APPENDIX 2. Notes on the main characteristics of the six acacia species of special interest

A. albida is widespread in tropical and sub-tropical Africa from Senegal and Gambia in the west to Egypt in the north east and southwards to Natal; it occurs also in Israel and Lebanon. A map of its distribution is given in Fig.5, and by Ross (1966) and Wickens (1969). A. albida displays a number of characters unusual amongst African acacias. These features and their taxonomic implications are discussed by Ross (1966, 1979) and (Wickens 1969). Brenan (1959) recognized two well-defined geographical races within A. albida: one with generally smooth features (race A) and the other hairy (race B) with intermediate forms. All specimens from areas south of central Tanzania are referable to race B (Ross 1979).

A. albida is typically a large tree to 30m in height and up to 2m in diameter. The dense, rounded often wide-spreading crowns carry distinctive blue-green foliage during summer. The short straight thorns are in pairs, of reddish-brown colour with white tips. The species occurs as scattered individuals or in groves in a wide range of habitats, ranging from the vegetation on alluvial soils fringing perennial or seasonal water-courses to open savanna wood and cultivated lands. It prefers deep sandy soils where the water-table is accessible to the tree roots and it is therefore independent of local rain once established. Mean annual rainfall throughout the distribution of A. albida is in the range of 250–600 mm. While the species will withstand short periods of flooding, it does not do well under irrigation, especially on heavy soils. It is fairly frost resistant.

This tree is particularly valued in agricultural areas on account of its unusual habit of retaining its leaves during the hot weather and dropping them during the rains (FAO 1974a). The pods and leaves are very good fodder and the pods, prolific crops of which are produced annually, can be stored. The pod and leaf fall, together with the dung and urine of cattle that seek the shade of trees in hot weather, improve the nutrient status and organic content on the soil near established trees, so that yields of sorghum and other agricultural crops cultivated during the rains are considerably increased. The cattle-carrying capacity of the land is also improved where this tree is present, because of the excellent leaf and pod fodder produced at a time when grass is scarce. It is therefore an important tree in village economy and, in some regions, is declared a protected tree. The wood of A. albida is useful for general construction, but it is liable to stain and to attack by borers and is an indifferent fuel. The tree is also useful for soil stabilization and ornament. Its stumps coppice readily.

A. albida grows quickly after planting once the roots have established. Trees have reached 6.5 m after 4 yr and 10.5 m (with a 9.4 cm diameter) after 7.5 yr (NAS 1979). Notes on the silviculture of the species are given by FAO (1974a) and Giffard (1964, 1971). The planting of nursery stock is preferred to direct spotsowing. The seed (11 000 per kg) is collected in April and has high germination capacity. It is grown as a scattered tree and never in close plantations.

Mulga dominates the vegetation over large areas of arid and semi-arid Australia. The main distribution of the species (Fig.5) is in the central and southern parts of the continent from about 21–32½ °S and from near the coast of Western Australia to the mid-western areas of Queensland and New South Wales (Hall et al. 1979). The A. aneura complex comprises a group of species, only some of which have been named. Pedley (1973) clarified the position in the eastern part of the range and refers to four species that are sometimes confused with A. aneura.

A. aneura varies from a shrub 2–5 m tall, with markedly ascending branches, to a small tree up to 9 m, with a well-defined main stem and less oblique branches. The branching habit and silvery-grey phyllodes are distinctive. Mulga grows most plentifully on flood and erosional plains and in broad valley heads and only occurs scattered on hill slopes and ridges. In sandridge deserts it may occur in the dune swales. Soil types vary but the denser stands are usually found on red earths and sands or red clayey sands and, sometimes, on sandy gravels. It is found only scattered on less favourable soils such as lateritic and calcareous crusts or those of markedly skeletal nature. For the main area of occurrence the mean maximum temperature of the hottest month is 36–40°C and the mean minimum of the coldest month around 5–8°C. No part of the area is completely frost-free and the average number of heavy frosts a year is 1–12. The mean annual rainfall is mainly 200–250 mm, but in semi-arid Queensland and New South Wales it extends to 500 mm. Associated with the extensive distribution of the species rainfall incidence ranges from summer to winter maximum. Variability is very high and in the driest areas the lowest on record may be only 50–60 mm. The average number of days of rainfall a year is mainly 30–60 (-170).

