Phytogeography of the copper and cobalt flora of Upper Shaba ( Zaire ) , with emphasis on its endemism , origin and evolution mechanisms

Copper and cobalt ore deposits occur on at least a hundred outcrops scattered in the Shaban Copperbow, an area of 2 000 square kilometres, in the metallogenic Province of Southern Central Africa. With more than 200 species, this flora includes a large number (42) of endemic species of various degrees. Some species are known from only one site, many are located on neighbouring outcrops, others occur on all the ore deposits. Present migratory pathways have been traced for some species and are reported. The relative importance of palaeoendemism and neoendemism is discussed. The origin of these endemics, as well as that of other plants is to be found in several adjacent floras such as that of steppe-savannas developed on more or less poorly aerated soils (Kalahari sands or dambos overlaying laterite), dwarf vegetation on siliceous cellular rocks and miombo woodlands on poor hydromorphic soils. Examples are given for each vegetation type. Systematic details, leaf anatomy and phytogeochemistry data support these hypotheses, which are illustrated for several closely related taxa.

In Shaba, about 100 copper-cobalt deposits totalling some 20 km2 are scattered in a 20 000 km2 area of the Shaban Copperbow in south-east Zaire (Morrison et al. 1981).These deposits support a distinct vegetation.Both endemic taxa and a great number of more widely distributed species can be distinguished within this flora, origin of which is mainly to be found in the surrounding vegetation types, of which the main types were noted by Duvigneaud (1958).However, preliminary lists permitting an appreciation of respective origins, are not yet available.Moreover, as far as endemism is concerned, both palaeoendemism (Wild, 1971;Wild & Bradshaw, 1977) as defined by Stebbins (1942) and neoendemism (Antonovics et al., 1971) have been proposed, although the speciation mechanisms have never been precisely examined.The present study attempts to assemble data relating to this lack of knowledge.

ENDEMISM OF THE COPPER-COBALT FLORA OF UPPER SHABA
The number of endemic species of the coppercobalt flora of Upper Shaba may be estimated at present at some 42 taxa.This number will undoubtedly change as a result of new systematic studies and further collecting.Revisions may reveal some taxa, until now considered as endemics, to be synonymous.For instance, Geerinck (1971), whose wider concept of the idea of species differs from the narrower view of Duvigneaud and Van Bockstal (Duvigneaud, 1959;Duvigneaud & Denaeyer-De Smet, 1963), reduces the number of Gladiolus occurring on copper soils to six, with only one endemic species, Gladiolus robiliartianus Duvign.et al., 1982).This plant, considered for a long time as a 'copper flower', is no longer endemic to copper outcrops, even if it remains a very useful indicator of copper anomalies in Shaba.Nevertheless, these preliminary remarks permit certain deductions.
Copper-cobalt flora endemism shows several aspects (Fig. 2).Endemic species, clearly separated from neigh bouring taxa not present in Shaba, and which are moreover widely distributed on copper deposits, probably belong to paleoendemism, as is the case of Ascolepis metallorum, Acalypha cupricola and Rendlia cupricola.The first of these three species can still be found on the Kengere deposits, where it resists high zinc-lead concentrations (Duvigneaud & Denaeyer-De Smet, 1963) whereas Rendlia cu pricola, has been observed outside the Shaban Copperbow at Dikulushi, on copper deposits in the vicinity of Lake Moero.We also noted its presence on anthropic metalliferous sites such as the alluvious soils polluted by waste water of the factory at Likasi, along the Panda River, an ecological niche occupied by Pteris vittata L. in the area surrounding Lubumbashi.Neoendemism, on the other hand, should be considered for plastic species, where intermediate forms may still be observed, notably mine ecotypes corresponding to the differentiation of 'small' species, generally having a limited distribution.Silene cobalticola, several species of Becium, Haumaniastrum timpermanii (Duvign.& Plancke) Duvign.& Plancke subsp.kambovianum (Duvign.& Plancke) Duvign.& Plancke, etc belong to this last category.In conclusion, the coppercobalt endemics of Upper Shaba contain a large number of neoendemic species to which may be added a small number of palaeoendemics.This dual origin can be explained (1) by the age of copper-cobalt mineralization in Upper Shaba, which dates from late precambrian era and has therefore been in existence for more than 620 million years and (2) by the extreme climatic changes which Ascolepis metallorum is found on all the ore characterized the tertiary era and which therefore created numerous vacant ecological niches at the beginning of the quaternary.These may have favoured the differentiation of species little able to compete in normal conditions, such as Haumanias trum katangense, which reveals a clear weakness to fungal attacks when copper is absent (Morrison et al., 1979).

