Acclimatization and utilization of phyllodineous acacias from Australia in Senegal**

O. Hamel

Directeur du Department des Recherches Forestières et Hydrobiologiques de l'Institute Sénégalais de Recherches Agricoles. Center National de Recherches Forestièeres BE 2312, Dakar, Senegal.

Introduction

1. Nursery production

2. On-site behaviour of species

2.1. Bambey site

2.2. Bandia site

2.3. Keur-Mactar site

2.4. Linguere site

2.5. Mbiddi site

3. Suitability of species

3.1. Browse

3.2. Windbreaks

3.3. Soil protection and regeneration

3.4. Browse and other production

4. General conclusions

Annex Ia

Annex Ib

Annex Ic.

Annex II

Annex III

Annex IV

Annex V

Introduction

The Senegalese Forestry Research Institute has planted a collection of acacias previously unknown to Africa on a number of stations characteristic of the Sudanian and Sahel area of Senegal. A crop of Australian seeds was used, procured in 1973 by the Centre Technique Forestier Tropical in cooperation with the Australian Forestry Timber Board. Most of the seed introduced came from the Northern and West Australian territories, the climatic conditions with regard to the Australian seeds and the Senegalese sites chosen for introduction are briefly described in Annex I to this paper.

The species introduced were as follows:

a) from Australia: A. bivenosa, A. coriacea, A. dunnii, A. farnesiana, A. holosericea, A. inaequilatera, A. aff. Linarioides, A. monticola, A. mountfordae, A. plectocarna, A. pyrifolia, A. sclerosperma, A. spathulata, A. tenuisissima, A. tetragonophylla, A. tumida;

b) from the Maghreb: A. cyanophylla, A. Cyclops, A. salicina.

Nursery production

Raising these Acacia spp. in the nursery poses on particular problems. Direct pot sowing (2 seeds per pot) gives very good results, with an excellent germination rate.

Depending on the site it may be necessary to use shading during the period before the phyllodes appear, i.e. for about a fortnight, but usually the resistance of these species means that this precaution is not necessary. The nursery stage lasts about 3 months. The only precautions necessary are on the health side, against cutworm and nematodes. The latter precaution is important since, like all acacias, these species are sensitive to nematodes, and the risk of infestation on virgin land resulting from the introduction of these species is considerable. Furthermore, nematodes prevent the formulation of rhizobia nodules and thus inhibit their nitrogen-fixative capacity. Nematicide treatment also accelerates the development of the plants in the nursery.

Ongoing research at the ORSTOM Soil Microbiology Laboratory in Dakar in collaboration with the Centre National de Recherches Forestières/ISRA is focusing on the selection of species with good nitrogen-fixative properties, on the selection of successful rhizobia strains and on inoculation in the nursery. Inoculation will enable the nursery stage to be shortened while still providing high quality plants. The behaviour of rhizobia when planted during the dry season is also under study.

Since 1979, direct drilling trials in the natural environment have given highly encouraging results, raising hopes that planting the species will prove to be inexpensive.

On-site behaviour of species

2.1. Bambey site

Soil: the soil is known locally as "deck dior", consisting of deck soils (vertisols with a temporarily moist soil climate, lithomorphic soils with massive intergrade surface structure consisting of ferruginous soils) but with a much lower clay content in the upper horizons. Plant spacing: 3m × 3m; the plants were holed manually in large seed holes (60 cm × 60 cm × 60 cm).

The plants introduced in 1974 and 1976 (see Annexes 1 and 2 for detailed results) have enabled an initial idea of the behaviour of these species to be gained:

a) A. cynophylla plants died;

b) A. farnesiana, spathulata, tenuisissima and tetragonophylla are in the process of dying;

c) A. coriacea, dunnii, inaequlatera, monticola, mountfordea, plectocarpa and pyrifolia are surviving but their performance is mediocre;

d) A. bivenosa, holosericea, linariodes, sclerosperma and tumida are behaving satisfactorily.

