ACACIA ANEURA SEED COLLECTIONS FOR INTERNATIONAL PROVENANCE TRIALS

by

S.J. Midgley and B.V. Gunn
Division of Forest Research, CSIRO
P.O. Box 4008, Queen Victoria Terrace
Canberra, A.C.T. 2600, Australia

INTRODUCTION

Acacia aneura F. Muell ex Benth (mulga) is a nitrogen-fixing woody perennial displaying polymorphism in characters such as growth habit and size and shape of phyllodes. Everist (1949) distinguished 4 common forms of mulga from small branchy shrubs of 4 m in height to erect trees of over 15 m. The more unusual forms include small straight trees with horizontal lateral branching (“Christmas tree” form), low spreading shrubs, 2–5 m in height with small resinous phyllodes (“desert form”) and erect small trees to 6 m with long pendulous phyllodes (“weeping form”). (Hall et al, 1979).

The distribution of mulga is over a wide range of the arid and semi-arid areas of Australia but predominantly in the central and southern parts of the continent (Figure 1) where the mean maximum temperature of the hottest month is 36–40°C. Over the greater part of the area the average number of frosts (0°C or less in the screen) is 1–12. The mean annual rainfall is mainly 200–250 mm but in the semi-arid eastern occurrence it extends to 500 mm. In the driest areas direct precipitation figures may give a misleading impression of the amount of water the trees get as mulga often receives “run off” water from surrounding areas. Table 1 summarizes the climate for a few representative stations in Australia.

The vegetation of Australia's arid and semi-arid zone of some 5.3 million km² is largely dominated by the acacias and communities including mulga occupy some 1.5 million km². Mulga dominates the vegetation in many areas with the major vegetation formations being tall shrublands, tall open shrublands or low woodlands with the tall tree-form occurring as low open-forests or sometimes as open-forest in the most favourable environments (Williams 1979) (Figures 2a-d). It grows on many soil types with the densest stands usually found on red earths and sands or red clayey sands, rarely on alkaline soils and almost never on black cracking clays.

A. aneura is probably the most important woody fodder species in Australia because it is palatable, abundant and widespread in regions of low rainfall, providing valuable sheep and cattle fodder especially during drought periods. Its foliage has a low to moderate digestibility but has a high crude protein content and a low phosphorus content. Palatability is generally good, although variation has been recorded in the chemical constituents and palatability of phyllodes according to the type of phyllode and locality (Niven 1983). In some areas specialized techniques of lopping have been developed to minimise mortality as a result of foliage harvesting during drought periods. It appears necessary to leave a small apical crown after lopping lateral branches if the tree is to survive. The seed is gathered, ground to flour and used as food by Australian Aborigines. The wood is hard, dense and durable in the ground; it is used for small posts and poles and is a highly regarded fuelwood. The growth habit and persistence of A. aneura give it potential for erosion control and shelter.

A. aneura has already been introduced to other parts of the world such as Kenya and India where the high demand for seed reflects a substantial interest in this species (Hall et al. 1979). Results have shown growth rates to be generally slow (2 m in height after 3 years) but little attention has been given to improvement of growth or other attributes through the use of better provenances. The morphological variation within mulga, its wide geographic occurrence and recorded variability as a fodder make it essential that, in any introduction of this species to a new environment, provenance trials be established to identify the most suitable sources in Australia from which to obtain seed for large-scale planting on different sites.

The potential of A. aneura for use by rural communities in developing countries was recognised by the FAO Panel of Experts on Forest Gene Resources in 1977 (Palmberg 1981a). FAO's Forestry Department initiated a project in 1979 aimed at the conservation and better utilization of genetic resources of arboreal species for the improvement of rural living. The main emphasis was for species of potential for fuelwood in arid and semi-arid regions and A. aneura was given a high priority (Palmberg 1981b). The Tree Seed Centre of the CSIRO Division of Forest Research undertook to assemble the provenance collections with FAO assistance.

