EXPLORATION AND SEED COLLECTION OF ACACIA SENEGAL IN SUDAN

by
B. Kananji
Tree Seed Centre
Forest Research Centre
Box 7089, Khartoum,
Soba, Sudan

INTRODUCTION

Sudan is situated between latitudes 21° 55' and 3° 53'N longitudes 21° 54' and 38° 31'E and has a total area of 2,505,813 km³. Of this, 1,879,359 Km³ (75%) is hot and dry (729,685 Km³ desert and 476,104 semi-desert), 476,104 Km³ (15%) high rainfall savanna and montane and 250,582 Km³ (10%) is swamps or areas liable to flooding.

A need for increased reforestation and afforestation in Sudan was reflected in the Forestry Sector Review of 1986. Comparing the growing stock with annual allowable cut and consumption of wood gave a deficit of 9.95 million m³ for Northern, Eastern, Central and Khartoum regions combined. Khordofan had a surplus of 3.75 million m³ and Darfur had a 17.64 million m³ surplus. The Sudan South had a surplus of 50.57 million m³. However, the distances involved and the conditions of the roads economically prevent transport of wood from surplus areas to deficit areas. The demand of seed for reforestation and afforestation was estimated by Forest National Co-operations to be between 80 and 120 tons.

Identified needs lead to the establishment of the Tree Seed Centre in 1989 with support from United Nation Sahelian Office (UNSO), UNDP and DANIDA. The Seed Centre is based in Soba some 20 Km from Khartoum, with a regional centre in El Obeid, Kordofan state. The aim of the Tree Seed Centre is to procure and provide forest reproductive materials (mainly seed) of high physiological and genetic quality.

The Tree Seed Centre handles many species, and species regularly collected are listed in Table 1. For tree improvement programmes the Tree Seed Centre had to establish priorities as such programmes require long-term costly commitment. The first priority species for tree improvement is Acacia senegal (L) Wild, but also Acacia nilotica (L) Wild and Balanites aegyptiaca (L) Del will be considered.

Table 1. Species regularly collected by the Tree Seed Centre (species not listed can usually be collected upon request)

* exotic

ACACIA SENEGAL

Of the 900 species of Acacias botanically identified in the world, 500 are known to grow naturally in the Sahelian Region in Africa. Out of these species only three species are major sources of gum arabic and are commercially exploited for this commodity: A. senegal, A. seyal and A. laeta. About 80% of all commercial gums arabic is derived from A. senegal and 10% from A. seyal the remaining 10% from A, laeta, A. camplycantha and A. depanolobium (Anderson 1977). The World's supply of gum arabic comes from Africa including Sudan, Senegal, Mali, Mauritania, Niger, Chad and Nigeria. Over 80% of this supply comes from Sudan and accounts for 10% of Sudan's export proceeds (Beshai, 1984).

Despite its importance in the Sudan, improvement of Acacia senegal (L) Wild has been carried out exclusively through silvicultural means (Nasroun, 1989), although progeny trials were established to investigate possibilities for genetic improvement already in 1967, and gum yield variation from 6 different seed sources has been studied (Kananji 1993).

With the establishment of the Tree Seed Centre, breeding of A. senegal mainly for gum arabic production and to some extent for fodder, fuelwood and drought tolerance was initiated. This article presents the germplasm exploration and collection that was done as the first step in such a programme. Seeds of Acacia senegal were collected mainly as single tree seedlots. Some of the seed lots can be made available to other research institutions upon request.

The basis for selection and improvement of forest trees, both indigenous and exotic, is the natural variation present. In order to properly utilize and at the same time conserve this variation (generally called genetic resources) there is need to explore the species both botanically and genecologically. It is essential that only seed of known origin are used for species trials, tree breeding and genetic studies.

Some species cover a wide range of environmental conditions within which the variation may be as large as between closely related species. Other species however, have a limited natural distribution, which may consist of isolated provenances adapted to specific environmental conditions (i.e. drought, salinity etc). Acacia senegal covers extensive areas especially in the drier parts south of the Sahara, throughout the Sahelian zone from Senegal to Somalia. In Sudan it extends between 16° N and 10° N latitude, with the gum production zone being concentrated around 11°N and 14°N latitude (see Figure 1).

Figure 1. Map of the gum belt in Sudan (from Kananji 1993)

Genetic variation does not always manifest itself in the natural conditions. Therefore, systematic sampling throughout the area of natural distribution, and evaluation in the field trials over a wide range of potential planting sites are essential to determine of the kind and extent of the within-species variation and of its potential and adaptability to varying environments.

Therefore, a germplasm exploration, collection and field evaluation was considered a necessary first step in the tree improvement programme of Acacia senegal in the Sudan.

