ACACIA ROBUSTA BURCH. AND ACACIA CLAVIGERA E. MEY. IN NATAL, SOUTH AFRICA.

KATHLEEN D. GORDON-GRAY

Bews Botanical Laboratories, University of Natal, Pietermaritzburg, South Africa.

Brenan (1957, p. 365) stated that the Southern African material of Acacia robusta Burch. at Kew was not uniform and could be divided into two entities, which, he believed, should be considered as distinct species, A. robusta Burch. and A. clavigera E. Mey. These species he distinguished from one another on the basis of the indu- mentuln of peduncles, calyx lobes and leaf rachides;pinna and leaflet number; pod width, the degree of eurature of the pod, and the alignment of the seeds within it. Of these characters, indumentum and pod form were stated to be the most significant.

According to Brenan, A. robusta has so far been found only in the Transvaal and in Beehuanaland. Reports of this species from other localities refer mainly to A. clavigera, which occurs along the southeast and east coast of South Africa from the mouth of the Great Kei River northwards into Portugese East Africa, the Trans- vaal, and Southern Rhodesia. In Portuguese East Africa A. clavigera becomes diffi- cult to distinguish from a species previously known as A. usambarensis Taub. which Brenan (1.e.) prefers to regard as A. clavigcra subsp, usambarensis (Taub.)Brenan. The Natal plants are thus, following Brenan, A. clavigera E. Mey. subsp, clavigera, except for occasional specimens in northern Zululand that appear to favor drier lo- calities and which may be A. clavigera subsp, usambaren,sis.

Prior to Brenan's work, another authority, Codd (1951, p. 48-50), had recognized the heterogeneity within the Southern African material previously referred to A. robusta. The two authors are not entirely in agreement, however, for Codd regards the trees of A. clavigera subsp, clavigera as larger than those of A. robusta, while Bre- nan states, "a generally greater robustness is shown by 'A' (A. robu~ta sensu stricto)". Codd suggested (pers. comm.) that A. robusta sensu strieto might be represented in the valleys of the Bushman's, Mooi, and Tugela rivers of Natal, as well as in Beehuanaland and the Transvaal.

In an endeavour to determine whether, on the basis of the diagnostic criteria pro- posed by Brenan, the Natal plants are A. clavigera subsp, clavigera and distinct from the Transvaal and Beehuanaland A. robusta, or whether both "species" are repre- sented and form an intergrading series, this investigation was undertaken.

A. clavigera subsp, clavigera (sensu Brenan) in Natal. This species, which is often found in moist situations, occurs in a narrow coast strip generally below 1,000 ft in altitude and some 10 miles in width which, north of the Tugela river, broadens to a flat plain 20-50 miles wide. The plants of the coast zone probably represent a single population, for the trees are mostly scattered at short intervals so that genie inter- change is possible. The species also extends inland up the larger river valleys for a distance of up to 100 miles. These river valley populations connect with one another and with the coast population. Occasional scattered groups are also to be found on the dissected hinterland between the Umgeni and Illovo rivers. Whether these

1In the text the word "random" has been placed in quotation marks lest the sampling method used infringes statistical requirements. Great difficulty was experienced in finding an accepted method of randomized branch sampling that was practical for large numbers of trees often growing in difficult terrain. The sampling method was tested by different operators sampling the same trees on different days. Some operators had no scientific training, nor knowledge of the problem under consideration. For single trees, results for each character were subjected to the Variance R~tio Test and in every case no significant difference between samples was obtained.

BRITTONIA 17: 202--212. Jul 1965.

-202-

1965] GORDON-GRAY: ACACIA IN NATAL 203

groups are the result of natural distribution, or whether they developed originally from planted trees, is difficult to determine. Whatever their origin, they probably represent genetically isolated allopatrie populations.

A. clavigera subsp, clavigera is generally deciduous, losing all or the majority of its leaves during winter. Its leaves are of two kinds, primary leaves developed be- tween stipular spines oll young branches and secondary, or fasciculate leaves devel- oped from the abbreviated axillary stems or dwarf shoots that expand, as a rule, only after the pr imary leaves have been shed. I t is among the first of the Natal Aca- cias to bloom in spring, so that, from late August to the end of September, trees may be found bearing mature pods belonging to the previous season's growth, as well as flowers, fully formed leaves and, in some cases, nfinute pods of the present season. Blooming is often sporadic, not all trees of the same population flowering every year.

The pods develop to almost maximum width 3-4 months after flowering, but at this stage the ovules are still nfinute. Enlargement and maturat ion of seeds takes place rapidly in the fifth and sixth months just before the fruits darken at drying off.

