Ecology of a key African multipurpose tree species, Balanites aegyptiaca (Balanitaceae): the state-of-knowledge

Forest Ecology and Management, 50 ( 1992 ) 1-30 Elsevier Science Publishers B.V., A m s t e r d a m

Ecology of a key African multipurpose tree species, Balanites aegyptiaca (Balanitaceae): the state-of-knowledge J o h n B. H a l l

School of Agricultural and Forest Sciences, UniverJity College of North Wales, Bangor, LL57 2UW, UK (Accepted 30 May 1991 )

ABSTRACT Hall, J.B., 1992. Ecology of a key African multipurpose tree species, Balanites aegyptiaca (Balanitaceac): the state-of-knowledge. For. Ecol. Manage., 50: 1-30. Published information on the distribution of Balanites aegyptiaca (Linnaeus) Del. was assembled and summarised as a distribution map interpreted using soil and vegetation maps of Africa and the Middle East, meteorological data and information in the ecological literature. 8alanites aegyptiaca is reported from most African countries and also from the Arabian peninsula and adjacent parts of the Middle East. Within Africa the range is from Mauritania in the west to Somalia in the east and from Egypt southwards to Zimbabwe. Occurrence south of the Sahara desert is generally related to rain-fed situations with a mean annual rainfall of 400-800 ram, but there has been spread to disturbed sites in wetter climates. Further north, Balanites aegyptiaca is present in numerous localities where sub-optimal rainfall is augmented with water from other sources. Other conditions associated with the species are a mean annual temperature > 20°C and freedom from frost. Typical sites are level and low-lying, with relatively fertile loamy or clayey soil of low salinity but, particularly it', the northern Sahel, Balanites aegyptiaca is also widely present on lighter, better-drained soil. Balanites aegyptiaca is characteristic of vegetation in which woody species are widely spaced, with full crown exposure. Associated woody species vary through the range but listings frequently include the families Caesalpiniaceae, Capparaceae, Combretaceae (especially Combretum ) and Mimosaceae (especially Acacia). When present in abundance, the number of individuals more than 5 cm diameter at breast height may exceed 25 h a - t, representing > 15% of woody individuals present. Attention is drawn to numerous relationships with animals (including Man and domestic livestock) and the significance of these in the conservation and management context. Practicable action in terms of provenance trials, which would lead to improved understanding of the potential of the species as a resource, is outlined and suggestions are made for positive management measures and planting initiatives. Correspondence to: J.B. Hail, School o f Agricultural and Forest Sciences, University College o f North Wales, Bangor, LL57 2UW, UK.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$05.00

2

J.B. HALL

INTRODUCTION

In the last 20 years, disconcerting implications for environmental degradation in the semi-arid regions of tropical Africa have been acknowledged. Loss of woody plant cover is perceived as a contributory element and the relative poverty of knowledge of trees adapted to semi-arid conditions has been exposed. Studies on such species as Acacia nilotica (Greaves, 1984a), Acacia tortilis (Greaves, 1984b), Adansonia digitata (Wickens, 1982) and Faidherbia albida (Felker, 1978; Centre Technique Forestier Tropical, 1988) represent efforts to unify current knowledge as a basis for management initiatives with tree species adapted for areas of Africa with low rainfall. It is, however, questionable for any of the tree species which are characteristic of semi-arid Africa, so far studied, whether there has been organized and specialized use sustained for as long as in the case of the desert date, Balanites aegyptiaca (Linnaeus) Del. - Sprague referred in 19 i 3 to the significance of this species in burial rituals in Egypt over 4000 years ago. Balanites aegyptiaca subsequently was one of the first woody species of the Middle East and northeast Africa to come to the attention of western science. This was through a description provided in 1592 by Prosper Alpinus, who is identified (Sands, 1990) as the collector of the type specimen subsequently named Ximenia aegyptiaca by Linnaeus ( 1753 ). By the end of the nineteenth century, even views on the ecology of the species had been expressed: a woody plant of wadis in the A'ir range, Niger, dependent on subterranean water (Schimper, 1903, p. 617). In the present era, sharply increasing interest in Balanites aegyptiaca has led to many papers in the pharmacological literature (e.g. Hardman and Sofowora, 1970, 1971 a, 1972 ). The growing interest in industrial potential which followed was appraised for Sudan, the country richest in the species, in a report (Abu-AI-Futuh, 1983) to the United Nations Industrial Development Organization (UNIDO). The report received wide publicity in development agency literature (FAO, 1985; Anon., 1987a, 1987b). Informal reports also suggest the tree continues to play a vital nutritional role in the drier parts of its African range. One assertion is that during 1985, in part of the Sudan, together with Boscia senegalensis, Balanites aegyptiaca contributed as much calorific value to local diets as did international food aid sent to ameliorate the impact of the severe drought of that year. Remarkably, while FAO publications were among the first to call attention to the UNIDO report's confirmation of its multipurpose potential, Balanites aegyptiaca was omitted from the Africa forest genetic resources priority listing agreed at the sixth session of the FAO panel of experts on forest gene resources (FAO, 1988). Despite the diverse and sustained interest in Balanites aegyptiaca, information that could guide those seeking to develop it into a formally managed resource remains fragmentary and scattered and lacks the broad context needed for effective use. The provision of a unified account of the distribution and ecology of Balanites aegyptiaca is therefore attempted

ECOLOGY OF BALANITES,,IEG YPTIACA ( BALANITACEAE)

3

here, mainly using information extracted from a comprehensive range of floras and ecological and vegetation accounts. Wherever well-represented, Balanites aegyptiaca (Fig. 1 ) is a highly distinctive component of the vegetation because of its size, tendency to retain foliage throughout the year, massive spines and bifolio!ate leaves. Confusion with other species is possible only where other species of Balanites occur in the same vegetation on the eastern side of Africa - an area for which, fortunately, floras are available indicating authenticated voucher specimens ofBalanites aegyptiaca. A distribution map has been compiled and interpreted through comparison with continental scale maps of environmental factors, primarily climatic, and consideration of comments on the immediate growing conditions provided by authors. Active taxonomic revision at Kew prevented use of herbarium specimens, but MJ.S. Sands (personal communication, 1990) has confirmed that the resulting map is consistent with the distribution pattern indicated by voucher specimens. Account has also been taken of authors" remarks on other aspects ofthe ecology of the species.

Fig. 1. Balanites aegyptiaca: a tree on the Mkata Ranch, Tanzania (photo, E. Shanks, February t989).

4

J.F.. HALL

RANGE

Few African tree species are as widely distributed as Balanites aegyptiaca, which occurs in almost every African country north of the equator and in several countries in the southern hemisphere. Generalized maps of occurrence in West Africa and the countries of the Red Sea seaboard have been published (Aubr6ville, 1950; Qu6zel, 1965, 1978) and there are maps on a more local basis for Chad (Grondard, 1964), Central African Republic (Boulvert, 1980) and Zimbabwe (Palgrave, i 977 ). The latitudinal extremes of range are Zimbabwe ( 1 9 ° S ) and the Middle East ( 3 5 ° 2 5 ' N ) and the longitudinal extremes are Mauritania ( 16 o 30' W ) and Somalia (49 oE ) in Africa and, according to Zohary (1973), further east in Iran, probably at about 53°E. The range (Fig. 2 ) is fairly continuous although occurrence becomes increas2 0 °W

0°

20" E

40" E

20°N

O"

20~S

Fig. 2. Balanites aegyptiaca: distribution (dots) of the species in relation to the main phytochoria of White ( 1983 ): ( 1 ) Guineo-Congolian centre; (2) Zambezian centre; (3) Sudanian centre; (4) Somalia-Masai centre; (5) Cape centre; (6) Karoo-Namib centre; (7) M,~:diterranean centre; (8) Afromontane/Afroalpine centre; (9) Guinea-Congolia/Zambezia transition; (10) Guinea-Congolia/Sudania transition; ( 11 ) Lake Victoria mosaic; ( 12 ) Zanzib~.r-lnhambane mosaic; (13 ) Kalahari-Highveld transition; (14) Tongaland-Pondoland mosaic; (15 ) Sahel transition; (16) Sahara transition; (17) Mediterranean-Sahara transition; (18) West Malagasy centre; (19) East Malagasy centre.