GERMPLASM EXPLORATION

Germplasm exploration took place from September to December 1992, and covered the states of Kordofan, the Central and Eastern states, Darfur and Upper Nile. For analysis, the gum belt was divided into rainfall zones using isohyets. Five rainfall isohyets were drawn at 200 mm rainfall intervals, from 200 mm up to 1000 mm. Using existing maps, the isohyets map was superimposed on soil maps, and the following generalizations were drawn:

1. the 200mm isohyet overlaps predominantly with the lateritic catena soils of Khartoum state, North Central state, northern Kordofan and northern Darfur state.

2. the 400mm isohyet overlaps mainly with the clay soils of Kassala and northern Darfur, and with the mixed clay/sandy soils of Kordofan, and Darfur.

3. the 600mm isohyet overlaps with the clay soils of Kassala, Blue and White Niles: with the mixed clay/sandy soils of northern Kordofan and southern Darfur, and with the hill catena soils of southern Darfur.

4. the 800 mm isohyets overlaps mainly with alternating clay and sandy soils in Upper Nile and Bahr el Ghazal; with the mixed clay/sandy soils of Southern Darfur and with the clay soils of Eastern state.

Information on soil type, rainfall pattern and elevation, formed the basis for the selection of seed collection sites. Table 2 gives more information on the collection of Acacia senegal. The seed sources are described in detail in the following section.

SEED COLLECTION

For Sudan the most important character of A. senegal is gum arabic production, however, this character can not be instantly assessed at the time of the seed collection. Whenever possible the knowhow of farmers who had tapped gum for a long time in the seed collection area was used to select consistently high yielding trees in both natural populations and plantations. This local knowledge was, however, not available at all collection sites, and therefore three selection criteria were used:

1. Farmer identification: Farmers were asked to identify high yielding trees. It should be noted that in the Sudan, even natural populations of A. senegal are tapped for gum by rural people.

2. Vigour: This criterion was used where local information on gum yield was not available. Only vigorous trees were selected for seed collection.

3. For some populations none of the above criteria could be used. Trees were selected merely by the criterion that the distance between trees from which seed was collected should be at least 100m apart.

In all cases, tree health was taken into account, and seed was collected only from healthy trees.

All individual tree seed collections were maintained and labelled. The seedlot identities were maintained during processing, cleaning and storage. Special care was taken to remove, by hand, all seeds that showed signs of insect infestation.

PROVENANCES DESCRIPTIONS

Provenance descriptions with additional information on the seed collection site are given below. The descriptions are summarized in table 2. Each provenance was given a specific code. The number on the code relates to site number and the letters relate to the name of the site (table 2).

Table 2. Seed sources of Acacia senegal

N.B. Some of the above seed lots can be made available by the Tree Seed Centre, Sudan to other research institutions upon request.

Wad Dafta Forest (code 1-kw).
15km south of Wadelhuri village in the eastern state. Lat. 13°50'N and long. 35°20'E. Within the 600 mm rainfall regime. Flat terrain with black cracking clay soils. Natural populations of A. senegal are found in mixture with A. seyal. Thirty high yielding trees were collected from. Generally, the pods were big on this site. The site has a good access road.

Saraf Saeed Forest (code 2-KS)
This is a plantation of A. senegal within a natural A. seyal forest. The site is some 36km south of Doka (capital of Gallab province) in the Eastern state, lat. 13° 20'N and long. 35°50'E. It is within the 800mm rainfall isohyet, with black cracking clay soil and flat terrain. It is on the edge of the gum belt boundary. It is a good site for A. seyal seed collection. With the help of farmers, thirty high yielding gum arabic trees were selected for seed collection.

Wadelmahi Village (code 3-BW).
50km south-east of Damazin in Central state, lat. 11°33'N and long. 34°35'E, just outside the gum belt boundary. The soil type is clay. The rainfall is 800mm. 30 trees were selected for seed collection based on vigour. The population is natural, scattered and phenotypically little impressive.

Jebel Samina (code 4-BS)
Situated some 23km east of Wademahi village, in Central state, the site is on the hill side of Shenima Hill facing eastward lat. 11°35'N and long. 34°40'E. The rainfall is nearly 1000mm. Stony catena hill soils. The natural A. senegal is found in association with Sterculia spp. It is a small population of A. senegal and the fruits have small pods. 30 trees were selected and collected based on vigour. The site is outside the gum belt and is near the Ethiopian border.

Bogal (code 5-BB)
A valley between hills, Bogal is 80km south west of Damazin. Located outside the gum belt, lat. 11°00'N and long. 34°25'E. Black cracking soils with stony pebbles due to the presence of hill catena soils on two hills on both sides. It is a small natural population which is phenotypically little impressive. The trees are scattered. 30 trees were collected at random, without selection, but with a minimum spacing of 100 m.