The method of sampling. The parameters recorded by Brenan for A. clavigera sub- sp. clavigera were obtained from herbarium specimens. In the present study popu- lations were studied in the field and an endeavour was made to ensure that the trees sampled were representative of the populations from which they were taken and that the leaves, flowers, ~nd fruits constituting a sample from an bldividual tree were representative, in turn, of leaves, flowers, and fl'uits produced by the tree at that particular season. Sampling was carried out mainly in spring. The method of samp- ling for individual trees was uniform throughout and consisted in cutting the distal 1-2 ft from not less than 20 branches taken at" random ''j from the crown. Occasional trees were so tall tha t the top of the crown could not be reached, but here it was con- sidered that most field workers collecting fl'om the tree would experience similar difficulty. In many cases insufficient mature pods were present on the cut branches. More pods were thus collected at " random" from the crown.

The branches were wrapped in polythene and transported to the laboratory where they were stripped. Primary leaves, secondary leaves, young pods, mature pods, and flowers were segregated each into a separate group. The constituents of each group were thoroughly mixed; then 20 specimens were taken at " ranaom" trom the group. The 20 specimens each of primary leaves, secondary leaves, mature pods, and flowers constituted the sample from the tree. Preliminary counting showed that in both primary and secondary leaves, the lowest and second uppermost pairs of pinnae respectively gave extreme estimates of the least and greatest number of pinnule pairs per pinna for the leaf as a whole. These pinnae were consequently used in counting pinnule number.

For the sample from each tree, the following measures and estimates were re- corded :

Primary and secondary leaves number of pinna pairs per leaf minimum and maximum number of pinnule pairs per pinna pubescence of rachides number and position of glands on rachides

Pods and flowexs width of pod at its broadest point pod curvature orientation of seeds within pod number of flowers per inflorescence pubescence of peduncle

204 BRITTONIA [VOL. 17

Estimation of rachis and peduncle pubescence, and of the degree of pod curvature were based on the following arbitrary scales:

Pubescence

G - entirely glabrous p - scattered hairs visible under X l0 lens. D - pubescent to the naked eye, densely so

under X 10 lens.

Pod curvature

S - s t r a igh t C -s l ight ly curved F - fa lea te

Measurments of the longest stipular spines visible on the tree were also taken.

Sampling of "Populations". Trees from different localities in Natal were sampled according to the given procedure to determine the variation among trees in any one locality and from one locality to another. The number of trees sampled in any area depended upon the number of trees growing there and their importance in repre- senting a distributional extreme within the Province. Thus in the Tugela River Val- ley near Tugela Ferry, 20 trees were sampled at "random" along the roadside, 20 being regarded as a reasonable number for statistical analysis (Youden 1951, p. 18- 21). This "population" is an extensive one and was regarded as likely to give a repre- sentative indication of variation near the inland limit of the species in Natal. Twenty trees were also sampled from the coast "population" by following the road from Scottburgh, approximately 40 mi south of Durban to Verulam approximately 15 mi north of Durban.

Five to ten trees only were sampled also from each of the Muden, Weenen, Um- geni Valley (Inanda Location), and Ashburton areas.

DISCUSSION

i) General. Before the results are discussed in detail, some general comments must be made. Brenan (1957) made no mention of whether the leaves counted in his study were primary or secondary. It is possible that both types were represented in the material from which his parameters were obtained, but secondary leaves probably predominated. In general the primary leaves of A. clavigera subsp, clavigera (sensu Brenan) are larger and may sometimes carry more pinnae, but not necessarily more pinnules than do the secondary leaves. Their petioles are shorter and stouter and generally carry, somewhere along their length, a large gland that is, with practically no exceptions, absent from the secondary leaves.

Quite early in the present work, it became evident for the following reasons that primary leaves were less satisfactory than fasciculate leaves in estimating leaf par- ameters :

i) at all seasons of the year a mature tree carries more secondary than primary leaves; it is only on the new season's growth that primary leaves are present. Occa- sionally a tree may carry no primary leaves at all.

ii) malformed or imperfectly developed specimens are more frequent among pri- mary than among secondary leaves.

In the present paper, in view of the above findings, whenever comparisons have been made among leaves, it is always the secondary leaves that have been con- sidered.

Brenan also failed to state whether the limits given for leaflet pairs per pinna are maximum and minimum numbers per leaf, or whether the range applies to the var- iation encountered for the maximum number of pinnule pairs only. In the present paper maximum and minimum numbers have been given in order to indicate the complete range.

