Sessile oak (Quercus petraea)

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Ancient sessile oak tree covered with ferns and lichens

Top facts

  • The sessile oak is named for the fact that its acorns are not supported on stalks, instead sitting directly on the branches.
  • In a good year, an individual sessile oak may produce up to 50,000 acorns.
  • A long-lived tree species, the sessile oak may potentially reach ages of up to 1,000 years.
  • The sessile oak supports a great variety of other species, making it an important habitat in its own right.
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Sessile oak fact file

Sessile oak description

KingdomPlantae
PhylumAnthophyta
ClassMagnoliopsida
OrderFagales
FamilyFagaceae
GenusQuercus (1)

Like other oak trees, the sessile oak (Quercus petraea) has a special place in culture and folklore, and is a much-loved symbol of strength (3). The crown of this magnificent native tree is domed, with branches that radiate outwards and are straighter than those of the related pedunculate or English oak (Quercus robur) (4) (5). Another distinction between these two oaks is that the crown of the sessile oak appears more open when in leaf, as the leaves are evenly spread rather than in clusters as they are in the pedunculate oak (4).

The bark of the sessile oak is greyish in colour and has mainly vertical fine fissures and ridges (4) (5) (6). The dark green leaves are smooth on the upper surface but pale green and hairy below. They usually have five to eight rounded lobes on each side, which gives a typical ‘wavy-edged’ outline (2) (5) (6). The leaves of the sessile oak are borne on stalks up to two centimetres in length, whereas in the pedunculate oak the leaves only have very short stalks (5).

The male flowers of the sessile oak grow in clusters of hanging ‘catkins’ which measure up to four centimetres in length, while the female flowers are tiny and bud-like, with hairy scales (5) (6). The sessile oak is named for the fact that its acorns are not supported on stalks (‘peduncles’) as they are in the pedunculate oak (3) (5) (6).

Also known as
Cornish oak, durmast oak, French oak, Welsh oak.
Synonyms
Quercus sessiliflora.
Size
Height: 20 - 45 m (2) (1)
Leaf length: 6 - 12 cm (2) (1)
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Sessile oak biology

The sessile oak flowers between April and May, and produces acorns which ripen from September to October (6). Both male and female flowers occur on the same tree, and the flowers are pollinated by the wind (5). The seedlings of the sessile oak are more tolerant of shade than those of the pedunculate oak (Q. robur), which enables the sessile oak to regenerate in woodlands (7). This deciduous tree is long-lived, often living to around 500 years old and potentially even reaching 1,000 years of age (5).

As well as being long-lived, the sessile oak also grows quite slowly, with individuals not producing their first good crop of acorns until they are around 40 to 50 years old. Acorn production varies between years, but in a good year this species may produce up to 50,000 acorns. Despite this, very few of the acorns successfully go on to grow into mature trees (5). Oaks have an intriguing method of boosting their chances of reproduction and beating the acorns’ many predators. This is done by a relatively low production of acorns for three to four years on average, before the production of a huge crop. This is called a ‘mast year’, and with so many acorns on the ground it is likely that more will survive predation (8).

Acorns were once widely used to feed pigs, and were also ground down to make a substitute for coffee and even types of bread, biscuits and a meal that was added to stews and soup (2) (8). A good crop of acorns was used to predict a good harvest, and a heavy fall of acorns was thought to signal an impending harsh winter (2). However, sessile oaks do not yield as many acorns as pedunculate oaks, and their timber has been less highly valued. Coppicing of these oaks was common in the north and west of the United Kingdom (3), and involved cutting the tree at the base and then leaving it to regrow. This practice produces many thin poles of wood, which would then be harvested every 10 to 14 years (8). The wood was burned in the iron-smelting industry and the bark was used in the leather tanning industry as a source of tannin (3).

Like other oak trees, sessile oaks support a staggering variety of wildlife, and are habitats in their own right (5) (9). During the life of the tree and even after its death, an oak can play host to many hundreds of different species, and is therefore often referred to as a ‘cornerstone species’ (8). The open canopy of the sessile oak also lets light through to the ground, which favours the growth of a diverse ground flora (9).

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Sessile oak range

The sessile oak is found in most of Europe, from southern Scandinavia south to the Mediterranean, and from the United Kingdom east to Russia (5). Within the United Kingdom, it is found mainly in the west and north of the country (4).