Mazimum (code 6-M)
This is some 122km south east of Gebalien. Lat. 12°20'N and long. 33°30'E. The terrain is flat with dark cracking clay soils. Rainfall is in the 600mm range. The site is a good natural population. It is recommended for in situ gene conservation. Seed production is abundant. 30 trees were chosen for collection based on vigour, health and seed productivity.

Ladpior Village (code 7-WL).
Twenty four kilometres south of Renk, lat. 11°30'N and long. 32°47'E. Desert soil type in the 600mm rainfall range. Based on gum yield, 30 high yielding trees were selected and seeds collected.

Masabal Village (code 8-WM).
This site is on the west bank of White Nile river, opposite site code number 7-WM, lat. 11°45'N and long. 32°40'E. Desert soil type, rainfall range 600mm. 30 trees were marked and seed collection was based on high yield.

Ummbusa Village (code-10 WN)
Some 27km north-west of Kosti City. Desert soil type with rainfall range of 200mm. The trees did not bear seeds when stand was visited.

Naeem Qoz (code 11 WN)
110km south-east of Kosti City. The soil is desert type with a rainfall range of 400mm. The trees had no seed when stand was visited.

Elnuhud (code 12-FA)
Some 23km west of El Obeid lat. 12°40'N and long. 28°25'E. Currently a lot of development projects are being carried out in this area. The site is near Araita Waillya village some 20km north of Elnuhud. It has sandy soils, and is within the 400mm rainfall range. With the help of local farmers some 18 high gum yielding trees were chosen for seed collection. Although the area is an A. senegal area, it was difficult to find many high yielding ones. The population seemed to be a mixture of natural regeneration, coppice and planted trees. Most trees did not produce enough seed for collection, which could be due to the locust outbreak that had occurred in the area earlier.

Sunjukiya Bridge (code 13-FS)
The collection was done along the road from Elobeid to Dilling. Starting at about 30km from Dibibat, which is 140km from El Obeid. Collection continued along this road up to the Sunjukiya bridge, lat. 25°25'N and long. 29°40'E, spacing the trees collected from 1 km. Soil hill type catena, reddish in colour. The rainfall is above 800mm. The area is not an A. senegal area but it was considered desirable to collect from it because it is close to the extreme of the species's altitudinal range. Seed was collected from 12 trees without selection (100 m spacing). Most trees had not enough seed. The seed was bulked.

Abassiya (code 14-FT)
Lat. 12°15'N and long. 31°20'E, some 24km from El Obeid in south east direction. The actual site of collection was some 20km north of Abassiya near a village called Tofeni on the foot of Tugida hill. The soil is hill catena with some pebbles. Rainfall above 800mm. It is a population of coppice regeneration of some 700 feddans (approx. 294 ha) in mixture with Albizzia species and others, including A. mellifera. Based on vigour, 30 trees were selected for seed collection. Some trees were good seed producers.

Umsumima (code 15-FN)
This site is 140km north east of Elobeid with sandy soils. The site just on the edge of gum belt, lat. 13°35'N and long. 30°55'E, rainfall approx. 200mm. Trees were selected at random. The population is scattered, the few trees found each produced a lot of seed. The seed was bulked.

IDENTIFICATION OF POSSIBLE SEED SOURCES FOR SOME OTHER SPECIES

To save resources, it was envisaged that the seed collection of A. senegal could be combined with collection of other species including Balanites aegyptiaca, A. seyal and A. nilotica. However, it was not possible to fully combine the collection of A. senegal with collection of these other species. Nevertheless, some populations of Balanites aegyptiaca and A. nilotica var. nilotica were identified in Saarta as possible future seed sources. Similarly, promising seed production populations of A. seyal were noted at Rawashda between Gadarif and Kassala, Wad Dafta Forest and Sarafi Saeed Forest near Gadarif and south east of Dilling.

REFERENCES

Anderson, D.M. (1977) Water soluble plant gum exudates -1: gum arabic. Proc.Biochem.12:24–25.

Beshai, A.A. (1984). The economics of a primary commodity: gum arabic. Oxford Bulletin of economic and statistics 46(4): 371–380.

Kananji, B. (1993). Variation in gum arabic production of six sudanese Acacia senegal seed sources. In: Tree Seed Problems with special reference to Africa, Proceeding of the IUFRO Symposium held in Ouagadougou, Burkina Faso, 23–28 Nov 1992: pp. 118–127

Nasroun, T.H. (1989). Forestry Research priorities in the Sudan. Sudan Silva 27(VIII):5–11.

Forest Genetic Resources No. 22. FAO, Rome (1994)
Manuscript received May 1994.