Chestnut Trees (Castanea sativa)

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The Sweet Chestnut (Family: Fagaceae, Beech family)

Above a giant Sweet Chestnut tree, Castanea sativa (on the Fredville Park estate, Kent, UK). I didn't take actual measurements, but the diameter of the trunk must be about 3 m. Note the buttresses to support the great weight and in particular to keep the tree stable when subject to lateral forces such as high winds. On the main branches, the younger bark displays the typical spiral pattern of youth, so characteristic of sweet chestnuts, which spirals either clockwise or anticlockwise up the tree.

Trunk. Also characteristic of old sweet chestnuts is the large diameter of the trunk relative to the height. One of the neighbouring chestnuts had an open hollow bole, with the dead heartwood burned away and it was easily large enough for several people to stand inside. Chestnuts are renown for the thickness of their trunks, especially when pollarded, and the thickest trunk in the world is thought to belong to a pollarded chestnut in Europe (though the thickest species is in general the baobab tree of Africa). They become to resemble living caverns more than trees and the trunks may eventually split into a ring of separate trees.

Fruit. The sweet chestnut is a native of the Mediterranean region and the Romans probably introduced the sweet chestnut to Britain for its nuts (seeds) which are quite edible raw or cooked and can be made into flour or used as a coffee bean substitute. Often, however, in the current British climate the fruit abort before become fully developed and many are shed as hollow skins without accumulating any edible flesh. The nuts usually occur in threes, enclosed within a green, fleshy and spiny fruit (cupule).

Leaves. The leaves are long, frequently up to 22 cm (9 inches) with sharply toothed margins. Each leaf vein enters a tooth and emerges from it as a terminal bristle.

Flowers. Both male and female flowers are borne on the same slender, yellow upright spikes (resembling pipe-cleaners) on the ends of the branches, with the female flowers at the base and open in July. Male only catkins may also develop near the base of the branch. Chestnut (and horse chestnut) trees are deciduous, shedding their leaves in Autumn (Fall). The floral formula is typical of the Fagaceae, the family to which
Castanea belongs, along with Quercus (oak) and Fagus (beech).

Male: K(4-7) C 0 A 4-many G 0;
Female: K(4-7) C 0 A 0 G(3), inferior ovary.

Specifically, for Castanea sativa, Ronse de Craene (Floral Diagrams: an aid to
understanding flower morphology and evolution, 2010, Cambridge University Press) gives the formula for the male or staminate flowers as: * K3+3 A3x3+3 G(3-sterile and the female or pistillate flower as: * K3+3 A(all sterile staminodes)3+3 G(6), inferior ovary. The male flowers spiral around the catkin axis and are grouped into bunches of 4-9 flowers. The female flowers occur in groups of 1-3.

Bark. The bark is smooth and greyish-green when young, becoming deeply fissured by spiral grooves with age (each bark ridge/fissure often forms a clockwise or anticlockwise spiral extending for several feet). Burrs are often formed high up in the bole (see photos).

Wood. The wood is as strong and tough as oak, though more easily split. The sapwood forms a thinner layer than in the oak and the medullary rays are very fine and hard to see. It has been used in fencing, posts and stakes. The winter buds are yellow-green or brown and alternate. The twigs are five-sided (angular) and vary from shiny olive-green to purplish-brown.

Sweet chestnuts grow well in the South-East of England, with the warmer summers and the warm sandy soils. Here they are one of the dominant trees and are naturalised, growing from seed.

Above: a montage of another ancient chestnut at Fredville, with 'elephant's feet' or bulbous roots which have probably reacted to the mechanical stresses of the weight of this slanting tree.

Above: chestnuts of Castanea sativa. Photo credit: Fir0002/Flagstaffotos, Wikimedia Commons. Of the three nuts, the one in the center has aborted, failing to develop the enclosed ovule and so appears flattened. No more than one ovule, and its enclosed embryo, develop in each nut. Each nut is a whole fruit, the spiny cupule being developed from bracts. Aborted fruits contain no seed, but air filled instead with silky hairs.

More pictures of the Castanea sativa trees at Fredville Park in early April (click images for full size):

Above: This particular chestnut tree is also called the Step Tree as it was once adorned by steps and a platform.

Later in the season ...

Above: close-ups of part of the upper bole of one of the Fredville chestnuts. The Fredville estate is privately owned, but part of it is opened to the public where public paths cross through it. Enclosed within the private part is one of the largest oak trees in England, Majesty (the Fredville Oak), but permission from the owners must be sought in order to visit the oak.

Above: more ancient chestnuts at Fredville

Above: the 'Step Tree' - an ancient chestnut at Fredville, so-called because a platform with steps leading up. It has layered itself - embedding in the ground in two places, with a new 'tree' growing from the branch tip.