Since 1976 a comparative trial on behaviour and production between local and Australian acacias has been launched, giving us a clearer idea of their suitability (see the results recorded in Table 1)

Table 1. Comparative trial on behaviour and production of local and Australian acacias: results of measurements at Bambey site, 1976.

	12/1976		7/1977	12/1977		5/1978		12/1978		6/1979		12/1979
	H	%	H	H	s	H	s	H	s	H	s	H	s	%
A. seyal	108	94	154	168	13	181	9.8	239	22.3	283	29.0	294	30.8	93
A. senegal	68	98	124	160	10	188	11.5	240	26.3	272	29.6	279	31.8	98
A. raddiana	89	100	129	161	9	189	13.0	228	24.0	278	32.4	283	33.7	100
A. holosericea	60	97	138	200	13	200	13.9	305	33.0	332	40.0	336	44.0	96
A.linarioïles	88	88	199	229	17	186	21.4	251	61.3	256	65.3	264	67.1	81

H = height (in cm) s = section (in cm ²)

Variance analysis has demonstrated that since December 1977 the production of Acacia linarioïdes and holosericea has been significantly higher (5% theshold) than local acacias.

In this trial the spacing used was 3m × 3m and it appears that for all species competition become acute in December 1979. The stands were therefore thinned out to 3m × 6m in February 1980, and the data gathered at that time are recorded in Table 2.

Table 2. Comparative trial on behaviour and production of local and Australian acacias: data collected during thinning of stands from 3m × 3m to 3m × 6m at Bambey site, 1976.

Species	Weight (kg) of dry leaves per tree	Weight (kg) of dry leaves per hectare	Weight (kg) of wood per tree	Production (t) of wood per hectare	Production (t) of wood per ha/year	Production in steres per hectare	Production in stere per ha/year
A. seyal	208	229	4.39	4.8	1.6	18.3	6
A. senegal	60	66	9.92	10.9	3.6	36.5	12
A. raddiana	130	143	5.89	6.5	2.1	24.1	8
A. holosericea	2660	2926	12.03	13.2	4.4	36.5	12
A. linarioïdes	1993	2192	10.65	11.7	3.9	54.7	18

It should be noted that measurements were carried out in the middle of the dry season, a time at which local acacias have lost a large proportion of their leaves, whereas Australian varieties mostly keep theirs.

It emerges clearly that wood production is higher in Australian varieties and that leaf production at this time of the year could contribute significantly to livestock feeds.

2.2. Bandia site

Soil: vertisols and brown eutrophic soils on alluvial clays and zoogenic limestones. The plant spacing used was 4.5 m × 4.5 m, after subsoiling with cross lands to a depth of 50 cm.

The plants introduced in 1977 (see Annex 4 for detailed results) gave results very similar to those at Bambey. Thus:

a) A. cyanophylla, Cyclops and tenuisissima died;

b) A. dunnii, monticola, pyrifolia and tumida may or may not survive;

c) the performance of A. plectocarpa and salicina has to be verified;

d) lastly, A. bivenosa, coriacea, holosericea, linarioïles and sclerosperma gave interesting results (A. bivenosa, coriacea and sclerosperma are forming hybrids).

A. farnesiana, mountfordae and inaequilatera were only introduced at Bandia in 1979; their behaviour is therefore not yet significant.

Comparison of the best Australian varieties with local plants gives the results found in Table 3, but it should be empahsized that here the aim of the trial was to study the behaviour of each species individually and that, as a result, there were no replicates for comparison. Thus, Acacia nilotica and Prosopis juliflora are high as Acacia linarioides, but A. holosericea is the most productive of all.

Table 3. Results of comparison of best Australian with local species.