SEED COLLECTIONS

Details of five field trips undertaken from 1978 to 1983, aimed primarily at obtaining seed collections of A. aneura, are summarised in Table 2. The 1982 and 1983 trips were conducted to some of Australia's remotest regions under extreme climatic conditions and required considerable preparation. Priority was given to sampling the range of mulga in its typical form for international trials with seed from the more uncommon variants being collected for mainly taxonomic research when available.

The techniques and materials used for collecting mulga seed have been described in detail by Doran et al. (1983). The collections represented new work for the Tree Seed Centre and many new techniques, especially for seed cleaning and testing, had to be developed. A major difficulty in the collections was predicting the availability of seed crops and timing the collections to coincide with seed maturity in remote areas. Flowering depends upon favourable environmental conditions and may occur at various times throughout the year or not at all (Preece 1971). Only late summer flowering followed by winter rain leads to seed set (Davies 1976) and the seed is shed over a two-week period between October–December. Seed crops at the northern extent of the natural range generally matured four to six weeks before those in the southern areas and were most prolific in areas of enhanced soil moisture such as roadsides and adjacent table drains.

Despite careful long-range planning including the study of meteorological records and past reports, reconnaissance and seed collections were sometimes frustrated by unpredictable factors such as unseasonal hot or cool weather, strong winds, unseasonal rains or insect attack. As knowledge was accumulated about A. aneura and its seeding patterns it became evident that success in seed collections was more likely if large areas of the natural distribution were traversed between mid-October and early November. The different distances travelled in each of the trips reflect the evolution of this collection strategy (Table 2). Although good seed crops of mulga were not always available, some excellent collections from other arid land species with potential for agroforestry and fuelwood were made. Two such species were A. cowleana Tate and A. holosericea A. Cunn. ex G. Don which are shrubs to small trees up to 7 m in height and sometimes occur in association with mulga in the northern part of its range. They have a wide distribution in the hot arid to hot sub-humid zones of northern Australia.

Because of the variable seeding pattern of mulga, the primary criterion for selection of mother trees was the presence of adequate seed. Where this criterion was met, mother trees were spaced at a distance of at least four mean tree heights from one another to decrease the possibility of collecting from related trees. When seed crops of A. aneura were plentiful a small proportion was kept separate by individual trees and the remainder bulked by provenance. Where seed crops were small then bulk collections from a minimum of 5 trees were made. Individual provenance collections represent the form of mulga predominant in a particular area. All seedlots have been cleaned, fumigated and tested for viability in the laboratory and viability tests have shown that 400–1200 viable seeds per 10 g (av. 76%) can be expected. Immersion in boiling water for 5–30 seconds or physical scarification of the seed coat are the preferred pre-sowing treatments necessary to break dormancy in A. aneura seedlots (Doran et al. 1983).

The seed accessions have been divided into 7 broad provenance groupings (Figure 1) based upon a bioclimatic analysis by Nix and Austin (1973), our own experience and seed availability. Table 3 gives details of the grouping of the twelve seedlots available in sufficient quantities for inclusion in the international trials. It is anticipated that if a seedlot from a particular group is exhausted it will be replaced by a seedlot from within the same group.

Seedlots from a few trees exhibiting mulga's great morphological variation - including the weeping form, desert form and ‘Christmas tree’ form - are available to those with a special interest in the taxonomy of the mulga ‘complex’.

AVAILABILITY OF SEED

Seedlots of A. aneura were distributed in 1984 for testing to cooperators in the FAO Project on Genetic Resources of Arid and Semi-Arid Zone Arboreal Species. Other interested parties wishing to participate in internationally coordinated provenance trials of this species should address their requests to the CSIRO Division of Forest Research, Australia (see address at the beginning of the article), with a copy to the Director, Forest Resources Division, FAO (Via delle Terme di Caracalla, I-00100 Rome, Italy). Detailed information on the proposed testing site(s) should be attached to the request (latitude, longitude, altitude; climatic and soil conditions) and an indication given on preferred date of receipt of the seed; and possible import permits required. All seedlots will be accompanied by information of origin and viability.