ECOLOGY OF 8ALANITES AEGYPTIACA (BALANITACEAE)

ingly closely linked to coastal and rift valley conditions in the Middle East and the adjacent part of Africa. The most disjunct occurrence appears to be that in Angola where, in the absence of more information on the circumstances, the possibility that the species was introduced cannot be ruled out. Aubr~ville (1950) indicates a satellite occurrence in West Africa (Togo), perhaps of relatively recent origin, but Adjanohoun (1964) interprets an isolated population in Ivory Coast in terms of considerable antiquity. RELATIONS WITH ENVIRONMENTAL FACTORS

Climate Reference to the extensive compilctions of meteorological data recently published (Meteorological Office, 1983; FAO, 1984, 1987) enables interpretation of range-wide distribution in terms of rainfall and temperature.

Rainfall Relationships with rainfall (Fig. 3) are complicated by dispersal by Man and livestock into wetter climates and by numerous occurrences where very 20"W

0"

20°E .-:-.

",

40"E

-

.

"e..

j,

.....

%20°N

•

db

(Y 1000 km

400 "~ fmsl occurs

~

20"S

~i

Fig. 3. Balanites aesTptiaca: distribution (dots) of the species in relation to rainfall (400, 800 and 1439 mm isohyets shown ). Areas at low elevation subject to frost are demarcated by broken lines.

6

J.B. HALL

low rainfall is c o m p e n s a t e d by locally high h u m i d i t y (coastal areas) and groundwater. F r e q u e n t occurrence over extensive areas totally d e p e n d e n t on direct rainfall is principally in the 4 0 0 - 8 0 0 m m y e a r - 1 rainfall belt through sub-Saharan Africa from Senegal to the Sudan, here t e r m e d the ' m a i n belt'. As a thinly scattered rainfed shrub growing to 2 m in height, Balanites aegyptiaca is present north of this belt where m e a n annual rainfall is as low as 250 ram. The n u m e r o u s occurrences in regions of lower m e a n a n n u a l rainfall involve compensatory factors. South of the 4 0 0 - 8 0 0 m m y e a r - ~ rainfall belt there is progressively reduced p r o m i n e n c e as the flora b e c o m e s d o m i n a t e d by m o r e southerly phytogeographical elements as bushland a n d w o o d e d grassland (White, 1983 ) grade into w o o d l a n d formations. Aubr6ville ( 1 9 5 0 ) indicates the 1000 m m y e a r - 1 rainfall isohyet as the natural limit to the range in West Africa. U n d e r conditions of disturbance ( a n d probably by conscious introd u c t i o n ) , Balanites aegyptiaca has spread beyond this limit: isolated occurrences u n d e r m e a n rainfall 1 3 0 0 - 1 4 0 0 m m y e a r - ~ in G u i n e a Bissau and the Central At~ican Republic have resulted. F r o m Ethiopia southwards, m e a n annual rainfall patterns are m o r e intricate t h a n further north a n d there is no well-defined relationship with any closely circumscribed rainfall belt. Nevertheless, an association with m e a n rainfall 4 0 0 - 1000 m m y e a r - ~ r e m a i n s apparent. Balanites aegyptiaca var. quarrei ( D e W i l d . ) G. Gilbert (hereafter "var. quarrel"), localised within the southern part of the species' range, departs from the overall pattern in occurring naturally u n d e r relatively high rainfall ( 10001300 m m y e a r - ~), typically on termitaria, where m o i s t u r e stress creates conditions favouring species which are m o r e xerophytic t h a n typical o f the comm u n i t y generally.

Temperature and frost M e a n a n n u a l t e m p e r a t u r e s exceeding 20 ° C prevail over most of the range. The low elevation of West Africa results in m e a n annual temperatures of m o r e t h a n 24 °C while over the elevated parts of the range in eastern a n d southern Africa, values of 2 0 - 2 5 °C apply. N o r t h w a r d from the m a i n belt, m e a n annual t e m p e r a t u r e s decline to 2 0 - 2 3 ° C u n d e r coastal influences a n d fall to 19 ° C at the n o r t h e r n limit in Morocco, although at the poleward extreme in the Jordan Valley, Israel, m e a n a n n u a l t e m p e r a t u r e is above 23 ° C. Even the S a h a r a n m o u n t a i n localities enjoy m e a n annual temperatures of m o r e than 20 °C. South of the m a i n belt, m e a n a n n u a l t e m p e r a t u r e s associated with Balanites aegyptiaca l~all as low as 19 °C only in the rift t h r o u g h the E t h i o p i a n highlands a n d in southern Z i m b a b w e . Absolute m a x i m u m t e m p e r a t u r e s north of the m a i n belt c o m m o n l y exceed 4 6 ° C a n d reach 5 4 . 5 ° C in T i m b u k t u , Mali. W i t h i n the m a i n belt, absolute

ECOLOGY OF B41.o4NITESAEGYPTI,4C.4 (BALANITACEAE)

7

maxima are in the range 40-46 ° C. South of Ethiopia, absolute maxima rarely exceed 40°C (parts of Zambia and Zimbabwe), where the species has been reported, and are typically 35-40°C. Almost all the range is frost free (Fig. 3 ) and occasional frost occurrence approximates the northern and southern boundaries of the distribution. There are exceptions in the cases of the Saharan mountains (notably the Hoggar and Tibesti ranges), where regular frost is likely at the altitudes from which Balanites aegyptiaca has been repo,'ted, and also where var. quarrei occurs. Populations on Jebel Marra (Sudan) in southern Zambia and in Zimbabwe may experience occasional frosts.

Substrate Balanites aegyptiaca grows in soils derived from diverse rock types: volcanic materials (Glover et al., 1962 ), old crystalline rocks (Fairbairn, 1943; Kassas, 1956), sandstones (Ramsay, 1964), quaternary deposits of sand (Ramsay, 1958 ) and black clays (Bogdan, 1958 ). This ability to become established on varied soils has diverted attention from clear localised contrasts in the representation on different soil types. Although soils supporting the species are repeatedly described as sands, sandy loams or clays, it is apparent that in the 450-900 mm mean annual rainfall belt of the Sudan the species is better represented on sandy loams and on sandy clay loams than on lighter soils (Ramsay, 1958 ). Under relatively high mean annual rainfall (900 mm) in northern Nigeria, multivariate analysis of data from a large number of samples of savanna also showed that the soil associated with the highest stocking of the species was heavy (Ramsay and De Leeuw, 1965). However, for the drier parts of the Sahel zone (250-450 mm mean annual rainfall), Balanites aegyptiaca has been described as one of the chief woody species (White, 1983 ), a view supported by Le Hou~rou's (1980) assertion that it is the most typical tree or shrub of the Sahel. White makes clear that in this case occurrence is on the deep sands which cover large areas immediately south of the Sahara desert. Groups of major soils The FAO-UNESCO (1977) Soil Map of the World reveals many links between groups of major soils and climatic factors. It follows that a tree species whose distribution relates closely to climate is likely to occur where certain groups of major soils are mapped. In the climatic conditions which appear most favourable for Balanites aegyptiaca, the soils concerned are acrisols, arenosols, luvisols and a "vertisolic' complex (fluvisols, gleysols, planosols and vertisols - in the strictest sense).