Species	12/1977		6/1978	12/1978			6/1979		11/1979
	H	%	H	H	s	%	H	s	H	s	%
A. bivenosa 1538	57	95	93	145	10.1	93	179	14.9	216	28.1	93
A. coriacea 1460	58	100	105	171	9.8	100	219	20.8	264	31.9	100
A. holosericea 1459	43	100	144	278	24.4	100	284	19.2	382	63.7	100
A. aff. linarioïdes 1466	90	96	120	255	19.4	96	265	13.9	355	45.7	96
A. sclerosperma 1525	59	97	99	154	9.1	97	210	16.8	251	27.2	97
A. nilotica var. adansonii	65	99	104	195	12.2	97	230	14.2	305	41.5	97
A. raddiana	55	100	53	121	3.2	100	174	6.1	258	18.7	97
A. albida	30	99	35	70	–	98	108	–	121	–	92
Prosopis juliflora	87	99	125	243	11.8	99	307	23.7	379	40.1	99
Prosopis cineraria	40	99	45	92	–	96	124	–	200	–	92

Direct drilling trials gave the results shown in Table 4. These results may be considered highly encouraging.

Table 4. Results of direct drilling trials.

2.3. Keur-Mactar site

Soil: this site, located on the edge of a salt pan, has a large variety of soils with more or less pronounced salinity and hydromorphic features. They are characterised by a gradual transition from bare soil with Salontchaks to a plant cover consisting of Combretum, via a grass cover and an Acacia seyal stand. The original mangrove swamp and its surrounding vegetation probably vanished during the last century. Spacing: the spacing used was 3 m × 3 m; the soil was tilled manually and large seed holes were dug (60 cm × 60 cm × 60 cm).

The 1977 trial (for detailed results see Annex 5) was intended to enable a number of Australian acacia species to be selected for colonizing soils of this kind. To this end six types of soil cover were defined:

b) salt pan with grass cover (Sg);

c) soil with Acacia seyal (Sa);

d) soil with Combretum (Sc);

e) soil with Parinari (Sp);

f) bottom-land (BI);.

The results recorded were as follows:

a) A. holosericea and linarioïdes behaved quite remarkably throughout the entire range, especially A. holosericea, the performance of which on salt pan is astonishing, to say the least, and may be even suspect; this variety was also the only one to tolerate bottom-land (see Figure 1).

b) A. bivenosa and sclerosperma gave interesting results on soils with A. seyal, while A. plectocarpa did well on Combretum soils.

c) A. tumida, pyrifolia and monticola did not appear to adapt.

The 1978 trial dealt with the forestry methods applicable to these species, from the point of view of both browse and forage production on ferruginous hydromorphic soils with a pre-existing plant cover consisting of Combretum. The first trial tested spacing of 3 m × 3 m, 3 m × 6 m and 6 m × 6 m, using a Latin square layout with Acacia linarioïdes as a control batch.

Spacing	12/1978		6/1979		12/1979
	H	%	H	%	H	s	%
3 m ×3 m	82	100	177	99	251	45	99
3 m × 6 m	81	100	169	99	268	67	93
6 m × 6 m	84	100	165	99	268	77	95

Figure 1. Behaviour of A.. holosericea and A. linarioides.

The differences on the section recorded in December 1979 are highly significant. It may be noted that these individuals are already as well developed as the Acacia linarioïdes plants of the 1976 trial at Bambey. We may thus assume that foliage production is also about identical after 18 months.

A second trial launched in 1978 focused on the different possible methods for harvesting the wood and the foliage biomass (felling, cutting back, trimming, stripping, etc). Different forms of utilization were applied for the first time in February 1980 and are to be staggered until July 1980. The first results for trees 18 months old subjected to utilization in February and to a spacing of 3 m × 3 m were as follows:

	Weight of leaves per tree	Weight of leaves per ha (3m × 3m)	Weight of wood per tree	Weight of wood per ha (3m × 3m)
A.linarioïdes	1845g	2029g	5.25 kg	5.77t

Thus it was concluded that the weight of leaves after 18 months was approximately identical to that of the Bambey plantation in 1976 after 36 months.

A third trial launched in 1979 was intended to test direct drilling techniques in the field.

Rates of germination resulting in viable plants.

Soil: edge of salt pans.

Species	Treated seed	Untreated seed
A. holosericea	51%	28%
A. bivenosa	34%	14%
A.linarioides	45%	42%
Prosopis juliflora	27%	7%
A. seyal	8%	1%

Ferruginous soils: soils with Combretum

Species	Treated seed	Untreated seed
A. holosericea	2%	1%
A. albida	27%	14%

It was found that the resistance of young plants is closely linked to the soil type. Other kinds of trial will be launched this year.

