L,D. PRYOR is Professor of Botany at the Australian National University, Canberra. He has carried out numerous assignments for FAO.
THERE IS good reason for interest in tree species which perform well under extremely difficult climatic conditions, and the Australian Acacia pence is one which is quite remarkable in this way. It occurs in small stands scattered here and there around the Simpson desert (Figure 1) where the total population of the species is quite small. Unlike many tree species in arid areas, it is neither associated with water drainage channels nor fixed sand dunes but reaches a height of about 16 meters (50 feet) on rather shallow soil of sandy loam over underlying rock.
FIGURE 1. - Map of Australia showing the position of the Simpson desert: 5- to 20-inch rainfall isohyets are shown.
FIGURE 2. - Monthly rainfall and temperature chart Charlotte Waters station, .
Nothing is known of the growth rate of the tree except as a seedling, it being easy to raise from seed once that is obtained. It produces -in most specimens a quite good central stem and a log up to about 46 centimeters (18 inches) diameter at breast height. The wood is very heavy, dense and hard, and on one occasion in Australia has been used in a building, the old Andado station at the western edge of the Simpson desert where for 40 years it has been exceedingly durable and resistant to termites.
The climatic conditions under which it occurs are indicated by the station Charlotte Waters in the Northern Territory (Figure 2), somewhat to the west of the area of natural occurrence. It will be seen that the rainfall is very low on an annual basis; it is also very erratic. The tree is not subjected to more than occasional very light frosts but exceedingly low humidity through much of the year is a feature of the habitat.
Two stands have been examined recently and seed obtained from one of them - at North Bore, Andado, and at a location kilometers (10 miles) north of Birdsville, Queensland. The difference in form of the tree on the two sites is itself interesting as is illustrated by Figures 3 and 4, the Andado stand being distinctly better than that near Birdsville The tree at Andado is known locally as "casuarina," a misapplication of the generic name for desert oak, Casuarina decaisneana, which it superficially resembles. Near Birdsville the tree is known as "waddy," an aboriginal word for a fighting stick, and. no doubt it would make a good one. The Andado trees are taller and of better form than those near Birdsville, and the soil is less sandy at Andado.
FIGURE 3 - Acacia peuce. Mature tree 46 feet (15 meters) tall and 17.5 inches (46 centimeters) d.b.h. North Bore, Andado.
FIGURE 4. - Acacia peuce. Mature tree about 25 feet (7.5 meters) tall and 15 inches (38 centimeters) d.b.h. 10 miles north of Birdsville.
FIGURE 5. - Botanical specimen with pods of Acacia peuce.
FIGURE 6. - Bark of mature tree at Andado.
FIGURE 7. - Seedling 2 months old showing cotyledons, pair of pinnate primary leaves and the first three primary phyllodes.
The tree has long slender phyllodes which are tetragonous in section, and produces pods with a good deal of variation in size but the biggest of which are among the largest of the phyllodineous acacias in Australia (Figure 5). The bark is flaky-fibrous and gray-brown in color (Figure 6). The seedlings have two primary pinnate leaves before developing juvenile type phyllodes which are broader than the mature ones and flat rather than tetragonal (Figure 7). If the stern is cut, the newly emerging shoots have shorter upright and stiff pungent phyllodes.
Both of the stands examined show considerable variation in tree form which at times at Andado is seen even in trees growing side by side. The difference is illustrated in Figure 8. It is unlikely that selection can be made effectively in the field of the type with the straighter and more prominent single stem but it does offer a prospect for selection under cultivation if this is achieved.
In the spring of 1966, 500 grams of seed were collected (there are about 8,000 per kilogram) from the Andado site, the Birdsville site having scarcely any fruit at all. Since germination is good and the plants are easy to raise by the methods used for other Acacias particularly by simply giving hot-water treatment to the seed before sowing, a sample of as few as 50 seeds should be adequate to allow preliminary tests with this tree over a wide area, since a good many such samples are now available.1
1Requests for seed should be addressed to the Director, Forestry and Forest Products Division, FAO, Rome, Italy.
There are few woody plants which reach tree size under climatic conditions such as those in which this occurs and it is by far the tallest in the locality. Indeed the only other trees which are seen ordinarily anywhere near this area are Eucalyptus microtheca, Coolabah, which in these situations is found either on the lower slopes of fixed sand dunes where it is a very small tree virtually of no value or as a somewhat larger spreading umbrageous short-stemmed tree in areas in which there is occasional water drainage. Since Acacia peuce has not been planted and subjected to trials under plantation conditions, there is no way of knowing whether it will fulfill a useful place as a plantation tree in arid areas on a world basis. Its extraordinary performance, however, in the soils and conditions under which it occurs naturally make preliminary trials from the precious small amount of seed obtained well worth the effort.
FIGURE 8. - Acacia peuce. North Bore, Andado. (Left) narrow crowned rigid single-stemmed upright tree. (Right) spreading pendulous many-stemmed tree.