8

J.B. HALL

The large number of localities where the species has been reported and vertisolic soils mapped is noteworthy, underlining the tendency to occupy valley, flood plain and watercourse sites. Mostly these soils occur as linear units, many too restricted in extent to be represented at the resolution of the F A O U N E S C O map ( 1 : 5 000 000). Arenosols are widespread in the West African part of the range, but of much more restricted extent in other parts. There is close association of vertisolic soils and arenosols, and some instances where the latter are mapped (mainly in Nigel a~d Senegal), may reflect only the impracticability of showing separately very small areas of vertisolic soils. Acrisols and luvisols are mapped in few areas where the mean annual rainfall is most typical for the species and are more characteristic of wetter climates. There are few occurrences ofBalanites aegyptiaca on either of these soils and in the case of acrisols (represented mainly in the East African part of the range), the inherently low fertility is probably a contributory factor. Despite occasional references in the literature (e.g. Glover et al., 1962) to occurrence on soils derived from volcanic ash, the major areas of andosols mapped ( F A O UNESCO, 1977) appear to lack the species: the high porosity of andosols is an unfavourable quality although the nutrient status is high. North of the main belt, absence from the extensive areas of regosols, xerosols and yermosols - all typical desert/arid land soils - is evident and known localities are mainly in the areas of extreme soil heterogeneity that characterise the mountain massifs. Reports referable to drainage lines and basins remote from the mountains are generally on pockets of specialised soils too restricted in extent for mapping. There is little doubt, however, from collectors' notes and published comments, that the soils of most such places have much in common with vertisolic soils that support the tree elsewhere. U n d e r mean annual rainfall heavier than that of the main belt, scattered occurrences are referable to additional major groups of soils and it is noteworthy that these are mostly the relatively fertile cambisols and luvisols. Occurrences associated with ferralsols, notorious for low cation exchange capacities and paucity of weatherable minerals, occur only in transition zones to the more fertile luvisols except where var. quarrei is concerned. Ferralsols are mapped over the part of the range where plants are referable to var. quarrei but the tendency for this variety to grow on termitaria ensures rooting in a substrate of considerably higher nutrient status than the surrounding earth (Mal:~isse, 1978 ).

Site Catenary/toposequence relationships underline the importance of smallscale environmental variation in the occurrence of Balanites aegyptiaca. Such

ECOLOGY OF BALA N I T E S AEG YPTIA CA (BALAN ITACEAE )

9

relationships are apparent in catenary diagrams (Fig. 4 ) for several localities subject to mean annual rainfall of 400-1000 m m (Morison et al., 1948; Germain, 1952; Radwanski and Wickens, 1967; Wickens, 1976 ). In most of these catenas, clayey or loamy soils typify the sections where Balanites aegyptiaca is concentrated. Where there are concentrations on sands, these are in lowlying positions and of alluvial origin. The absence of an association with the steeper slopes and coarser soil phases of the catenas implies that very freely drained sites or sites with little ability to retain moisture are unfavourable for the species, except in the case of var. quarrel Published comments on the growth of Balanites aegyptiaca also indicate association with deep soils low in the landscape and with little gradient - valley floors, river banks and at the

400~mm I

"

I

"

i

¢'=

I

~

I

650 mrn r-

^

I

R

I

5d

700 mm

900 mm

1000 mm

o A

.

_

W

-

-

Sd

I

Fig. 4. Balanites aegyptiaca in a catenary context under mean annual rainfall 400-1000 mm (values upper left of each diagram ): 400 ram, Sudan, basaltic rocks (Wickens, 1976); 650 ram, Sudan, basement complex rocks (Wickens, 1976); 700 mm, Sudan, basement complex rocks (Radwanski and Wickens, 1967 ); 900 mm, Zaire, basement complex rocks and quaternary deposits (Germain, 1952 ); 1000 mm, Sudan, quaternary deposits (Morison et al., 1948 ). Catenary zones: A, alluvial; C, hill creep; R, relict soils of earlier weathering cycle; Sd, sedentary; Sk, skeletal; W, hill wash. Soil textures: 1, clayey; 2, loamy; 3, sandy; 4, gravelly; 5, stony/rocky. Positions of Balanites aegyptiaca within each catena, as indicated by the original author, are shown as stylised trees.

10

J.B. HALL

foot of rocky slopes ( G o o r and Barney, 1976; Szolnoki, 1985). Suliman and Jackson ( 1959 ) noted poorer growth on sandy soils than on heavier soils in the vicinity. The ability of Balanites aegyptiaca (although not var. quarrei, which is a smaller tree - Gilbert, 1958 ) to attain large sizes on heavy vertisolic clay soils subject to prolonged flooding is noteworthy. Maximal develo p m e n t as an individual is on low-lying, level alluvial sites with deep sandy loam soil and uninterrupted access to a source of water. Establishment along water courses and on flood plains is thought to ensure periodic nutrient replenishment where soils would not otherwise sustain vigorous growth (Irvine, 1961; Burkhill, 1985). Tolerance of salinity is only modest: a tolerance value of 6 0 0 0 / t m h o s given by Firmin (1971 ) suggests the species cannot be described as salt-tolerant, while Zohary ( 1 9 4 4 ) and White (1983), respectively, report oasis occurrence only where water is fresh or groundwater is of low so,It c o n t e n t

BALANITES AEGYPTIACAAS A CONSTITUENT OF THE VEGETATION Chorology Balanites aegyptiaca is a pluri-regional linking species in the sense applied by Hopkins and White (1984), typifying the Sudanian and Zambezian centres of endemism and transitional phytochoria adjacent to these (White, 1965, I983). Vegetation formations Balanites aegyptiaca, with the exception of var. quarrei, typically grows widely spaced and with full crown exposure being particularly characteristic of bushland and thicket, shrubland and wooded grassland formalions, as described by White (1983). Other occurrences are in an invasive context in woodland with an open canopy, and as isolated or open stands of individuals where accessible groundwater compensates for low rainfall within desert regions. Reports of presence in riparian forest, as far as can be checked, refer to vegetation with an incomplete canopy from woody species or linear riparian formations where crowns are well-exposed to lateral light. Var. quarrei occurs in woodland under a canopy cover as extensive as 70% developed by much taller ( 15-20 m ) species (Freson, 1973; Malaisse, 1973). Associated species The wide range, paucity of quantitative surveys and diverse methodologies and objectives applicable to field activities where Balanites aegyptiaca has been encountered, limit scope for examining associated species. Neverthe-

ECOLOGY OF BALANITESAEGYPTIACA ( BALANITACEAE)

11

TABLE 1 Woody species which are noteworthy associates ofBalanites aeg),ptiaca in different parts o f t h e range Species

North ................................................................. South A BC

Moringa peregrina (Forsskal) Fiori

D E F G H I J

K L M N O Pt

A

( Mori ngaceae )

