Long before the dinosaurs, about 400 million years ago, trees first appeared on Earth. They were similar to modern tree ferns and cycads, although none of the original species survive today. Most trees long outlive people, 100-200 years being a typical lifespan.

ECOLOGICAL VALUE
During the process of photosynthesis all green plants take in carbon dioxide and give off oxygen. Primitive plants were responsible for converting the poisonous atmosphere of early Earth into an oxygen-rich atmosphere that supports animal life. Trees help to maintain low levels of carbon dioxide, thereby reducing the greenhouse effect which threatens to make the Earth uncomfortably warm (see Enviro Facts "Global Warming").

Soil benefits from trees, as their far-reaching roots hold the soil in place, preventing erosion. Trees improve soil quality as their leaf litter makes perfect compost. Some trees, for example acacias, have bacteria living in their roots. The bacteria convert nitrogen from the air into nitrates which the tree can use to grow and reproduce, whilst the soil is also enriched.

The leaves of trees are eaten by many insects as well as monkeys, elephants, giraffes, kudus and bushbucks. Even fallen dead leaves are eaten - they are a favourite food of the blue duiker. Flowers are eaten by monkeys, and nectar by birds, insects and bats. Many fruits are eaten by animals, some of which aid seed dispersal.

Trees provide nest sites for birds. The leafy branches make good hiding places and are difficult for most predators to reach - even non-breeding birds roost in trees at night. Woodpeckers, barbets and hornbills nest in holes in trees. Very large, old trees frequently develop a hollow centre, a favourite breeding and roosting place for bats. When dead, their rotting wood is a source of food for insects and their predators.

COMMERCIAL VALUE
Trees provide timber. A widely-used building material, its strength and lightness often gives it the advantage over concrete and steel. Modern buildings still require timber in roof construction. Timber is used in furniture manufacture, and for various other products ranging from tool handles and sporting equipment to matchsticks. Pulped wood is used to make paper.

Wood was our first fuel, and is still the main energy source for many people. Sawdust and offcuts are an important fuel in industry, and may be processed to produce alcohol and chemicals.

Bark of some trees provides cork and can also be made into simple cord - the main building material of half the world's people. However the main importance of bark is as a source of chemicals and medicines. Tannin, derived from wattle bark, is the basis of the leather tanning industry. Bark, and many other parts of the tree, are used in traditional and modern medicine. Quinine and aspirin, for example, are made from bark extracts.

The inner bark of certain trees provides latex, the main ingredient of rubber. Acacias produce sap used in gum manufacture, and the maples of North America yield edible maple syrup. Several palms produce a watery sap which is drunk as palm wine, or can be fermented and distilled into a powerful spirit.

Trees are great producers of edible fruits - apples, bananas, plums, papaws, avocados, olives, nuts, oranges, litchis - the list of commercially grown fruits is almost endless. Many wild species are of economic importance, including the Brazil nut and maroela. Wild fruits eaten by rural people include monkey oranges, amatungulu and wild plum.

SHELTER
The shade of trees is welcomed by man and beast alike, providing essential shelter in the hottest climates. Trees are often used as windbreaks to shelter sensitive crops.

AESTHETIC VALUE
For all their values to which a price tag can be attached, trees have one more contribution to make: their beauty and variety of form. Some species are tall and thin, others flat-topped and spreading, leaves come in every shape and size, flowers and fruits are frequently decorative. These qualities make trees ideal for beautifying gardens, cities, and even industrial estates.

DID YOU KNOW?
* Trees are the largest of living things. Some species grow 100 metres tall and may weigh 600 tonnes.

* Baobabs live for up to 2000 years and some of the Bristlecone pines in North America are nearly 5000 years old - about the same age as the pyramids in Egypt.

* About half of the world's population use wood as fuel for cooking and heating.

* The Botanical Society supplies its members with seeds of many indigenous plants, including trees, at no charge!

* The seed store of the Dept. Environment Affairs supplies seeds to people wanting to grow trees from seed. Address below.

WHAT YOU CAN DO
* Plant suitable, local indigenous trees in your school grounds, public parks and unused municipal land (see Enviro Facts "Plant a Tree Today").

* Make a map of all the trees in your area. Include individual trees which deserve conservation because of their size or other special value. Prepare a report stating why the trees are important, and present it to your school and town authorities.

* Trees make wonderful, long-lasting gifts: give a living indigenous tree or a R15.00 gift voucher (redeemable at participating nurseries) from Trees for Africa - address below.

FURTHER READING

EYEWITNESS GUIDE TO TREES. Dorling Kindersly, London. 1990

POCKET GUIDE TO COMMON TREES OF SOUTHERN AFRICA. E. and G. Moll and N. Page. Struik, Cape Town, 1989.

WWF ATLAS OF THE ENVIRONMENT. G. Lean and D. Hinrichsen. Helicon, U.K. 1992.

All books available from Russel Friedman Books, PO Box 73, Halfway House, 1685. Tel. 011-7022300/1.

USEFUL ORGANISATIONS

Dendrological Society. P.O. Box 104, Pretoria 0001.Tel. 012-574 009.

Tree Society. P.O. Box 4116, Johannesburg, 2000. Tel. 011-782 5473.

Botanical Society. Kirstenbosch, Claremont, 7735. Tel. 021-7972090/1/2/3. Branches nationwide.

Trees for Africa. P.O Box 2035, Johannesburg, 2000. Tel. 011-803 9750.

National Botanic Institute. P/Bag X7, Claremont, 7735. Tel. 021-762 1166. Eight National Botanic Gardens nationwide.

Dept. Environment Affairs and Tourism. P/Bag X447, Pretoria, 0001. Tel. 012-310 3425.