You can view distribution information for this species at the National Biodiversity Network Gateway.
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Sessile oak habitat

The sessile oak occurs mainly in semi-natural forests (3) on shallow, well-drained and acidic soils (7). This tree is the dominant species in upland oak woodlands (7), but in scrub, plantations and hedgerows it is typically replaced by the pedunculate oak (Q. robur) (3).

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Sessile oak status

The sessile oak has not yet been classified on the IUCN Red List.

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Sessile oak threats

In the United Kingdom, upland oak woodlands have declined by 30 to 40 percent over the last 60 years or so as a result of re-planting with conifers, conversion to grazing land, overgrazing by sheep and deer, and unsuitable management (10). The decline in the ancient technique of coppicing has resulted in oak woodlands becoming more shaded, and acorns do not germinate as well in these conditions. Many oak forests also have a skewed age structure, as young trees are not able to regenerate (3).

The sessile oak may potentially be affected by a condition known as ‘acute oak decline’, which has recently been identified in the United Kingdom and is known to kill native oaks. The causes of this condition have yet to be fully understood (11).

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Sessile oak conservation

Upland oak woodland was previously a priority habitat under the UK Biodiversity Action Plan (UK BAP). An action plan was produced to guide the conservation of this habitat, and this plan aimed to maintain current upland oak woodland and improve its condition, as well as to expand it (10). The UK BAP was succeeded by the ‘UK Post-2010 Biodiversity Framework’, which sets out priorities for protecting the United Kingdom’s biodiversity (12).

Not only is the sessile oak a beautiful and majestic species, but it also supports important communities of plants, animals and fungi (9). Conserving this iconic tree is therefore of utmost importance for a range of different species.

There may be further information about this species available via the National Biodiversity Network Gateway.
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Find out more

Find out more about the sessile oak and oak woodland:

More information on UK tree conservation:

You can see the sessile oak by visiting the Quantocks, Somerset, UK:

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Authentication

Authenticated (14/03/13) by Richard Browne.

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Glossary

Biodiversity
Short for biological diversity, biodiversity is a term used to define the great diversity of life on earth, or more specifically, in a particular habitat or ecosystem.
Coppicing
A traditional form of woodland management in which trees are cut close to the base of the trunk. Re-growth occurs in the form of many thin poles. Coppiced woodlands are cut in this way on rotation, producing a mosaic of different stages of re-growth.
Deciduous
A plant that sheds its leaves at the end of the growing season.
Germinate
To begin to grow, usually following a period of dormancy and in response to favourable conditions. For example, the sprouting of a seedling from a seed.
Pollination
The transfer of pollen grains from the stamen (male part of a flower) to the stigma (female part of a flower) of a flowering plant. This usually leads to fertilisation, the development of seeds and, eventually, a new plant.
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References

  1. Species 2000 and ITIS Catalogue of Life(February, 2013)
    http://www.catalogueoflife.org/annual-checklist
  2. Godet, J-D. (1986) Collins Photographic Key to the Trees of Britain and Northern Europe: A Guide to Identification by Leaves and Needles. Collins, London.
  3. Mabey, R. (1996) Flora Britannica. Sinclair-Stevenson, London.
  4. Mitchell, A. (1974) A Field Guide to the Trees of Britain and Northern Europe. William Collins Sons and Co., London.
  5. Trees for Life: Species Profile - Oak (February, 2013)
    http://www.treesforlife.org.uk/forest/species/oak.html
  6. Sutton, D.A. (1990) Trees of Britain and Europe. Kingfisher Publications PLC, London.
  7. Preston, C.D., Pearman, D.A. and Dines, T.D. (2002) New Atlas of the British Flora. Oxford University Press, Oxford.
  8. Browne, R. (March, 2013) Pers. comm.
  9. Forestry Commission - Sessile oak (October, 2003)
    http://www.forestry.gov.uk/forestry/infd-5nlj46
  10. UK Biodiversity Action Plan: Habitat Action Plan - Upland oakwood (March, 2003)
    http://jncc.defra.gov.uk/page-5718
  11. Forestry Commission - Acute oak decline (February, 2013)
    http://www.forestry.gov.uk/fr/INFD-7UL9NQ
  12. JNCC - The UK Biodiversity Action Plan (UK BAP) (February, 2013)
    http://jncc.defra.gov.uk/page-5155
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Ancient sessile oak tree covered with ferns and lichens  
Ancient sessile oak tree covered with ferns and lichens

© David Tipling / naturepl.com

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