Ziziphus spina-christi (Linnaeus) Desf. (Rhamaaceae) Acacia nubica Benth. ( M i m o s a c e a e ) Caiotropis procera (Aiton) Aiton f. (Asclepiadaceae) Hyphaene thebaica (Linnaeus) Marlius (Palmae) Maerua oblongifolia ( Forsskal ) A. Rich. (Capparaceae) Salvadora persica Linnaeus (Salvadoraceae) Leptadenia pyrotechnica ( Forsskal ) Decne. (Periplocaceae) Capparis decidua (Forsskal) Edgew. (Capparaceae) Maerua crasMfolia Forsskal ( C a p p a r a c e a e ) Acacia iaeta R. Br. ex Benth. ( M i m o s a c e a e ) Acacia ehrenbergiana H a y n e ( M i m o s a c e a e ) Faidherbia albida ( Del. ) A. Chev. ( Mimosaceae ) Acacia nilotica (Linnaeus) Willd. ex Del. (Mimosaceae) Commiphora africana (A. Rich. ) Engl. (Burseraceae) Boscia senegalensis (Pers.) Lam. ex Poiret (Capparaceae) Mitragyna inermis (Willd. ) Kuntze (Rubiaceae) Acacia tortilis ( F o r s s k a l ) H a y n e (Mimosaceae) Bauhinia rufescens Lain. (Caesalpiniaceae) Acacia polyacantha Willd. ( M i m o s a c e a e ) Anogeissus leiocarpus (DC.) Guill. and Perr. (Combretaceae) Combretum aculeatum Vent. (Combretaceae) AcaciaseyalDel. ( M i m o s a c e a e ) Dichrostachy~ cinerea (Linnaeus) Wight and Am. ( M i m o s a c e a e ) Acacia hockii De Wild. ( M i m o s a c e a e ) Bombax costatum Pellegrin and Vuillet (Bombacaceae) Combretum collin,tm Fresen. (Combretaceae)

A B B A

C B B B C D

A

C

F

A

C D D D D

F

E D

F

D

F E

A B C D

F

I J

F E G E F G F B C

E F E

H I H

K

12

J,B. HALL

TABLE I (continued) Species

North ................................................................. South A B C D E FG

Combretum glutinosum DC~ (Combretacv~.e) Entada africana Guill. and Perr. ( Mimosaceae ) Guiera senegalensis J. Gmelin (Combretaceae) Lannea schimperi (Hochst. ex A. Rich. ) Engl. (Anacardiaceae) Strychnos spinosa 1,am. (Loganiaceae) Acacia senegal (Linnaeus) Willd. (Mimosaeeae) Heeria reticulata (Baker f. ) Engl. (Anaeardiaceae) Maerua angolensis DC. (Capparaceae) Opilia ceitidifolia (Guill, and Perr,) Endl. ex Walp. (Opiliaeeae) Ziziphus mauritiana Lain. (Rhamnaceae) Acacia etbaica Schweinf. (Mimosaceae) Acacia persicifolia Pax ( Mimosaeeae ) Diospyros mespiliformis Hoehst, ex A, DC. (Ebenaceae) Capparis tomentosa Lam. (Capparaceae) Erythrina abyssinica DC. (Papilionaceae) Gardenia ternifolia Schum. and Thonn. ( Rubiaceae ) Delonix elata (Linnaeus) Gamble ( Caesalpiniaceae ) Acacia sieberana DC. (Mimosaeeae) Lannea humilis (Oliver) Engl. (Anacardiaceae) Ziziphus mucronata Willd. (Rhamnaceae) Sclerocarya birrea (A. Rich. ) Hochst. (Anacardiaceae) Tamarindus indica Linnaeus (Caesalpiniaceae) Zanthoxylum chalybeum Engl. (Rutaceae) Acacia mellifera (Vahl ) Benth. (Mimosaceae) Sterculia setigera Del. (Sterculiaceae) Strychnospotatorum Linnaeus f, ( Loganiaceae ) Acacia drepanolobium Harms ex Sjostedt (Mimosaeeae) Albizia anthelminthica Brongn. ( Mimosaceae ) Acacia robusta Burchell (Mimosaceae) Albizia harveyi Fourn, (Mimosaceae) Albizia petersiana ( Bolle ) Oliver (Mimosaceae)

H IJ

KLMN

O P~

G G G G G D

F

I J

L

H H H C

F G

I K

M N

I I

E

M

H

K L J

H

K L

N M N O P

K

M

H

P N

J

L P 0

M M M

ECOLOGY OF BALANITESAEGYPTIACA (BALANITACEAE)

1:3

TABLE I (continued) Species

North .................................................................South A BC

Euphorbia bilocularis N.E.Br. ( Euphorbi~ceae ) Manilkara mochisia (Baker) Dubard (Sapotaceae) Kigelia africana (I.am.) Benth. ( Bignoniaceae ) Allophylus africanus Pal. Beauv. (Sapindaceae) Combretum more R.Br. ex G. Don (Combretaceae) Aibizia amara (Roxb.) Boivin ( M imosaceae ) Haplocoelumfoiiolosum (Hiern) Bullock ( Sapindaceae ) Vitexfischeri Giirke (Verbenaceae) Berchemia discolor ( Klotzsch ) Hernsley (Rhamnaceae) Acacia nigrescens Oliver (Mimosaceae) Boscia mossambicensis Klotzsch (Capparaceae) Colophospermum mopane (Kirk ex Benth. ) L~onard (Caesalpiniaceae) Combretum eleagnoides Klotzsch (Combretaceae) Dalbergia melanoxylon Guill. and Pert. ( Papilionaceae ) Euphorbia candelabrum Tremaux ex Kotschy ( Euphorbiaceae ) Terminalia sericea Burchell ex DC. (Cornbretaceae) Xirner~ia americana Linnaeus (Olacaceae). Adansonia digitata Linnaeus (Bombacaceae) Afzelia quanzensis Welw. (Caesalpiniaceae) Combretum imberbe Wawra (Combretaceae) Xeroderrisstuhlmannii (Taubert) Mendon~a and E.P. Sousa (Papilionaceae)

D E FG

H ! J KL

M N O P~ M M

K

P N N O

M

N N

'A, oases in Israel; B, west coast of the Arabian peninsula; C, drainage lines in the Saharo-Saheliar, ecotone; D, rain-fed sites in the Sahara-main belt transition; E, sites with impeded drainage in the main belt; F, rain-fed sites in the main belt (area subject to 400-800 mm mean annual rainfall north of the equator at the southern margin of the Sahara 1; G, main belt-sub-humid area transition; H, Zaire/Nile watershed (Burnndi, Rwanda, Zaire); I, Ethiopian rift valley; J, East African rain-fed sites under mean annual rainfall comparable with that in the main belt; K, East African sites with good drainage bu: access to supplementary moisture; L, East African sites prone to inundation; M, termitaria in Tanzanian woodland; N, termitaria in Shaba, Zaire (var. quarrel); O, Zambezian mopane woodland in Zambia and Zimbabwe; P, seral vegetation in Zambian river valleys.