A. aneura is considered to be the most important fodder tree in Australia, not because it is the most nutritious but because it is so widespread, abundant and palatable (Everist 1949). There is, however, considerable variation in palatability and, in Australia, local observations are necessary in planning lopping of mulga for stock in times of drought. The wood is hard, dense and durable in the ground; it turns well and takes a high polish. Australian aborigines used it to make weapons, and it is still used for fencing where other material is scarce. Present-day use is mainly for making small ornamental articles, although it is used locally for fuel.

Mulga has potential for use in arid regions for erosion control, shelter, forage for domestic animals and fuelwood (Hall et al. 1979). Introduction and testing is proceeding in many areas including North Africa where the species is showing considerable promise. Because of the wide natural distribution of mulga and variations in form, palatability to stock and adaptions to different environments, careful attention should be given to seed sources in any introduction program.

A. caven occurs in the arid to semi-arid zones of South America including those in Bolivia, central Chile, northern Argentina, Southern Paraguay, western Uruguay and southern Brazil. Flinta (1960) provides an account of the species and its uses.

Caven is a small, spiny, deciduous shrub or tree 2–6 m tall with an open crown and stem diameter to 20 cm. It grows on sandy loams in both well-drained and wet situations. Annual rainfall is concentrated in the summer months and varies from 100–1000 mm. Mean annual temperature is about 17°C with an absolute minimum down to -8°C.

The principal use of A. caven is for the production of high quality charcoal. The hard and heavy wood is also used locally for fuel and fence posts while the tree is used for soil stabilization and ornament. The flowers are used in the perfume industry and are a useful source of honey. The pods contain tannin.

A. caven coppices vigoriously and regenerates abundantly by natural seeding. Caven is planted in Chile and Argentina using nursery seedlings produced from seed or stem cuttings.

Acacia nilotica is widely distributed in Africa, Arabia and the Indo-Pakistan sub-continent (Fig. 5). This thorny acacia is very variable and currently nine sub-species are recognised (Ross 1979) and can be identified from one another using the shape of the plant, the shape of the fruit and the hairiness of the fruit and branches. A key to the subspecies indicating their countries of origin is given by Ali and Qaiser (1980).

Egyptian thorn is usually a small to medium sized tree about 10 m tall, but reaching 20 m in favourable locations. It often branches low down to form a compact rounded to flat-topped crown, the width of which often exceeds the tree's height. This species occurs mostly on seasonally innundated flood plains, although subsp. adstringens is adapted to grow on dry sites away from rivers (FAO 1974a). It is highly drought resistant with a minimum water requirement of about 400 mm per year, although some subspecies (e.g. subsp. nilotica in Sudan and Pakistan) are restricted to sites that are inundated with flood water for several months each year. Although preferring fertile alluvial soils it will grow well on heavy black cotton and clay soils. A. nilotica trees withstand extremes of temperature but are frost tender when young.

The two most widely grown subspecies for fuel are subsp. indica in India and Pakistan and subsp. nilotica in Africa (NAS 1980). The dark red wood is hard and dense, durable and termite resistant and popular for a wide range of uses. The leaves and pods are widely used as a fodder. The tannin from the bark and pods are used in the leather industry and this species is a commercial source of gum arabic. In India, it is one of the most important species in social and farm forestry.

