Sessile oak

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Sessile oak (Quercus petraea) is a deciduous broadleaf tree native to the UK and most of Europe.

It differs only subtely from English, or pedunculate oak, and the two spceies often hybridise. Sessile oak is widely planted outside its natural range.

What does oak look like?

Sessile oak is a large decidulous tree up to 20-40m tall.

This species differs from English, or pedunculate, oak (Quercus robur) in having stalkless (sessile) acorns. It also has a more upright trunk and straighter branches than English oak, and the leaves have longer stalks.

As 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, and their smooth and silvery brown bark becomes rugged and deeply fissured with age. 

Sessile oak is monoecious, meaning male and female flowers are found on the same tree. Male flowers are green catkins and female flowers are inconspicuous clusters of bracts (modified leaves), which resemble red flower buds. After pollination by wind, female flowers develop into a shiny seed held in a scaly wooden cup, commonly known as an acorn. Young acorns are green, maturing to brown before they fall.

Interesting fact: the sessile oak is so-named because, unlike the English, or pedunculate oak, its acorns are not carried on stalks (peduncles) but directly on the outer twigs (sessile). 

Value to wildlife

Whether sessile or pedunculate, oak trees support more wildlife than any other native trees. They provide a habitat for more than 280 species of insect, which provides food for birds and other predators. The bark also provides a habitat for mosses, lichens and liverworts, and deadwood cavities for nesting birds and roosting bats. The acorns are eaten by a number of birds and mammals including the jay, badger and red squirrel. 

The soft leaves break down with ease in autumn and form a rich leaf mould beneath the tree, supporting beetles and numerous fungi, such as the oakbug milkcap. 

Mythology and symbolism

The oak is held in high regard across most cultures in Europe. It 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 performed rituals in oak groves and cherished the mistletoe that grows on 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 available. 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. Traditionally the leaves, bark and acorns were believed to heal many medical ailments including diarrhoea, inflammation and kidney stones. Modern uses of English oak include flooring, wine barrels and firewood.

Historically humans 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. 

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 damages the foliage of trees and increase oak’s susceptibility to other diseases, but its fine hairs can lead to itching and/or respiratory problems in people 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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