Australian Dry-zone Acacias for Human Food

By CSIRO PUBLISHING

Australia's unique and diverse woody flora has become socially, economically and environmentally important in many other countries. The seed of some Acacia species showing promise in planting programs in semi-arid areas has been a part of the traditional diet of Australia's Aboriginal people. The dry seed may be ground to flour, mixed with water and eaten as a paste or baked to form a cake. Forest tree breeding has focussed on wood production, selecting taller, faster-growing varieties.

The same principles of selection and improvement can be applied to improve seed yields and nutritional properties of shrubs. The selection criteria would include seed characters such as taste, seed coat thickness and nutritive value to maximize their food value. The book looks at the possibility of building upon the traditional knowledge of Aboriginal Australians, using modern scientific methods, for the benefit of people in the world's dry areas.

Australian Dry-zone Acacias for Human Food documents the proceedings of a workshop held at Glen Helen, Northern Territory, Australia. The purpose of the meeting was to examine the idea of developing the food value of the seed of Australia's dry-zone acacias.

This book covers a summary of the workshop conlcusions, the invited papers, and recommendations of the working groups.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Jan 1, 1992
• ISBN: 9780643102408

Book preview

INVITED PAPERS

Acacia colei ms, south of Halls Creek, Western Australia. Left, tree form; right, detail of pods with seeds. Photos: Lex Thomson.

Acacia tumida, Niger. The bushy habit facilitates the harvesting of seeds. Photo: Lex Thomson.

Australia’s subtropical dry-zone Acacia

species with human food potential

Lex Thomson

¹

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ABSTRACT

Based on reports of their utilization as food by Aborigines and other salient features, this review has identified 44 species of Acacia from Australia’s subtropical dry-zone which have potential for production of human food. Information on each of these species, including important components of their natural distributions (climate and soils), growth characteristics and their taxonomie relationships to other Acacia species has been tabulated. Three species in section Juliflorae, viz. A. colei ms, A. cowleana and A. tutnida, appear to have outstanding potential for production of human food in the Sahelian zone of sub-Saharan Africa. These species have already demonstrated rapid growth and early production of heavy, easily harvested and utilized seed crops when planted on otherwise non-arable lands in West Africa. Other promising Juliflorae acacias for human food include A. adsurgens, A. sp. aff. cowleana and A. oligophleba. Two species in other sections, viz. A. coriacea var. pendula (section Plurinerves) and A. glaucocaesia (a close relative of A. victoriae, section Phyllodineae), have very high potential for production of human food but have yet to be evaluated outside Australia.

There is an urgent requirement for thorough biochemical analyses, especially possible anti-nutritional or toxic components, of the seeds of those species with greatest human food potential. There is also an important need for biosystematic studies to be undertaken on species and species groups of high potential socioeconomic importance (e.g. A. holosericea group, A. tumida, A. coriacea, A. victoriae group). These studies must be based on comprehensive and well-documented seed collections.

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INTRODUCTION

Australia has a vast tropical and subtropical, dry (< 500 mm) zone covering 1.7 x 10⁶ km² (north of 24°S). The 130 Acacia species present in this zone represent an important, but largely untapped genetic resource for reforestation in other tropical, dry parts of the world. A few species, in particular A. colei Maslin & Thomson (manuscript name, fide Maslin & Thomson in prep.) (formerly under the name A. holosericea) and A. tumida, have demonstrated rapid growth and adaptability to difficult and degraded sites in the drought and famine-prone Sahelian zone of west Africa (Cossalter 1987), as well as east Africa (e.g. Somalia and Kenya) and southern Africa (e.g. Zimbabwe, Gwaze 1989).

In addition to rapid growth and high survival on infertile, sandy sites species such as A. colei ms, A. cowleana and A. tumida

fix atmospheric nitrogen and therefore improve soil fertility for other crops,

lower windspeeds and reduce sand-blasting of adjacent crops,

produce large quantities of wood which are excellent fuels and amenable to conversion into charcoal (CTFT1985),

resist insect attack (including termites and locusts), and

have low palatability to grazing livestock (including goats).

The seeds of many dry-zone Acacia species were traditional food items of Australian Aborigines. The nutritional status of dry seed of Australian Acacia species is excellent, being typically high in protein (17-25%), fat (4-16%) and carbohydrate (30-40%) (Brand & Cherikoff 1985). The majority of species have an impermeable seed coat and the seed can be stored indefinitely (> 10 years) without deterioration, and will not rot or germinate if wetted, i.e. they constitute an ideal human famine food reserve.