DISCUSSION

A. aneura belongs to the group of arid land plants known as the “endurers”, i.e. plants which are adapted to tolerate extremes in temperature and drought. Plants in this group are generally not noted for their fast growth, but they have proved useful for shelter and erosion control. Another group of arid-land plants, the “opportunists”, respond quickly to limited favourable conditions and are often capable of rapid growth. It appears that in suitable conditions species from this latter group will produce biomass faster than species from the “endurer” group. For this reason researchers participating in the international provenance trials of A. aneura may wish to consider assessing two “opportunist” species, A. cowleana and A. holosericea, in conjunction with A. aneura. The Tree Seed Centre will provide research seedlots of these two species on request.

ACKNOWLEDGEMENTS

The authors wish to thank Mr. J.C. Doran and the staff of the Tree Seed Centre, Division of Forest Research for their support during the collection expeditions and in the preparation of this manuscript. Mr. E.G. Cole led the Cobar collection in 1978. Dr. J.W. Turnbull made unpublished information on Acacia aneura, A. cowleana and A. holosericea available.

REFERENCES

Davies, S.J.J.F. 1976 Studies of the flowering season and fruit production of some arid zone shrubs and trees in Western Australia. J. Ecol. 64, 665–87.

Doran, J.C., Turnbull, J.W., Boland, D.J. and Gunn, B.V. 1983 Handbook on seeds of dry zone acacias. (FAO, Rome) 92 pp.

Everist, S.L. 1949 Mulga (Acacia aneura F. Muell) in Queensland. Qld. J. Agric. Sci. 6: 87–139.

Hall, N., Turnbull, J.W. and Doran, J.C. 1979 Acacia aneura F. Muell. ex Benth. Australian Acacias No. 7. CSIRO Division of Forest Research (P.O. Box 4008, Canberra A.C.T. 2600, Australia). 2 pp.

Hall, N., Wainwright, R.W. and Wolf, L.J. 1981 Summary of Meteorological Data in Australia. Divisional Report No. 6. Division of Forest Research, CSIRO.

Maslin, B.R. 1981 Acacia. pp. 115–142. In: Jessop, J. (Editor in chief). Flora of Central Australia. The Australian Systematic Botany Society. 537 pp.

Niven, D. 1983 Mulga supplementation of sheep in southwest Queensland. Qld. Agric. Journal 109, 207–209.

Nix, H.A. and Austin, M.P. 1973 Mulga: a bioclimatic analysis. Tropical Grasslands 7: 9–21.

Palmberg, C. 1981a Genetic resources of arboreal fuelwood species for the improvement of rural living. FAO/UNEP/IBPGR Technical Conference on Crop Genetic Resources. Rome, Italy. 6–10 April 1981.

Palmberg, C. 1981b A vital fuelwood gene pool is in danger. UNASYLVA 33 (133): 22–30.

Preece, P.B. 1971 Contribution to the biology of mulga. 1. Flowering. Aust. J. Bot. 19: 21–38.

Williams, O.B. 1979 Ecosystems in Australia. pp. 145–212. In: Goodall, D.W. and Perry, R.A. (Editors) ‘Arid-land Ecosystems’ Vol. 1. International Biological Programme 16. Cambridge University Press, Cambridge.

Table 1. Meteorological summaries from representative stations within the natural distribution of A. aneura (after Hall et al., 1981).

Table 2. Summary of Acacia aneura seed collection expeditions 1978–83

Table 3. Details of provenances available for trial

[Queensland (Qld),
New South Wales (NSW),
Northern Territory (NT),
Victoria (Vic),
Western Australia (WA),
South Australia (SA)]

Figure 1 - The natural distribution of Acacia aneura showing available provenances in broad bioclimatic groupings

Figure 2.

a. A. aneura typical form, Charleville, Qld. (Seedlot No 13481)

b. Stand of tree form A. aneura, St. George, Qld.
(Seedlot No 13480)

c. A. aneura, Jameson Ranges. Typical arid zone form, W.A.
(Seedlot No 14079)

d. A. aneura, Weeping form, Central Australia
(Seedlot No 13724)

Manuscript received July 1984