Another fine Castanea sativa from a different site:

The Horse Chestnut (Family: Sapindaceae)

The name 'chestnut' applies equally to what is properly called the horse-chestnut, Aesculus hippocampus, an import from the Balkans, introduced into Britain in the 16th century. Also called the 'horse chestnut' in native Turkey, the nuts (conkers or obblyonkers) were fed to horses and used as horse medicine, acting as an anti-inflammatory to treat sprains and the like. The horse-chestnut is in a different genus, indeed a different family, to the sweet chestnut and the two are quite different trees, though both encase their nuts in green spiny cupules (which turns brown when ripe in the horse chestnut). Indeed Castanea belongs to the Fagaceae or beech family, whereas Horse-chestnut belongs to the soapberry family, Sapindaceae, Usually only one conker occurs within each cupule. Conkers are bitter and mildly poisonous, though they have been ground into flour and boiling water used to extract the bitterness. Both conkers and sweet chestnuts are a rich 'chestnut-brown' in color.

Originally introduced into private estates, by architects and land-scapers like capability Brown and Christopher Wren, the horse chestnut was eventually introduced into towns and cities where the game of conkers became nationwide.

The leaves are consist of 5 to seven large leaflets with serrated margins, arranged in a palmate fashion on a central stalk. When the leaves dehisce (detach or are shed) they leave horseshoe-shaped scars on the twig, studied by teh sealed ends of vessels (like 'nails' in a horseshoe). Each leaflet broadens toward the tip before suddenly narrowing to a point, and may be up to 30 cm (12 inches) long.

The flower spikes are especially attractive, upright cones, like candelabras, with white or pink flowers. Each spike may exceed 30 cm in height and bear more than 100 flowers. The tree may self-seed in Britain, but is not generally naturalised, being largely planted as an ornamental. Each flower has a yellow blotch on its petals when newly opened, which turns crimson after pollination. this signals to pollinating insects to ignore those flowers which have already been pollinated.

The bark is dark grey-brown and smooth in youth, dividing into irregular pink-brown scales with age. The wood is white and soft and brittle. The trunk is often fluted and spreading or buttressed at the base.

Extracts from horse-chestnuts are used to make certain anti-inflammatory medicines. The active ingredient, found in the essential oil, is the saponin aescin. Saponins are soapy compounds produced by many plants. Soaps are salts of fatty acids and so are amphipathic molecules, meaning that they have one readily water-soluble end, the hydrophilic or 'water-liking' end and one lipid-soluble hydrophobic or 'water-avoiding' end. This allows them to form micelles, tiny molecular spheres that can enclose fat-soluble (and water-insoluble) 'dirt' (see lipids). Similarly saponins are amphipathic, and will also foam in water, producing micelles. In this instance the hydrophilic part is formed of three glucose sugar-residues, whilst the fat-soluble hydrophobic part is a triterpine derivative - a large planar molecule formed from cyclic hydrocarbons joined together.

Terpenes are built from five-carbon isoprene units (left). Two isoprenes make a monoterpene, six make a triterpene. These terpenes may be linear chain molecules, of they may become circularised and undergo other modifications.

One of the drug-actions of aescin  is to alter the rate of synthesis of nitric oxide (nitrogen(II) oxide, NO) in endothelial cells lining the lumens of small blood vessels. NO is a key signalling molecule that regulates vessel diameter (via smooth muscle cell contraction) which is a key mechanism in the inflammatory response.

Horse-chestnut (Aesculus hippocastanum) a member of the Sapindaceae or soapberry family.

Other Chestnuts

In North America, 'chestnut' refers to the American Chestnut, Castanea dentata, a close relative of the European sweet chestnut (Castanea sativa). The Chinese Chestnut, Castanea mollissima is also a close relative. The American Chestnut is a natural woodland species, favoured for its nuts, though it is increasingly rare in the wild. The Chinese Chestnut is also grown for its nuts. All are stately trees, reaching about 100 feet (30-35 m) in height.

Wood Structure

Castanea belongs to the beech family, along with Fagus (beech) and Quercus (oak). Hardwood may be roughly divided into two structural types, depending on the arrangement of the parenchyma rays: storied (stratified) and nonstoried (nonstratified) wood. Recall the rays are short plates or strands of living cells that radiate from the centre of the trunk to the periphery. In storied wood, the rays do not overlap vertically, and so form distinct horizontal layers or tiers down the trunk:

Examples of nonstoried wood include the sweet chestnuts (Castanea), Quercus (oak), Juglans (walnut) and Fraxinus (ash).

Examples of trees with storied wood include the horse chestnut (Aesculus), Ficus (fig) and Tilia (lime). In nonstoried wood the rays overlap vertically with their neighbours, so that the tiers are indistinct:

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Article updated: 4 May 2019, 6 June 2022