This paper focuses on those Acacia species which produce heavy and easily harvested seed crops from an early age and which have the greatest potential to provide human food in dry, tropical environments, especially those in Africa. Africa faces massive human and environmental problems over the next few decades. More than 100 million Africans are living in a semi-permanent state of starvation. Food production has failed to keep pace with the rapid population increase, and had declined by 12% even before the 1984/85 and 1987/88 famines (Harrison 1987). In many areas the diet of African children is chronically short of energy and protein.

The purpose of this paper is to summarise existing knowledge of Australia’s subtropical dry-zone Acacia species with human food potential in order to aid their future research and utilisation.

METHODOLOGY AND RESULTS

The main sources of information for this paper have been published and unpublished literature dealing with the species (see References) and my field observations of these species in their native habitats in northern Australia, and trial plantings in west Africa and India. Information from various unpublished sources including herbarium label data, unpublished research reports and observations of Australian and overseas researchers has been sought to supplement the meagre published literature for little-known and recently described or undescribed species.

For the purposes of this review, species considered to have human food potential were those which:

(i) produced moderate or heavy seed crops which could be readily harvested and processed into food; and which were

(ii) recorded as having been utilized for human food by Aborigines.

This review identifies 44 Acacia species, originating in Australia’s subtropical dry zone, which have human food potential. The characteristics of these species are summarized in Table 1. The more promising of these species are discussed in the following section. Temperate and subtropical dry-zone Acacia species whose seed is reported to have been utilized for food by Aborigines, but which have not been discussed in the text of the paper, are listed in Table 2.

DISCUSSION

Most promising species

Acacia colei ms

A. colei ms is a hexaploid taxon presently included under A. holosericea. Sterile plants of A. colei ms can be distinguished from diploid and tetra-ploid taxa also presently under A. holosericea on the basis of phyllode characters. Mature phyllodes are obovate and slightly asymmetrical, 10-19 cm long and 2-5.5 cm broad, with 2-3 prominent and basally confluent longitudinal nerves. Secondary venation is longitudinally reticulate producing a pattern of slightly elongated (≤ 3 times as long as wide) nerve islands. A more prominent, nearly straight, longitudinal secondary nerve runs for nearly the length of the phyllode between the major nerves. A dense, felt-like covering of appressed, short sericeous hairs over the entire phyllode surface gives the plant its characteristically silvery blue appearance. The strongly and openly curved pods of A. colei ms are easily distinguished from the straight or shallowly curved pods of A. cowleana and the tightly inter-woven, coiled (2-3 complete spirals per pod) pods of A. neurocarpa (diploid) and A. holosericea sens. str. (tetraploid).

A. colei ms is widespread across inland regions of northern Australia. It extends from the Pilbara and southern Kimberley region in northern Western Australia, across the Great Sandy and Tanami Deserts (Northern Territory) to the Gulf country and Simpson Desert in western Queensland.

It grows in hot, semi-arid (270-690 mm) tropical zones, where it frequently forms dense, nearly monospecific, stands along dry, stony or sandy drainage lines. During cycles of wet years it proliferates and may extend onto sand plains and stony ridges. It is a colonizing species and a component of many semi-arid, subtropical plant communities, especially the Acacia-dominated scrubs and tall open shrublands of north-western Australia. Dense regrowth populations often develop on disturbed sites such as road verges, gravel pits and burnt areas.

Soil types include red-brown stony clays, deep aeolian sands, red sandy loams and fine textured clays and silty clays. Soils are typically neutral but can range from acidic (pH 5.5) to alkaline (pH 8.5). Some populations occur on the margins of saline drainage systems, e.g. Lake Gregory and Eighty Mile Beach (WA).

A. colei ms has shown outstanding growth in trials and extension plantings in west Africa (Cossalter 1987) and has vast potential as a new food crop for dry, sub-Saharan Africa (Thomson 1989; see Figure 1). It has greater drought tolerance, heavier seed yields and more easily extracted seed than A. holosericea and A. neurocarpa (Thomson 1989).

Table 1. Characteristics of Acacia species from Australia’s subtropical (<24°S), dry (<500 mm) zone with human food potential.

Explanatory notes on Table 1.

Related species (& hybrids): The most closely related species are indicated by numbers (for those related species included in Table 1) or as follows:

Species known to form hybrids are indicated in brackets.