!4

J.B. HALL

20 ° W

0°

20 ° E

40 ° E

°

jl2o.N 0

1000

Rm

,20°S

I i

i

i

,

i

Fig. 5. Balanites aegyptiaca: geographical distribution of units within range for which noteworthy associates are listed in Table 1. A, oases in Israel: B, west coast of the Arabian peninsula: C, drainage lines in the Saharo-Sahelian ecotone; D, rain-fed sites in the Sahara - main belt transition; E, sites with impeded drainage in the main belt; F, rain-fed sites in the main belt (hatched); G, transition between main belt and sub-humid part of range; H, Zaire-Nile watershed; l, Ethiopian rift valley; J, East African rain-fed sites under mean annual rainfall as in main belt; K, East African sites with good drainage but access to supplementary moisture; L, East African sites prone to inundation; M, termitaria in Tanzanian woodland; N, termflaria in Shaba, Zaire (var. quarrei); O, Zambezian mopane woodland in Zambia and Zimbabwe; P, seral vegetation in Zambian river valleys. less, a p r e l i m i n a r y p i c t u r e h a s b e e n c o n s t r u c t e d . T h e r a n g e h a s b e e n s u b d i v i d e d into 16 g e o g r a p h i c a l u n i t s ( T a b l e l , Fig.5 ). W i t h i n e a c h o f these, w o o d y s p e c i e s are l i s t e d as a s s o c i a t e s o n the b a s i s o f r e p o r t s o f p r o m i n e n c e or, in better-studied instances, of consistent reports of association. S p e c i e s a d a p t e d , o f t e n t h r o u g h d i s t i n c t i v e x e r o m o r p h i c features, for cond i t i o n s o f low m e a n a n n u a l r a i n f a l l , a n d e x t e n d e d r a i n - f r e e p e r i o d s a n n u a l l y , d o m i n a t e t h e list o f a s s o c i a t e d species. A l t h o u g h a few species are a s s o c i a t e d w i t h Balanites aegyptiaca o v e r m u c h o f t h e range, m o s t are r e s t r i c t e d to sect i o n s o f it. O n l y o n e s p e c i e s ( Z i z i p h u s mauritiana, R h a m n a c e a e ) is l i s t e d for b o t h t h e m o s t n o r t h e r l y a n d m o s t s o u t h e r l y parts. A s s o c i a t e s w i t h i n the m a i n belt r e l a t e closely to t h o s e i d e n t i f i e d f u r t h e r n o r t h a n d a s e p a r a t e g r o u p typifies t h e e a s t e r n a n d s o u t h e r n parts. T h e C a e s a l p i n i a c e a e ( f i v e spp. ), C a p p a r -

ECOLOGY OF BALANITES AEG YPTIAC4 (BALANITACEAE)

I

aceae (seven spp.), Combretaceae (nine spp.) and Mimosaceae (22 spp.) are the most prominent families in the list: all are represented from the southernmost part to the main range and the Capparaceae and Mimosaceae extend further north. Two (Acacia, 16 species; Combretum, six species) of the three best-represented genera conlorm to the family ranges: the third (Albizia, four species) is reported as an associate only from eastern and southern Africa.

Population levels Reports of stocking of Balanites aegyptiaca are relatively few and there is no consistency in the size categories distinguished or in the areas assessed. Observations are generally confined to few individuals because plot sizes are small and estimates are not statistically robust. Nevertheless, regardless of the sizes given attention, expressed on a per-hectare basis, stockings in excess of 25 are apparently unusual although several have been reported (Table 2), all, with one exception (Elayu, Somalia), from the main belt. Because of the tendency to grow as dispersed individuals in stands oftrees with a discontinuous canopy, it is unusual for Balanites aegyptiaca to account for a high proportion of the trees constituting a community. However, the capacity to grow to larger sizes than most species which grow with it leads to increased contributions when attention is confined to larger diameters. TABLE 2 Balanites

aegyptiaca: concentrations o f more than 2 5 individuals ha-

Locality, sampled area (where stated) and reference

Individuals ( h a - ~) and size (where stated)

Somalia, Elayu ( I ! ° 30' N, 48 ° 30' E, 0.1 ha; Gilliland, 1952 )

807 251 72 240 190 > 200

Burkina Faso, Dori ( 14°05'N, 0°05'E, 0.1 ha; Menaut, 1983 ) Tchad, Wadi Rim6 ( 13 ° 30' N, ! 9 ° 00 °E; Grondard, 1964) Senegal, Ferlo ( 14°30'N, 14°O0'W; Bille, 1978) Sudan, Darfur ( 1 3 ° 0 0 ' N , 25°30'E, 6.1 ha; Ramsay, 1958) Sudan, Daffur ( 13°00'N, 25° 30°E, 6.9 ha; Ramsay, ! 958) Nigeria, Zamfara ( 12° 50'N, 6°55'E, 0.8 ha; Keay, 1949 ) Sudan, Dinder ( 1 2 ° 3 0 ' N , 34°00'E; Adams, 1967) ~dbh, diameter at breast height.

< 2.5 cm dbh I 2.5-7.5 cm dbh 7.5-12.5 cm dbh < 3.2 cm dbh >/3.2 cm dbh

79 3!

>/3.5 cm dbh "woody'

28

"woody"

25

>~1.6 cm dbh

25

40 cm mean dbh

16

J.B. HALL

Prominence Low per-hectare stocking is widely characteristic of the woody species that thrive under conditions favourable to Balanites aegyptiaca. Although widely dispersed in the vegetation, Balanites aegyptiaca is distinctive among these in being unusually prominent where it occurs because of its dense crown, evergreen or only briefly deciduous phenology and height. In Darfur, Sudan, under mean annual rainfall of 400-500 mm on coarse-textured soil, Balanites TABLE 3 Reported contributions (%) to stands o f vegetation o f Balanites aegyptiaca individuals. Criteria for inclusion: more than four individuals observed; contribution more than 9% Locality, vegetation and reference

Size (cm dbh ~)

Percentage o f individuals

Burkina Faso, Dori ( 14°05'N, 0°05'E, savanna; Menaut, 1983 )

all woody >~3.2 >~6.4 >~9.5 >'12.7 >" 15.9 <2.5 ~>2.5 >'7.5 >~1.6 ~4.8 >19.5 >" 19.1 >i 23.9 >'3.2 >-4.8 >~6.4 8.0 ~9.5 >~ 12.7 >i 15.9 40 ( mean )

67 90 89 93 93 100 69 25 11 10 16 22 35 50 17 26 32 42 44 41 40 22

Somalia, Elayu ( 11 ° 30'N, 48 ° 30' E, thicket; Gilliland, ! 952 ) Nigeria. Zamfara ( 12°50'N, 6°55'E, open savanna woodland on ill-drained soil; Keay, ! 949 )

Senegal, Ferlo ( 14°30'N, 16°00'W, savanna; Bill~, i 978)

Sudan, Dinder ( 1 2 ° 3 0 ' N , 34°00'E, woodland; Adams, ! 967 ) Nigeria, Zamfara ( 12°50'N, 6° 55'E, open savanna on deep sand; Keay, 1949) Sudan, Darfur ( 13 ° 00' N, 25 ° 30' E, thorn savanna on sand; Ramsay, 1958) Sudan, Darfur ( 13 ° 00' N, 25 ° 30' E, riparian thorn forest; Ramsay, 1958) Sudan, Daffur ( ! 3 °O0'N, 25 ° 30' E, riparian thorn savanna; Ramsay, 1958 ) 'dbh, diameter at breast height.