The tree is fast-growing in irrigated plantations (20–30 yr rotations) but suffers from being extremely thorny, poor form and negligible coppicing ability. Some silvicultural notes are given by FAO (1974a) and NAS (1980). A. nilotica is generally propagated by direct spot-sowing or broadcasting of pretreated seed at the rate of 6kg and 30kg of seed per hectare respectively. There are about 8000 seeds to the kilogram. Intensive weeding is necessary during the first two years and thinning to a 2 × 2 m spacing takes place when the plants are 60 cm tall.

A. senegal is found in a belt 300 km wide along the southern frontier of the Sahara Desert, from Mauritania to Somalia. It also grows in East Africa as far south as Natal; along the southern coast of Arabia and Iran; and in Pakistan and western India (Fig. 5). This acacia, as at present accepted, comprises four varieties described in detail by Ross (1979). The considerable variation both within and between varieties is not well understood and inadequate data is delaying redefinition of varietal limits.

Gum acacia is a shrub or tree to 15 m high with a slightly rounded or flattened and somewhat spreading crown, or a slender, spindly tree with irregular twiggy branches. This species is a native of hot, barren sub-tropical to tropical regions with annual rainfall as low as 200 mm with 8–11 dry months in the year. Best development is in the 300–450 mm per year zone with a range to 800 mm. It is intolerant of water-logged conditions. The species thrives on rocky hills, dry sandy flats or dunes. Altitudinal range is 100–1700 m and the species is marginally frost hardy. The natural life of the tree is usually 25–30 yr.

A. senegal is an important tree economically as the source of gum arabic for which it is extensively cultivated. The wood is hard and dense and used for poles, agricultural implements, fuelwood and charcoal production. The root fibres are utilized for rope, fish nets and lining wells. The foliage and pods are rich in protein and are browsed by domestic animals; and seeds are dried and preserved for human consumption. It is a useful species for shade, soil improvement and sand binding and is highly suitable for use in agro-forestry systems. Amongst its disadvantages are its potential to become a thorny weed, and the susceptibility of young plants to browsing damage by goats, sheep and other animals.

Brief silvicultural notes are given by FAO (1974a) and NAS (1980). The traditional method of cultivation for gum arabic and fuelwood is as a bush-fallow crop in conjunction with farming, depending on coppice regrowth from stumps following the agricultural phase. Increasing pressures for agricultural land have caused a decline in natural stands and considerable research has been directed towards artificial regeneration particularly in Sudan. A. senegal seeds abundantly and, although pods and seed may be attacked by insects, germination capacity is generally high. Direct spot-sowing is the normal method of establishment and plantations require intensive weeding over the first 2 yr.

Umbrella thorn is found in sub-Saharan countries from Mauritania to Sudan and in East African countries from Ethiopia to South Africa. It is also found in Egypt, Israel, Saudi Arabia and Yemen (NAS 1979). Distinctive features of this wide-spread acacia include a mixture of short recurved spines and long straight spines and spirally twisted or variably contorted pods (Ross 1979). Four subspecies are recognised in different ecological zones.

Acacia tortilis varies in form from a small shrub or bush to a tree to 21 m in height with a crown typically flattened and spreading although sometimes rounded. The species is best adapted to hot, dry, tropical lowlands. Throughout the range of the four subspecies annual rainfall varies from less than 100 mm to 1000 mm often with long, erratic dry periods. Temperature range is about 0–50°C. The tree favours alkaline soils, but grows well in sand dunes, sandy loam, rocky soils and other soils that drain well. It will also grow in shallow soils (less than 0.25 m) but must be widely spaced to allow room for widely spreading lateral roots.

Umbrella thorn is relatively fast growing over a range of extremely harsh conditions. Its dense wood makes superior firewood and charcoal and can be used for fence posts and other items. It coppices readily and the pods and leaves are a major source of fodder during the long dry season. The species is excellent for soil stabilization, provides shade and has ornamental value. Its main limitation is its thorniness.

A. tortilis subsp. raddiana is currently being planted in India and the Sudan for fuelwood and sand stabilization. It is raised easily from seed, although seed production is often severly reduced by insects (bruchids).