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Natural occurrence: Drainage lines ** species typically occurring in the lower parts of the landscape, in close association with drainage lines; * species which sometimes grow along drainage lines, especially in the drier parts of their range; - species not usually occurring in close proximity to drainage lines. Climate zone: 1 species principally of semi-arid sub-tropics; 2 species occurring frequently to infrequently in the semi-arid sub-tropics; 3 Species with marginal occurrence in the semi-arid sub-tropics; 1-3 species with very wide distributions; N,S,E denotes that distribution extends to the north, south and east of semi-arid sub-tropical zone. Frost Incidence: ** heavy frosts in most years over a substantial part of natural range; * low incidence of frost in some parts of natural range; - nil or very low incidence of frost throughout the range

Soil properties (underlining indicates dominant category): Texture S = sand, SL = sandy loam, L = loam, CL = clay loam, C = clay; Depth Sh = shallow (< 0.5 m), M = moderate (0.5-1.5 m), D = deep (> 1.5 m); pH Ac = acid (pH < 6), N = neutral (pH 6-7.5), Al = alkaline (pH > 7.5); Salinity ** high levels of soluble salts (saturation extract EC > 4 mS cm-1) in upper soil horizons in some locations, * high levels of soluble salts (saturation extract EC > 4 mS cm-1) in lower soil horizons in some locations, - low levels of soluble salts in most locations.

Species characteristics: Plant dimensions (the dimensions given are the range for fully-grown plants in their native habitat; MS = multistemmed) Growth rate 1 = fast 2 = moderate, 3 = low; Longevity 1 = long-lived (> 50 years), 2 = moderate life-span (10-50 years), 3 = short lived (< 10 years); Coppicing ability and root suckering habit 1 = high, 2 = moderate or variable, 3 = low, R = root suckering habit (brackets indicate a low frequency of root suckering); Human food source 1 high-yielding and easily harvested species, 2 species whose seeds have some food potential.

Figure 1. Climatically suitable environments for Acacia colei ms in Africa. Map produced by Dr Trevor Booth (CSIRO Division of Forestry) in consultation with the author.

Black areas satisfy following requirements:

* indicates selected factor(s)

Silvicultural features

A. colei ms has exhibited high survival and rapid growth on a wide range of soil types in the tropical, hot semi-arid and sub-humid (500-1000 mm) zones of West Africa (see Cossalter 1987). Whilst A.colei ms grows less rapidly during the first 1-2 years cf. A. holosericea (tetraploid) and A. neurocarpa it has consistently shown greater drought tolerance and adaptability than these species and A. cowleana in older field trials in the Sahelian zone of West Africa, e.g. Keur Mactar, Senegal; Dosso, Niger; Gonse, Burkina Faso (Thomson 1989). There do not appear to be major differences in growth rates between provenances: plants from Turkey Creek, Western Australia (CSIRO S14460) and Hooker Creek, Northern Territory (CSIRO S14637) have exhibited the most rapid growth in provenance trials in Burkina Faso (IRBET/CTFT 1989).

A. colei ms has the capacity to regrow vigorously after cutting. Research at Dara Miliki (Niger) and Thienaba (Senegal) indicates that cutting time and height are critical. At Dara Miliki survival of plants cut at 5 and 50 cm in April was only 4 and 38%, respectively. The figures for cutting in June, at the onset of the rains, were 51 and 95% (C. Kjellstrom, pers. comm.). At Thienaba the best period for coppicing was in the months preceding the wet season, i.e. May-June (M. Cazet, pers. comm.).

Management of A. colei ms for seed production will require planting at wide spacing, e.g. 5 x 5 m, and periodic rejuvenation of the crown, possibly every 1-2 years, through high cutting, e.g. at 80-100 cm. At closer spacings, e.g. 4 x 4 m, the growth of plants stagnates after 2 years, fruiting is greatly reduced and plants prematurely senesce during prolonged dry periods.

Flowering and fruiting

Plants flower from an early age, e.g. 6-7 months at four out of six trial sites in Thailand (Wasuwanich 1989). In northern Australia the peak flowering period is June-July, but flowering may occur from May to September. Large masses of loosely interwoven reddish-brown, curved pods are present between mid-September and mid-November.

Seed collection, processing and utilization for food

At dehiscence the seeds are exposed, dangling from the pod by a bright yellow aril. Good timing is essential for efficient seed collections. Pods mature more or less synchronously and pods/seeds fall from the bush within 1-2 weeks in hot, windy weather. The seed can be very rapidly collected by beating the dry, mature pods