1.6 )9.5 ~23.9 "woody'

9 19 21 16

"woody"

12

'woody"

I0

ECOLOGYOF B,ILANITES AEGYPTIACA (BALANITACEAE)

17'

aegyptiaca reaches heights of 5-6 m, while other woody species are no taller than 4 m: on more productive soils, Balanites aegyptiaca emerges as individuals 8-10 m high above a general woody canopy at 4-5 m (Ramsay, 1958). In rain..fed sites under lower mean annual rainfall (250-400 ram), Balanites aegyptiaca accounts for a high proportion of the widely scattered woody plants able to reach 6 m in height, when woody plants are generally less than 2.5 m tall (White, 1983). Where well-represented, Balanites aegyptiaca accounts for upwards of 20% of individuals exceeding about 10 cm diameter at breast height (dbh) (Table 3 ). With much smaller plants also taken into account, however, Balanites aegyptiaca usually constitutes a smaller proportion of the stand - often less than 10% of the individuals present, even where it is abundant. B A L A N I T E S A E G Y P T I A C A IN ECOSYSTEM I N T E R A C T I O N S

Effects on soil Effects on soil moisture and nitrogen status have been reported. The phenology, size, form and rooting characteristics of the species account for effects expressed more strongly in association with Balanites aegyptiaca than with other woody plants in the same community. Thus, Glover et al. (1962) noted that at Aitong, Kenya (mean annual rainfall approximately 890 mm, loamy soil derived from volcanic ash), concentration of intercepted rainfall into stem flow and drip from the crown periphery were reflected in zones of damp soil confined to the vicinity of individual trees. Shade from the crown slows subsequent water loss by evaporation, while the strong positive geotropism of the root system enables moisture percolation to depth and is considered an explanation for a flourishing field layer everywhere under the crown. Enhanced moisture status in the rhizosphere is the probable explanation of the increased nitrogen mineralisation detected (Bourli~re and Hadley, 1983) under 8alanites aegyptiaca trees at F~t~ Ol~, Senegal (mean annual rainfall 340 m m ) . There are no reports of bacterial root nodules in 8alanites aegyptiaca, although these occur (Sabet, 1946) in several genera of the Zygophyllaceae, with which the Balanitaceae is often thought to be closely allied.

Relationships with animals The reported relationships ofBalanites aegyptiaca with animals vary widely, from dependent roles, through passive roles to roles as an exploited resource. Most relationships are opportunistic. A role as a host for apparently monophagous invertebrates (e.g. phytophagous mites, thrips and white flies) may reflect only poor knowledge of the groups concerned and, among the species listed in Table 4, Anacridium melanorhodon (Johnston, 1932; Poppy, 1959)

18

J.B. HALL

TABLE 4

Balanites aegyptiaca: relationships with invertebrate species Species

Relationship

Arachnida Acari, Eriophyidae

Phyllocoptes balanites Soliman and Abou-Awad

lnsecta Diptera, M~scidae Glossina longipennis Corri G. palpalis fuscipes Newstead G. swynnertoni Austen Hemiptera (Homoptera), Aleurodidae Aieurolobus nilotica Priesner and Hosny Lepidoptera, Gelechiidae Ephysteris turgida Janse Lepidoptera, Saturniidae Bunea alcinoe Cram

Orthoptera, Acrididae A nacridium melanorhodon (Walker) Thysanoptera, Hoplothripinae Megeugynothrips efflatouni Priesner

Causing leaf curl (Soliman and AbouAwad, 1979 )

Habitat linkages (Swynnerton, 1936; Buxton, 1955 ) Phytophagous (Priesner and Hosny, 1934 ) Phytophagous (Vari, 1963) Phytophagous (Akanbi, 1973; Momoh and Akanbi, 1977; Malaisse, 1978) Phytophagous (Johnston, 1932; Popov, 1959 ) In leaf galls (Priesne~', 1929)

and Bunea alcinoe (Akanbi, 1973; Momoh and Akanbi, 1977; Malaisse, 1978) are known to be polyphagous. Whether pollinators are specific is unclear; floral character conforms with the entomophilous syndrome (Faegri and Van der Pijl, 1979), but no reports on the pollination process have been seen. Of those that have received attention, the most important role of animals is in dispersal. Ingestion by birds or mammals is not required for seed germination, but it does facilitate transportation away from the parent. Wild vertebrate dispersal agents are reported by Burtt ( 1929 ), Phillips (1930), Baumer (1983), Boudi~re (1983) and Von MaydeU (1986): among the mammals, primates (baboons, galagos), large herbivores (elephants, Thompson's gazelles) and carnivores (black-backed jackals, civet cats), all feed readily on the fruits. Today, however, the greatest contribution to dispersal can be attributed to free-ranging domestic stock, as cattle, goats, sheep and camels all consume fruits when available. Attention has been drawn to

ECOLOGY OF R4LANITES AEG YPTIACA (BALANITACEAE)

19

the incidental introduction of Balanites aegyptiaca along cattle trails (Grondard, 1964) and to the vicinity of water sources (Aubr6ville, 1950). There are likely to be differences in the effectiveness of differer.*~ mammals as dispersal agents. Cattle and elephants, which ingest seed, may evacuate these at some considerable distance. More local transportation may be due to Thompson's gazelle, which regurgitates seed from the stomach (Bunt, 1929). Sheep and goats may be even less effective as they spit seeds out after stripping off the pulp (Booth and Wickens, 1988 ), although Von Maydell (1986) implies, in contrast, that goats evacuate them. Whether many viable seeds pass through camels is open to question: Booth and Wickens (1988) report that this animal crushes the kernel in the course of feeding. An alternative form of dependence concerns the ro'.,: of animals in creating the conditions conducive to establishment and the successful subsequent growth reflected in the widely reported characteristic association with termitaria ('ant hills' of various authors ) in Ghana (Taylor, 1960), Nigeria (Ramsay and De Leeuw, 1964), Tanzania (Bunt, 1942; Bjornstadt, 1976). Zaire (Malaisse, 1978) and Zambia (Fanshawe, 1969). From other information available, it is clear that the termitaria, while often colonized, are not essential for the establishment of Balanites aegyptiaca populations, except in the case of the Shaba (Zaire) miombo (Malaisse, 1978 ) where var. quarrei apparently is confined to mounds raised by Macrotermesfalciger (Gerst~icker). Passive roles arise from concealment, protective shelter or microclimate generated by the dense, thorny crowns of the trees, individually or collectively within local concentrations of trees and shrubs. Morel and Morel (1976) found that Balanites aegyptiaca in Mauritania and Senegal was favoured as a nesting tree by the golden sparrow, Passer luteus (Lichteastein), while Johnston (1932) and Popov (1959) noted that frequently it harboured roosting locusts (Anacridium melanorhodon). Reservoirs of several species of tsetse fly have been identified in plant communities where Balanites aegyptiaca is prominent. The nutritional role of Balanites aegyptiaca with respect to wild and domestic large mammals is well known (Le Hou6rou, 1980). Although fruits are relished by numerous large mammals, Balanites aegyptiaca foliage is selected infrequently by cattle when alternative species are in abundance, but is subject to intensive browsing pressure at the drier extremes of its range or when range-carrying capacity is exceeded. The extent to which game species opt for Balanites aegyptiaca foliage as browse is unclear, but the leaves are eaten by Duma gazelle (Grettenberger and Newby, 1986 ) and giraffe (Lamprey et al., 1980; Ngog Nje, 1984).