Brenan included spine length among the criteria for distinguishing the two species under consideration and pointed out that in A. robusta, spines were stout and long,

1965] GORDON-GRAY: ACACIA IN NATAL 2 0 5

reaching 7-11 cm in length, whereas in A. clavigera subsp, clavigera they generally were up to only 6.5 cm. Field study has shown that spine length is an unreliable diag- nostic criterion. Mature trces from all localities investigated often bore stipular spines no longer than 2-3 cm. It is generally only young trees, young coppice shoots of old trees, or trees growing under more arid and extreme enivronmental conditions that have long, stout spines. These vary greatly in length on a single tree, so that the limits which Brenan gives for the two species do not hold. Spines 10-11 cm long have been recorded for trees in every locality so far investigated.

Brenan also used orientation of seeds within the pod as an ancillary diagnostic criterion. This, too, is not reliable. Investigation of young and of mature pods not yet dehisced, showed that the seeds lie longitudinally in the pod (exceptions of 1-3 seeds lying obliquely to transversely have been found, but the majority are always longitudinal). A slender, wiry funicle attaches the seed to the pod and at dehiscence this brings about movement of that seed, so that after dehiscence, the seed often ap- pears to have lain transversely to the longitudinal axis of the pod: especially does this apply in wider pods where the nearly mature seeds do not occupy entire pod width.

At the coast, probably because of narrower pod width, the mature seeds are closely packed and tend to overlap longitudinally so that pods are swollen and ahnost round in transverse section. Inland, the seeds are by no means so closely packed which results in the dry pods being thinner and almost fiat in transverse section. Apart from frequent deformities due to insect attack, pod length for any individual tree is surprisingly constant, but there is great variation among trees in any locality (Figs. 1 and 2). In general pods are longer at the coast than inland, but tree to tree varia- tion precludes use of this character in species diagnosis.

Counts of flower number within infiorescences showed differences from tree to tree. Because of this variation and because of the labor involved in counting, this character was also ignored as a possible diagnostic criterion.

ii) Variation in individual "populations". Results obtained from sampling in the Tugela Valley near Tugela Ferry and at the coast from south to north of Durban are shown in Tables 1 and 2 respectively. These results have been given in detail in or- der to indicate the variation existing among the trees within a population.

In diagnosing A. robusta and A. clavigera subsp, clavigera as distinct species. Brenan emphasized pod width and shape and the indumentum of rachides and ped- uncles. Consideration of both tables indicates that pod width and shapevaried mar- kedly from tree to tree. For example, among the Tugela Ferry trees pod width varied from limits laid down for A. robusta (tree 1) to those for A. clavigera subsp, clavigera (tree 6). Podshape was also variable (Fig. 1, trees 11 and 19). At the coast, the range in pod width and shape was not quite so great, but nevertheless, trees that fell predominantly within the limits of A. robusta (tree 7) and A. clavigera subsp. clavigera (trees 16 and 20) were represented (see also Fig. 3, trees 9 and 19 for pod shape).

The indumentum of rachides and peduncles was less variable (Tugela Ferry mostly glabrous, occasionally pubescent: coast predominantly densely pubescent, some- times glabrous to pubescent).

The numbers of pinna and pinnule pairs showed greatest uniformity for a popu- lation. If these features alone are considered, the Tugela Ferry trees fall predomi- antly within limits suggested for A. robusta, while it is difficult to be certain where some of the coast trees should be placed, trees 5, 6, 8 and 9. These must be considered intermediate.

Undoubtedly Brenan intended his specific distinctions to be used collectively. Tugela Ferry (tree 1) must then be named A. robusta because of its wide pods, glab- rous rachides, peduncles and calyx-lobes, and its small number of pinna pairs, which

206 ~RITTONIA [VOL. ]_7

Fia. 1. Pods from Tugela Valley near Tugela Ferry.

1965] GORDON-GRAY: ACACIA IN NATAL 207

FIo. 2. Pods from the co~st near Durbam

208 BItITTONI& [VOL. 17

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210 BRITTONIA [VOL. 17

means that specimens fulfilling the characteristics laid down by Brenan for this spec- ies do occur in Natal. The distinctions between the two species do not hold, however, when other individual trees are considered, because Tugela Ferry trees 3 and 6 showed pods of A. clavigera subsp, clavigera and leaves of A. robusta, while coast trees 18 also showed this combination of characters.

The Tugela Ferry and coast populations are not exceptional in the variation in morphological form exhibited by individual trees, nor in exhibiting features that are frequently intermediate between the linfits laid down for A. robusta and for A. clav- igera subsp, clavigera. All other populations sampled are comparable in these re- spects.

iii) Variation from one locality to another. The pictoriMized scatter diagram in which mean pinna pair nmnber is plotted against mean pod width for all trees sam- pled that bore both mature leaves and pods (Fig. 3) indicates that the Tugcla Ferry trees are clearly different inorphologically fronl those of the coast, because there is no overlap between these two populations. Fig. 3 also shows that these populations represent extremes among the Natal plants and that all other populations sampled are intermediate between these extremes. Parameters obtained indicate that this condition applies not only to pinna pair number and pod width, but also to indu- mentum of rachides and peduncles and to pinnule number.

