ease in autumn and form a rich leaf mould beneath the tree, supporting invertebrates, such as the stag beetle, and numerous fungi, like the oakbug milkcap. Holes and crevices in the tree bark are perfect nesting spots for the pied flycatcher or marsh tit. Several British bat species may also roost in old woodpecker holes or under loose bark, as well as feeding on the rich supply of insects in the tree canopy.

Mythology and symbolism

The oak is held in high regard across most cultures in Europe. The oak was sacred to many gods including Zeus (Greek), Jupiter (Roman) and Dagda (Celtic). Each of these gods ruled over thunder and lightning, and oak trees are prone to lightning strikes as they are often the tallest living feature in the landscape.

Druids frequently practised and worshipped their rituals in oak groves and cherished the mistletoe that frequents oak tree branches. Royalty has had a long association with oak trees too; ancient kings adorned themselves with crowns of oak leaves, King Charles II hid from his pursuers in an oak tree at Boscobel House and Roman Emperors were presented with crowns of oak leaves during victory parades.

In England the oak has for centuries been a national symbol of strength and survival. It has played an important part in our culture – couples were wed under ancient oaks in Oliver Cromwell’s time, the festive Yule Log was traditionally cut from oak, it features on the 1987 pound coin and is the inspiration for the emblem of many environmentally focused organisations, including the Woodland Trust.

How we use oak

Oaks produce one of the hardest and most durable timbers on the planet, even its Latin name, Quercus robur, means strength. However, it takes up to 150 years before an oak is ready to use in construction. It has been a prized hardwood timber for thousands of years, was the primary ship building material until the mid-19th century and remains a popular wood for architectural beams. Modern uses of English oak include flooring, wine barrels and firewood.

Traditionally the leaves, bark and acorns were believed to heal many medical ailments including diarrhoea, inflammation and kidney stones.

Historically humans also collected acorns and processed them into flour for bread making. These culinary techniques have mostly died out following the domestication of wheat production 10,000 years ago, leaving the harvest for wild birds and mammals.

Tannin found in the bark has been used to tan leather since at least Roman times. 

Toxicity: Tannic acid in the leaves is poisonous to horses if consumed in excess, damaging the kidneys. Acorns are poisonous to horses and cattle, though swine can consume them safely in moderation.

Threats

Despite their high numbers in Britain and protection from over-harvest, our oak trees are threatened by a number of pests and pathogens. The oak processionary moth is a non-native pest that has been found in London and Berkshire. Not only does it damage the foliage of the trees and increase the oak’s susceptibility to other diseases, it is actually a risk to human health. The moth's hairs are toxic and can lead to itching and respiratory problems if inhaled.

Acute oak decline (AOD) and chronic oak decline (COD) are serious conditions affecting Britain’s oaks, several contributing factors are linked to the diseases. Decline of mature oaks first aroused concern in the 1920s, today most cases are in central, southern and eastern England. Key symptoms include canopy thinning, branch dieback and black weeping patches on stems and lesions underlying the bleed spots.

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