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National Tree Week starts on the 26th of November, so what better time to pay homage to our two native oaks? English and sessile oaks are our largest native broad-leaved trees, reaching heights of between 20 and 40 metres. They soak up carbon dioxide and produce huge amounts of oxygen. Both support a huge range of organisms, as well as being important culturally and historically. As two of our commonest and most visible deciduous trees, many people hold them in great affection. Their distinctive leaf shape and their instantly recognisable acorns also make them easy to identify. So, let’s look at these two species in more detail.
English and Sessile Oaks
Our two native oaks are superficially similar, forming broad, rounded crowns. Both produce flowers in loose catkins during spring. There are some easy to remember differences, though. The most obvious one is the arrangement of leaves and acorns on each. The English oak’s leaves are attached almost directly onto its twigs, but its acorns are on long stalks. This gives it one of its alternative names, the pedunculate oak, as peduncle refers to a fruit-bearing stalk. The sessile oak, meanwhile, has the opposite arrangement with its leaves on longer stalks and stalkless, or sessile, acorns. Sessile oak leaves also have shallower lobes and taper down more smoothly to the bottom. English oak leaves, however, have two distinct lobes near the base. Sessile oaks also tend to be taller and have straighter branches.
The two trees’ ranges overlap to a great extent. Sessile oaks can cope with shallower, sandier soils and higher altitudes, though, than the English oak, which is more tolerant to flooding and prefers rich, heavy soils in lowland areas. Where they do co-exist, they can hybridise. Any offspring will have characteristics of both parent species. Botanists have crossed English oaks with non-native species. One unusual hybrid is that of a sessile and a holm oak, a Mediterranean species. Audley End House in Essex has one such tree dating from the 1700s that was until very recently the only example in the world of this mix.
Both of our native oaks support more species than any other tree, from tiny invertebrates and lichens to birds and mammals. A 2019 study by the UK Centre for Ecology and Hydrology (UKCEH), summarised by the Woodland Trust, showed just how important oaks are for biodiversity. Oak trees support a minimum of 2,300 species, with 326 species depending on them for survival and 229 species rarely found on anything other than oaks. The fact that they can live to be over 1,000 years old means that they can potentially offer this support for a very long time as well. In addition, they continue to support life after they die, providing habitat for lichens, fungi and invertebrates in decaying wood. Rotting leaf mould as autumn progresses is similarly valuable.
The most obvious way oak trees support wildlife is as a food plant. An array of animals eat acorns including deer, badgers, squirrels, jays and wood mice. Autumn is a good time to look out for jays and squirrels in particular burying acorns for later in the winter. Pigs also love acorns and large numbers were released into our woods and forests historically to eat them (along with beech mast and chestnuts) in the autumn in a practice known as pannage. Pannage still exists in the New Forest. A number of caterpillars, meanwhile, eat the leaves. These include those of the purple hairstreak butterfly and moths like the great oak beauty and scalloped oak. Fresh leaf growth also attracts aphids, which in turn attract wood ants. They climb up to feed on the honeydew the aphids produce. Many species of insectivorous bird, including blue and great tits, take advantage of the large numbers of invertebrates oak trees support.
A Place of Sanctuary
As well as being a valuable food source for numerous creatures, our two oaks are important as nesting and refuge sites. A few bird species prefer oaks above other tree species because of the way old oak trees decay, providing nice nesting cavities. Three summer migrants, the redstart, wood warbler and pied flycatcher, all favour oak woods. Many of our resident birds also take advantage of oak cavities, including our three woodpeckers, tawny owls and treecreepers. Bats and other small mammals likewise make use of them.
A number of different insect species lay their eggs in oak leaf or flower buds or on the leaves’ undersides. The resulting deformities act as a protective casing for the developing larvae. The galls fall off in the summer or autumn, allowing the mature larvae to emerge. The commonest types seen in the UK are apple, marble, knopper and spangle galls, all produced by different wasp species.
Oaks are also home to a vast number of lichens and mosses. Some species are only found on oaks, while others are less fussy. Various fungi species also favour oak, developing from a tree’s root system or on dead branches. Oak bracket and oak polypore grow at the base of the trunks, feeding on the tree’s heartwood. Species like the penny bun, meanwhile, are mycorrhizal fungi that trade nutrients with the tree’s root system. Their fruiting bodies appear through the leaf litter. Other species, like black bulgar and green elf cup, grow instead on dead and decomposing branches. All these mosses, lichens and fungi in turn provide food or habitat for other organisms, including slugs and beetles.
Witnesses to History
Because of their size, ability to reach great ages and their usefulness, English and sessile oaks have become hugely important throughout their range in mythology and as cultural reference points. In many countries, including England, Ireland and Latvia, one or other is the national symbol, whether officially or unofficially. Many considered them sacred trees and they feature heavily in folklore. The Greeks, Romans, Norse and various Celtic tribes all venerated them. Druids supposedly practiced their rituals in oak groves. Prominent individual trees have also gathered various tales around them due to their great age and size. The Major Oak in Sherwood Forest, for example, is said to have hosted Robin Hood’s meeting with his men, despite the fact it is almost certainly not old enough.
A story with more historical merit concerns Charles II hiding from Parliamentarians in a large oak at Boscobel House in Shropshire in 1651. I find it amazing to think about the length of time some of these two species live and what events occurred that to them were the equivalent of a blink of an eye. A tree known as the Veteran Oak at Ham Green near Bristol is half the age of some truly ancient ones. Yet at about 500 years old, it has still been alive at the same time as Columbus’ voyage to the Americas, the Spanish Armada, the English Civil War and the Industrial Revolution. Oaks are valuable timber trees becuse of their hard, strong wood, and they were often used for housebuilding, furniture and barrels. Most famously, they were used for constructing wooden ships. The Royal Navy named a number of its ships Royal Oak in recognition of its importance. Because it took so many trees to make one ship (an estimated 6,000 were used for HMS Victory, for example), we are lucky to still have so many ancient trees left.
Our two native oak species are immensely important to both wildlife and humans. As familiar and long-lived trees of our woods and hedgerows, they have become entwined in our history and folklore. Providing food and valuable habitat for an eye-watering number of other species, they are important for preserving biodiversity wherever they grow. In addition, oak trees soak up carbon dioxide while producing vast amounts of oxygen. Definitely worthy of celebration!
For a more comprehensive look at some of the species supported by oak trees, see this excellent Woodland Trust page which summarises UKCEH’s findings.
Through the prism of one ancient oak he visited over a two year period, James Canton explores these incredible trees and our relationship with them. The Oak Papers is a beautiful combination of personal reflection and natural history.
This beautiful book examines the oak’s biological history in Britain since the last ice age, as well as diving in depth into its place in human myth, legend and history. The book also profiles 50 of our most famous trees and the stories behind them.