Thus gradients in morphological characters exist from the coast to the inland limit of distribution in Natal. Pinna and pinnule pair number decrease inland, as does pubescence of raehides and peduncles. Pod size, despite tree to tree variation which is pronounced, shows some overall tendency to gradation, pod width increasing and pod length decreasing from the coast inland. These morphological gradients are evident from the distribution map (Fig. 4)

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1965] GORDON-GRAY: ACACIA IN NATAL 211

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Conclusions. The present study does not uphold the concept that the South Afri- can material previously referred to A. robusta Burch. can be divided into two dis- tinct species on the basis of differences laid down by Brenan (1957). Neither does it give support to Brenan's interpretation that the Natal trees must aH be referred to A. clavigera subsp, clavigera.

The gradients in morphological form disclosed by field study of the Natal plants have been further emphasized by study of herbarium material from other localities in Southern Africa. Thus parameters for the Tugela Ferry trees though they fit Bre-

212 BRITTONIA [VOL 17

nan's parameters for A. robusta better than those for A. clavigera subsp, clavigera, do not fit as well as do parameters obtained fi'om herbarium material of trees in Bechuanaland and parts of the Transvaal. It is significant also that A. robusta (sensu Brenan) is never coastal in distribution, being restricted to Bechuanaland and Trans- vaal, whereas A. clavigera subsp, clavigera is coastal from the Great Kei River north- wards into Portuguese East Africa. I t also extends into the Transvaal and South- ern Rhodesia where, presumably, it grades into A. robusta (sensu Brenan). Detailed field sampling still remains to be carried out to substantiate these tentative indica- tions.

There must be ecological, or ecological and geographical factors, governing these morphological gradients. Available moisture over the whole, or part of the year, may be suspected as at least contributory (unpublished work carried out on A. mearnsii de Wild. by the Wattle Research Institute, University of Natal--personal commun- icat ion-and on indigenous species in these Laboratories, is suggestive), but as yet no conclusive evidence exists. Huxley (1938) proposed the term "cline" for such morphological gradients in response to ecological or geographical conditions.

One other point is worthy of emphasis, namely the morphological variation from tree to tree in any locality. In every population some phenotypes produce more widely spaced pinna pairs, wider pods and often a less dense indumentum than do others. These differences are undobtedly genotypically based. Parameters indicate that such genotypes, because of the effect of the coast-inland cline, produce narrower pods, a greater number of pinna pairs and a denser indumentum at the coast than do sinfilar genotypes in inland situations. These genotypic differences may perhaps be traceable to an ancient hybrid origin for A. robusta in its widest sense.

Understanding of the causes of morphological variation within this species in Southern Africa is by no means complete. Cytotaxonomic work will undoubtedly provide further information, as will further field sampling, particularly in northern Zululand, the Transvaal, Southern Rhodesia, Bechuanaland, and the eastern Cape Province. Until this has been done, the majority of the Natal plants are best con- sidered taxonomically as forming part of a single variable species, A. robusta Burch., in which no subspecies or varieties are at present recognizable.

With the sinking of A. clavigera subsp, clavigera, the question arises of the relat- ionship to A. robusta of the northern Zululand plants regarded by Brenan as A. clavigera subsp, usambarensis. Pending detailed field sampling in northern Natal, Portuguese East Africa, and further north, A. clavigera subsp, usambarensis must revert to its previous status of A. usambarensis Taub.

Acknowledgements. The writer wishes to express appreciation to Mr. B. Nevin, Department of Mathematics, University of Natal and Professor A. A. Rayner, Department of Biometry, Faculty of Agriculture, University of Natal, for helpful discussion concerning sampling and the presentation of results. Thanks are also due to Miss B. Mitchell-Innes, Messrs. I. M. Chahners, D. Edwards, C. Gordon-Gray and others, for assistance with field work.

LITERATURE CITED

Brenan, J. P.M. 1958. Notes on Mimosoideae: IV. Kew Bull. 1957: 365-369. Codd, L. E.W. 1951. Trees and shrubs of the Kruger National Park. Bot. Surv. Mem. 26. Govt.

Printer, Pretoria. Huxley, J.S. 1938. Clines: an auxiliary taxonomic principle. Nature 142: 219-220. Youden, W.J. 1951. Statistical methods for chemists. John Wiley & Sons, Inc., New York and

London. 126 p.