Casual relations with Man In addition to Man's relationships with the species through pastoral practices and his livestock, increasing effects on Balanites aegyptiaca populations

20

J.a. HALL

are arising from conscious retention of trees on farmed land remote from the remnants of the original rangeland population, and from the use and control of fire. Retention of individuals of Balanites aegyptiaca in Nigeria (Keay, 1959 ) and Senegal (Giffard, 1974) is attributed to their economic value, principally as a fruit source. However, the logistical problems of removing this hardwooded and firmly rooted species with small hand tools probably contribute to its continuing existence, and have been identified by Adams (1967) as being equally important reasons for its persistence in places in the Sudan from which other species ~.~avedisappeared. Away from the regularly cultivated land surrounding large settlements, the frequency and intensity of fires often determine vegetation character and fire is the main means of effecting land clearance for cultivation. Established Balanites aegyptiaca individuals often survive fires because the fluted bole ensures that bark damage is not total. In addition, under mean annual rainfall of less than 1000 ram, the low amount of combustible material in the open communities which Balanites aegyptiaca typifies reduces fire intensity, while in many areas of mean annual rainfall less than 600 mm there is insufficient fuel for fires to be either intense or extensive. This explains Onochie's (1964) findings that at Bimassa, Nigeria (mean annual rainfall approximately I000 mm), Balanites aegyptiaca flourished under a range c,f burning regimes from different annual burning options to complete protection. Tchi6 ( 1989 ) at Kiboko, Kenya (mean annual railifa!l 600 ram), however, imposed burning regimes which suppressed Balanites aegTptiaca (and all other woody species). Where fire is possible, however, regeneration may be suppressed: Von Maydell (1986) suggests the ability to withstand fire takes at least 3 years to develop. Land rehabilitation involving exclusion of fire underlines this, with Balanites aegyptiaca becoming a very prominent woody component of the protected stand (Cameroun - Letouzey, 1968; Mauritania- Adam, 1968; Sudan - Obeid and Seif El Din, 1q71 ). More locally within the range, in the oasis areas of Israel, extension of irrigated land has created areas which Balanites aegyptiaca has quickly colonised, showing further evidence of efficient seed dispersal mechanisms, in this instance reinforced by prolific generation of root suckers. Balanites aegyptiaca and vegetation succession Dispersal, resilience and ecological amplitude enable Balanites aegyptiaca populations to expand into adjacent areas when grazing pressures intensify or the existing woody cover is cleared for farming, but the land is abandoned after a period of cropping. Whether such events represent initiation of conventional vegetation succession is unclear. Balanites aegyptiaca enjoys a reputation both as a weed quick to invade cleared ground (Miehe, 1986, Jebel

ECOLOGY OF BALANITES AEG YPTIACA (BALANITACEAE)

21

Marra, Sudan) and as one of the longest-lived savanna trees (more than 100 years, Bourli6re and Hadley, 1983). No serious problems with regeneration have been noted in the literature and under typical current burning regimes and levels of grazing pressure it is probable that populations of Balanites aegyptiaca would persist longer than those of most associated woody species. Free-ranging goats have failed to suppress Balanites aegyptiaca where it has been introduced in Curacao (National Research Council, 1983) and cattle in Senegal were reported (Giffard, 1974) to be eliminating the preferred Acacia spp., progressively improving the opportunity for the population of Balanites aegyptiaca to increase. Early suggestions that succession would eventually result in 'Balanites and other spp. communities' being displaced by "Combreturn and other spp. open woodland' in central Tanzania (Phillips, 1930) and of'Thorn savanna" and 'Thorn scrub" vegetation containing Balanites aegyp tiaca being replaced by 'Thorn forest" in Niger (Fairbairn, 1939), were probably inaccurate and are even less likely to have relevance given today's landuse pressures. DISCUSSION - A CONSERVATION PERSPECTIVE

The economic importance of Balanites aegyptiaca arises from its role as a source of fruit and foliage at the height of the dry season. The fruits have particular value, especially in years of reduced rainfall when available grass may be exhausted relatively early. For the Sudan, in large areas in which Balanites aegyptiaca is numerous, a strong case has been made for establishing agro..industries based on this tree (Abu-AI-Futuh, 1983). Despite the dramatic changes in African vegetation linked with increasing human and animal populations, and exacerbated by climatic events, only one report (Viswanatahan, 1986) expresses concern over Balanites aegyptiaca population status (in northern Ethiopia). The aggressive colonising behaviour and dispersal along cattle routes are, however, frequently mentioned and elimination from any area is indicated only by Adams (1967) who was specifically attempting this. Indeed, Balanites aegyptiaca is well-adapted to withstand environmental changes and pressures currently affecting most of the range. Balanites aegyptiaca is resilient to drought and browsing (to the latter particularly, as long as preferred alternatives such as Acacia spp. remain available), and withstands both the increased exposure as woodlands are transformed into tree savanna, and the typical burning regimes. As urban fuelwood demand brings indiscriminate tree cutting, publicity to ensure that Balanites aegyptiaca is spared will become increasingly important. Already, problems are developing near large towns where mature trees have been retained, but regeneration is suppressed in cropping activity, and where there is uncontrolled cutting of the trees for constructing 'zeribas" (livestock enclosures). As, however, conservation embraces steps to improve as well as to

22

J.B. H A L L

safeguard resources, Balanites aegyptiaca m e r i t s inclusion in resource conserv a t i o n measures. W i d e genetic diversity is likely to reflect the extensive range a n d ecological a m p l i t u d e , a n d the e l u c i d a t i o n o f this v a r i a t i o n to allow identification o f p r o v e n a n c e s suited to different c i r c u m s t a n c e s will be an integral part o f c o n s e r v a t i o n action. T h e wide d i s t r i b u t i o n , n u m e r o u s s y n o n y m s a n d inconsistencies a m o n g descriptions based on m a t e r i a l f r o m different parts o f the range, p o i n t to considerable intra-specific variation. T h i s is a l r e a d y recognized in the differentiation o f vat. quarrel a n d var. pallida Sands (Sands, 1990), a n d it is p r o b a b l e that research c u r r e n t l y in progress at the R o y a l B o t a n i c G a r d e n s , Kew, will result in formal status for a d d i t i o n a l intra-specific t a x a (M.J.S. Sands, personal c o m m u n i c a t i o n , 1990). Relating intra-specific v a r i a t i o n to provenances w o u l d p r o d u c e a f r a m e w o r k for m o r e effective use ofBalanites aegyptiaca as a resource (Fig. 6 ) . T h e r e is need to establish the extent o f v a r i a t i o n a t t r i b u t a b l e to w e s t - e a s t clinal effects t h r o u g h the m a i n belt a n d to extend a t t e n t i o n to the a p p a r e n t l y disjunct p o p u l a t i o n s in Angola a n d Iran. T h e ex20°W

0°

20°E

40°E

f

20°N

,

20"S

Fig. 6. Balanites aegyptiaca: suggested source areas of material for provenance studies. C, westeast series of sources (all with populations on vertisolic soils ) for assessment ofclinal patterns; D, widely disjunct populations; F, localities subject to frost; I, recognized imra-specific taxa; R, high rainfall extremes; r, low rainfall extremes; S, range overlaps with other species of Balanites; T, high mean annual temperatures; t, lower mean annual temperatures.