2.4. Linguere site

Soil: brown-red subarid intergrade soils and ferruginous tropical soils. Plants were introduced in 1974, but unfortunately the low quantity of seed which we had available at the time only enabled us to plant about 10 individuals for each species. In any case the poor planting, maintenance and follow-up conditions have led us to discount these introductions as insignificant.

2.5. Mbiddi site

Soil: brown-red subarid intergrade soils and poorly developed ferruginous tropical soils. The species introduced were A. holosericea, A. linarioïdes, A. bivenosa, A. pyrifolia and A. tumida. All species developed spectacularly during the first year, but unfortunately there were considerable losses during the second year for A. holosericea and tumida. A. linoarioïdes, bivenosa and pyrifolia are surviving at present without showing any noticeable progress. More detailed research will be carried out in this area during this year.

Following these different adaptability tests, which will moreover be continued, the following selection may be put forward:

a) highly interesting species: A. holosericea and A. linarioïdes;

b) interesting species: A. holosericea, A. sclerosperma and coriacea;

c) species which show promise but will have to be confirmed: A. plectocarpa and A. salicina;

d) species for which we do not have enough experience: A. inaequilatera and A. mountfordae;

e) species not selected for the moment: A. dunnii, A. monticola, A. pyrifolia and A. tumida;

f) species which do not adapt: A. farnesiana, A. spathulata, A. tenuisissima, A. tetragonophylla, A. cyanophylla and A. Cyclops.

3. Suitability of species

3.1. Browse

There is not doubt that the primary suitability of acacias is for browse. The persistence of the leaves throughout the year and the abundant foliage contrast strongly with our traditional acacias.

Feed value analyses have been carried out by the ISRA Laboratoire National de l'Elevage et de Recherches Véterinaires (see results detailed in Table 5).

Table 5. Feed value analyses (in% per kg of dm) of leaves collected on 12 December 1979 at Bandia.

Obviously there are variable differences between the species in terms of contents of primary elements, but all varieties contain valuable levels of nitrogen, higher than 12% and reaching 18%. Three species have a clearly higher level of minerals, especially calcium, than the others: first Acacia bivenosa, then Acacia sclerosperma and Acacia coriacea.

In addition pod harvests were carried out during February 1980 and are currently being analysed.

Finally, the palatability of all these acacia species was tested during December 1979, under the following conditions: a makeshift enclosure round four trees of the same species in which two head of cattle or three sheep were pastured for one or two days at the most. Thus the spontaneous palatability was tested and the results were as follows:

	Cattle	Sheep
A.linarioïdes	P	P
A. bivenosa	NP	NP
A. sclerosperma	NP	NP
A. coriacea	NP	NP
A. plectocarpa	P	P
A. salicina	PP	PP
A. pyrifolia	P	P
A. monticola	P	P
A. tumida	PP	PP
Prosopis juliflora	PP	PP
Prosopis cineraria	PP	PP

The results presented here are the first ones carried out at a time when the animals still have grassland available in satisfactory amounts and are in a generally good state, but probably they would behave differently if all they could obtain was dried grass which was scanty and poor in feed value. Also, two further tests will be carried out in March and June and it should be noted that, as regards Acacia holosericea the palatability test was positive when the leaves were offered to the animals dry.

Pending the results of the various palatability tests as well as of production experiments using harvesting by trimming, pitching and felling, and pending the launching of trials to determine overall feed biomass (grass cover plus browse) as a function of spacing — and hence the forestry technique to be used — it may be pointed out that some species, especially Acacia linarioïdes offer valuable dry-season browse possibilities which could be utilized either by introducing browse stands on farmland or by organizing forest for pastoral use. In 1980 a pilot scheme will be launched on the Sangalkam experimental farm of the Laboratoire National de l'Elevage et des Recherches Veterinaires (ISRA).

