Symbols of strength, endurance and longevity, oak trees appear in literature and art as well as in our gardens. The oak is the national tree of numerous countries, including the United States, England, France and Germany. Their leaves and fruit are frequent decorative motifs in design and illustrations, yet just what is an oak and how many different kinds are there?
What are Oak Trees?
Oaks are mostly trees, although a few are shrubs. They are found in Europe, North America, China and several other countries in the Northern Hemisphere, from cold areas to the tropics. There are over 500 different species, with about half of them found in the United States and Mexico. The oak has always been important to humans, for timber, for tanning leather, for cork and as food for pigs. In times of hardship acorns have even been used as food, after suitable treatment to remove the bitter tannin.
Oaks can be evergreen or deciduous, but what makes them ‘oaks’ are their unique seeds – the acorn, a nut sitting in a small cup, which is found only among the oaks. Acorns begin in spring, before the leaves appear, as tiny flowers. The tree also produces ‘male’ flowers that make pollen blown by the wind to ‘female’ flowers on other oak trees. When the acorn has grown for up to eighteen months, it falls to the ground, mostly to be eaten by wildlife. The leaves vary greatly, but are often ‘oak-shaped’, with several deep lobes on each side, giving the leaf a characteristic shape. Often there is a small, soft spine on the ends of each lobe.
Oak trees are a major source of lumber, and many of the world’s major naval battles were fought in ships of oak. One reason the British did not want to lose the American colonies was for the lumber to build their Navy. The forests of English Oak had already been mostly cut down for that purpose and a new source was needed from the vast forests of the New World. The development of metal ships meant that the forests could finally be left to themselves.
Different Oaks for Different Folks
Wherever you live you have certainly seen oak trees. Perhaps in fall you saw the glossy, rich red leaves of the Red Oak or the Scarlet Oak, standing out against the gold and orange of the maple trees. These American native trees grow throughout the north east, often in deep, rich soil near rivers, and can be grown in almost any garden as attractive and very long-lived shade trees.
In warmer areas, like the south and south-west, the Sawtooth Oak is often seen. This is not a native oak, but an introduction from the Far East, known for its rapid growth and early production of heavy crops of acorns that are feasted upon by all kinds of wildlife.
Although not often seen in gardens because of its slow growth and ultimate large size, the White Oak is a major part of the forests of the north-east. It can grow to 100 feet, with a wide spread and large trunk, so each tree yields a large amount of lumber, for which this tree is famous. Although called ‘white’ the bark is actually light-grey. Its main use is for making whiskey barrels, since the close grain is leak-proof. A million oak barrels are made in America every year and can cost $1,000 when new. They are sold all around the world for aging bourbon, whiskey and wine. When their use is over they can often be seen in gardens growing plants, perhaps even a young oak tree.
It is not just what is in the bottle that depends on oak, but usually what keeps it in the bottle too. Cork comes from a unique oak tree – the Cork Oak – that grows in Portugal and Spain. By a quirk of nature the layers in the bark of all trees that produce thin sheets of cork within the bark are all collected together on the outside of the bark in the Cork Oak, so that a thick, continuous layer is produced. Every 8-10 years the cork becomes thick enough to be split away from the trunk, which will then grow a new layer. Besides being used for stoppers, cork is used for wall-covering, ground up in linoleum and even used in steam-cleaning buildings. Over half of the world’s cork comes from the tiny country of Portugal.
The acorns of the cork oak have another use. They are fed to the black pigs that produce the best Jamon Iberico – Spanish ham – that is a delicacy coveted by gourmets everywhere. Acorns have always been the traditional food of pigs, before artificial feed was developed, and the high fat content in the acorn becomes the silky fat that gives the best ham its quality.
Not all oaks lose their leaves in winter, and around the Mediterranean grow the Holm Oaks, with leathery leaves that stay all winter. On the roots of this tree a fungus lives, which produces an underground ‘mushroom’ that is round and black. Because of their remarkable smell these truffles are the most valuable vegetable in the world. Dogs and pigs are specially trained to find where they are underground and in fall special markets take place in small villages where this ‘black gold’ changes hands and travels to the world’s top restaurants. The best ones sell for $1,000 a pound and the high value has led to the planting of Holm Oaks in Australia, South Africa and California, which have then been seeded with truffle spores. There are early signs of success in these plantations that could one day make this rarity a supermarket item – by which time of course its allure will have vanished.
Easily recognized by their acorns, the many types of oak are important to the landscape as they are for human use. There is probably no other type of tree that produces such a diversity of goods and gives us both useful and luxurious products. A world without oaks would be much poorer.