ECOLOGY OF 8ALANI TES AEGFPTIACA (BALANITACEAE)

23

tent of adaptation in populations associated with extreme temperature and rainfall conditions also warrants investigation. Balanites aegyptiaca grows with, or near, populations of other species of the genus in several areas: east and northeast Africa (other species: 8alanites glabra Mildbraed and Schlechter, Balanites rotundifolia (Tieghem) Blatter); Zambia/Zimbabwe (other species: Balanites maughamii Sprague, 8alanites pedicellaris Mildbraed and Schlechter). Provenances of Balanites aegyptiaca in these areas merit evaluation in conjunction with these congeners, some of which have already aroused interest as economic plants (e.g. Sp~ague, 1913; Hardman and Sofowora, 1971b; Hardman and Wood, 1972; Radunz et al., 1985). Familiarity with the ecological and geographical aspects of intra-specific variation should enable provenances, well-adapted for any area, to be identified. Appropriate morphology and growth will determine resource value at any locality and provenance trials offer the best prospect for distinguishing inherited characteristics from those markedly affected by the immediate growing conditions. Booth and Wicken~ (1988) draw attention to the variability of Balanites aegyptiaca (and the implications of this for selection of improved strains), but do not elaborate on the observation. Earlier (1976), however, Wickens reported the occurrence of an unarmed individual (Jebel Marra, Sudan); no other specific comments on variation within popuiations have been seen. However, variation on a geographical scale, even among the few features covered by conventional morphological descriptions, emerges clearly from flora entries for different parts of the range: in height at maturity (Exell and Mendon~a, 1951; Ozenda, 1977; Burkhill, 1985), in spine length (White, 1962; Geerling, 1982), in inflorescence form and flowers per inflorescence (Aubr~ville, 1950; Zohary, 1972; Geerling, 1982 ), and in fruit size and shape (Launert, 1963; Zohary, 1972; Ozenda, 1977). There are also marked differences in the concentrations of chemicals, such as amino acids, in the kernels reported for Senegal (Giffard, 1974) and Sudan (Abu-Al-Futuh, 1983 ). The frequency of profiles of the species in recent literature promoting multipurpose trees (e.g. Baumer, 1983; National Research Council, 1983; Booth and Wickens, 1988; Rocheleau et at., 1988 ) underlines widely held views that management strategies for Balanites aegyptiaca are desirable. Practical conservation on a local scale can be undertaken through management actions devised for sustained local resource use both at the sites where there are existing populations in rangeland, and in areas with no populations where there is scope for planting in association with more intensively used land. Effort made where populations exist must recognise the risk that the currently acceptable conservation status of Balanites aegyptiaca could quickly change if human and animal pressure on rangeland increased sharply. The rangelands can generally be viewed as a continuous phase of open-access land,

24

J.B. HALL

a common property resource, within which there are '~slands' under identifiable management authority - ranches, forest, game and grazing reserves, national parks. Only in these 'islands' is early implementation of any innovative exploratory management policy feasible. Nevertheless, socio-economic studies of nomadic and pastoral societies are increasingly revealing cultural mechanisms with strong positive resource conservation impact (E. Shanks, personal communication, 1990, citing control over use of riverain woodland in Turkana, Kenya, and concerning Balanites aegyptiaca specifically in Tabora Region, Tanzania). Progress in successful communication of management ideas through extension activity must be awaited for refinement and reinforcement of indigenous initiatives in these areas. The long-term aim should be to extend practices of demonstrable value developed in the 'islands' to the rangeland in general as a logical enhancement of evolving indigenous measures. Where a management authority is identifiable, the need is initially to ascertain the spatial and age structure of the Balanites aegyptiaca population and to monitor these over time. The immediate conservation objective should be to maintain or promote a desirable spatial distribution of the species. With the complex ecological relationships outlined earlier, local conditions (mean annual rainfall, reliance on accessory sources of water) need to be considered in deciding a target stocking level. Natural stocking in rain-fed areas where Balanites aegyptiaca is well-represented suggests targets of 25-50 individuals ha-~ would be realistic. In association with accessory water sources, a possible aim would be to achieve continuity of distribution by ensuring that intervals between nearest neighbours in the Balanites aegyptiaca population did not exceed 50 m. No more than a reconnaissance survey would be needed to identify areas where the population status was sub-optimal. Such land should be rehabilitated, with encouragement of Balanites aegyptiaca regeneration a specific rehabilitation objective. Reported past experience with enhanced protection shows that control over grazing intensity, where appropriate, in combination with control over fire, could be used to promote regeneration. Modest fire and grazing controls will probably suffice as long as there are relict living rootstocks or nearby seed sources. Failing this, enrichment planting can be undertaken. In most cases, extended periods of rigorous protection would be unnecessary if animal-carrying capacity was not exceeded and any burning was early. Total exclusion of fire is impracticable, but even early burning would only have to be applied under mean annual rainfall more than 600 ram. Management should be able to maintain range condition and minimise fuel accumulation and fire risk through control of animal movement by rotational access to different parcels of land, once a satisfactory range character is established. Where woody plants are particularly sparse and Balanites aegyptiaca is at unusually high risk because it is the only forage available over part of the year, more comprehensive protection will be needed: livestock must

ECOLOGY OF B A L A N I i ~ S AEG YPTIACA (BALANITACEAE)

25

be excluded from blocks under rehabilitation until the young Balanites aegyptiaca individuals are robust enough to survive browsing impact. According to Von Maydell (1986), 3 years of protection will suffice. Areas where the Balanites aegyptiaca population typically occurs on termitaria do not need different measures. Enrichment planting is an option available where there is doubt over the sufficiency of seed sources or relict individuals capable of responding positively to any easing of fire or browsing/grazing pressure. Transplanting of nursery-raised seedlings is preferable to direct sowing as reports (Herlocker et al., 1981; Weber and Stoney, 1986) indicate that germination may be slow (up to 7 weeks) and restricted ( < 70%). With provenances from Nigeria (Lapido, 1989) and Kitui, Kenya (S.J. Retallick, personal communication, 1990), vegetative propagation has proved simple and effective (60-70% striking), and has been shown to facilitate more rapid shoot growth than in seedlings, but it remains unclear if provenances elsewhere are equally amenable. Ex situ action should be concentrated within the range of Balanites aegyptiaca but where the natural vegetation cover has been replaced as land has come under more intensive management. The acceptability of Balanites aegyptiaca for incorporation into prevailing management systems is indicated by spontaneous planting of this species rather than alternatives formally promoted in afforestation measures in West Africa (Kerkhof, 1990). There is much scope for the linear plantings or scattered shade tree plantings appropriate for a species thriving in full exposure. Linear plantings of particular interest are as stock barriers (favoured by interlocking branching, low foliage palatability when alternatives are available, and evergreen tendency), in the lower layer of shelterbelts, to eventually serve as a fruit source, and as a rural health benefit beside waterways (fruits that have fallen into water release molluscicidal chemicals active against the bilharzia hosts Biomphalaria and Bulinus). As shoot growth is slow, use in barriers should be in combination with other species. With time, the long-lived Balanites aegyptiaca, which has prolific suckering ability, should increase in prominence in the barrier. Conscious selection of provenances displaying characteristics particularly appropriate for the role sought from the tree would be advantageous. Screening of a variety of provenances through trials should assist with identification of suitable seed/cutting sources. A general interest in growth-rate variations can be predicted, but if an erect and little-branched habit proves linked to fast growth, slower-growing and perhaps more resilient provenances with more bushy characteristics and vigorous root suckering would be preferred for barriers. Unarmed or relatively weakly armed strains appeal for isolated trees grown as fruit sources in areas from which domestic animals are excluded, and trees grown specifically to provide fruit for direct human or animal consumption should be high yielding, preferably with large individual fruits. In-

26

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HALL

terest, either for industrial processing or rural health programmes, centred on compounds synthesized in the fruits, invites chemical characterisation of material from different geographical origins. Curiously, in view of the extensive current interest in phytochemical variability within and between related species (e.g. Adams, 1990), Abu-AI-Futuh (1983), though examining in detail agro-industrial prospects based on the chemical constituents, is silent on this matter.

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