3.2. Windbreaks

Some Australian acacias have a natural habit and a crown which renders them particularly satisfactory as low-stratum components in a windbreak. Particularly advocated is the combination of Eucalyptus camaldulensis with Acacia holosericea. These two species are often combined in their original habitats in Australia:

a) Eucalyptus camaldulensis would provide the upper stratum of the wind-break; it is a tree over 12 m in height in the Sudano-Sahelian zone, but can reach 20 m or more in more favourable conditions. This tree has a first rate ability to put out new shoot and would thus permit regular utilization, while also providing farmers with firewood or timber in large amounts. When two new shoots are selected the wind-break may be built up very rapidly and is entirely satisfactory.

In order to avoid excessive competition with agricultural crops it is recommended that subsoiling along the windbreak be carried out before the dry season so as to cut outrunning roots.

b) Acacia holosericea (probably A. plectocarpa and A. salicina also) would provide the lower level in the windbreak, thanks to its low height (5 to 6 m in the Sahelo-Sudanian zone), but more especially to its plentiful and dense crown, which often very rapidly reaches 4 to 5 m in diameter at both the base and the summit.

The most satisfactory arrangement of these two species would be the following combination:

+ ₀+₀+₀ +

3.3. Soil protection and regeneration

As regards soil protection and particularly the prevention of runoff erosion, all the species are able to play a valuable part, although probably Acacia bivenosa is the most suitable. This bush covers the soil particularly well and its behaviour on difficult land is good. But we can also draw attention to Acacia coriacea, sclerosperma and linarioïdes.

For stabilizing shifting sand, or more accurately for protecting niayes (bottom-land suitable for marshland cultivation) against sand encroachment, a number of tests have been carried out under a UNDP project at Kebemer, using A. holosericea, tumida, linarioïdes, cyanophylla and Cyclops. The behaviour of A. holosericea and linarioïdes is on the whole satisfactory.

3.4. Browse and other production

After 5 years of experiments with these species, it appears that Australian acacias suited to our climate have a browse production which is equal to or higher than local species (see the results at Bambey and Bandia), but our knowledge as to wood quality is largely insufficient and studies on charcoal production and quality have only just begun, using recent utilizations.

Given the growth pattern of the trees it would appear that A. holosericea, plectocarpa and salicina are probably suitable for making good charcoal, since they are trees which have a main trunk, whereas A. linarioïdes, bivenosa, sclerosperma and coriacea are bushes with a large number of fairly thin branches, making them easier to use for firewood than for turning into charcoal.

As regards subsidiary products, studies have not yet begun but they will focus on rubber and honey production, and on any other product which is likely to be valuable to the local population.

4. General conclusions

The different varieties of Australian acacia provide a variety of phyllodes, habit, behaviour and growth which is truly astonishing. We can exploit this rich variety by adapting their features to our own objectives. Thus, if we have to select only one species for each use, we would identify the following in the light of our present knowledge:

a) Acacia linarioïdes for its feed value;

b) Acacia holosericea for windbreaks;

c) Acacia bivenosa for runoff erosion prevention;

d)Acacia holosericea again for firewood and charcoal.

Given our fairly recent experience of the species we cannot yet with any certainty say that their drought resistance is the same as that of local acacias, but if this is the case these acacias will only complete the range of utilization found in our own traditional varieties without in any way replacing them.

In any case our range of intervention in the Sudanian and Sahelian zones has been enriched by several species which can be used with discernment.

Annex Ia

Sites of Australian seeds

Annex Ib

Climatological data for Australian sites: Meteorological office, Melbourne ( Pls. make the columns of Annex Ib wide) 

Annex Ic.

Location of Senegalese research sites

Climatological data for Senegalese sites (Asecna) 

Annex II

Bambey site: 1974 introductions measured in Nov. 1979

Annex III

 Bambey site
 1974 introductions measured in Nov. 1979

Annex IV

 Bandia site
 1977 introductions

Annex V

  Keur-Mactar site – 1977 trials
  Soil beneath Acacia seyal

Soil beneath Combretum

Soil beneath Parinari

H = height in cm; S = Section in cm²

Bottom land

Pure salt-pans

Grassy salt-pans

H = height in cm; S = Section in cm²