ORIGIN OF THE COPPER-COBALT FLORA OF UPPER SHABA
As far as is known at present, the copper-cobalt flora of Upper Shaba comprises some 220 taxa including 42 endemics.The origin of this flora is to be found in several vegetation types (Fig. 3).According to Duvigneaud (1958), these plants are derived from four main sources: steppe-savannas on sandy high plateaux (dilungus) (A), seasonally inundated steppe-savannas overlying laterite (dambos) (E), woodlands on yellow compact soils (F) and formations on non-mineralized rocky outcrops (I).deposits, whereas some plants are restricted to one mine.
However we have also noted other ecological groups, namely grassland plants not subjected to seasonal flooding (B), grassland plants with a wide ecological amplitude (C), plants occurring on derived savannas (D), ruderal species (G) and basi or (and) xerophilous plants (H).Malaisse & Grégoire (1978) listed some ruderal plants occurring on copper mines and Malaisse et al. (1983) listed the contribution of the flora of high plateaux steppesavannas.Table 1 gives a preliminary list of plants belonging to each group.
Regarding endemics, their probable origin con firms the contribution of these vegetation types, as well as revealing the possible róle of plastic mine ecotypes in the speciation of new endemic taxa (Table 2).
The preliminary analysis of the copper-cobalt flora shows that some 81% of the observed species occur in other vegetation types of Upper Shaba.The three main types are steppe-savannas on high plateaux, woodlands of the Xerobrachystegion For 'grassland plants with large ecology' read 'grassland plants with wide ecological amplitude'.

H IG H P L A T E A U X S T E P P I S H S A V A N N A S . D E R IV E D S A V A N N A S W O O D L A N D S O N Y E L L O W C O M P A C T S O IL S H IG H T E R M IT
Alliance and steppes and bushes on rocky outcrops with three other vegetation types (wooded savannas, dambos, high termitariaj playing a secondary role.In total, also taking into account the ruderal plants that are frequent on old mine workings, these nine ecological groups represent about 80% of the flora.The remaining 19% is made up of endemic taxa of varying importance, many of which are derived from characteristic species of neighbouring steppesavannas, woodlands and rocky outcrops with a second fairly important group derived from mine ecotypes.

EVOLUTION AND SPECIATION IN THE COPPER-COBALT FLORA OF UPPER SHABA
It is generally accepted that most of the copper-cobalt flora is made up of tolerant taxa able to grow on heavy metallic toxic soils.The establishment of metal-resistant plants on coppercobalt deposits could have favoured the appearance of tolerant taxa, the 'mine ecotypes' of Antonovics et al. (1971).These deposits may also have functioned, at some time, as relay stations or secondary centres of origin in the course of migration or distribution of some species.Examples illustrating the hypothesis can be found, for instance, in the genera Ipomoea and Becium.Plastic species complexes could have evolved and given rise to several ecotypes or slightly different species.Malaisse et al. (1983) pinpoint the mechanism of speciation for Silene cobalticola originating from the widely distributed S. burchelli var.angustifolia.Evolution here, results in a reduction of leaf width, an increase in leaf thickness and a reduction or even a disparity in trichome linked to floral differences.Similar evolution has been observed for other species.Justicia elegantula S. Moore, for example, another variable species, presents an ecotype with very glabrescent and unusually narrow leaves, which is confined to soils with a high copper content in Zimbabwe (Jacobsen, 1970).In Shaba, speciation of the same plant leads to Justicia metallorum (Duvigneaud & Denaeyer-De Smet, 1963).In similar way, Dicoma anomala Sond.presents abnormally narrow leaves on rocky copper steppes occurring at Luita, Fungurume and Mashamba.It should be noted that this genus has given rise to an endemic on the serpentine soils of the Great Dyke of Zimbabwe (Wild, 1971).The Pandiaka carsoni-metallorum complex gives a further example of evolution and copper ecotype

Pteris vittata L.
High plateaux narrow fringing forest Delphinium dasycaulon Fresen speciation (Fig. 4).On copper outcrops, in Shaba, we have observed a range of forms originating from a broad leaf bearing two types of hair, which was observed at Chabara and Dikulushi among other sites, and giving rise to an ecotype with narrow, glabrous leaves occurring at Luiswishi.Such evolution may or may not be linked to particular, different accumulation level of copper by each taxon.Among the endemics, numerous hyperaccumulator species are found (Brooks ei al. 1980): nine of the twelve copper hyperaccumulators and five of the fifteen cobalt hyperaccumulators are actually listed i.e. more than a quarter of the endemics.The acquisition of such an adaptation may constitute the first step in new ecotype speciation.Thus the Buchnera henriquesii plants occurring at the Etoile Mine possesses a very high copper content, whereas those found at Mindigi or Chabara have low copper values.In a similar way, the Pandiaka metallorum plants of Fungurume accumu late ten times more copper than those of Luiswishi, both growing on soils of the same toxic level.Further research, comparing plants grown from seeds of different origin, is the next step in finding solutions to these speciation problems.

F
ig. 1.-Copper (and cobalt) ore deposits in the south central African metallogenic Province.

Fig. 3 .
Fig. 3.-Origin of the copper-cobalt flora of Upper Shaba.Species dealing with the nine groups A to I are listed in the text.N.B.For 'grassland plants with large ecology' read 'grassland plants with wide ecological amplitude'.

TABLE 1 .
-Contribution of different vegetation types to the copper-cobalt flora of Upper Shaba

TABLE 2 .
-Origin of copper-cobalt endemics of Upper Shaba