The Baobabs: Pachycauls of Africa, Madagascar and Australia || Phytogeography

Chapter 14Phytogeography

Phytogeography is defined by Good (1964) as ‘that branch of botany that deals with the spatial relationships of plants both in the present and the past’. While there is no clear demarcation between phytogeography and ecology, Good’s definition indi-cates a concern with the evolution of plant distribution in geological time as well as plant distribution today. Phytogeographical areas are governed by the world’s climatic belts and phytogeography is concerned with the floristics of the common pool resulting from historical factors; ecology is concerned with the segregation of species within the common pool by environmental factors (Wickens 1976).

1 Phytogeography

The need to divide the global flora and fauna into meaningful biogeographical units based on the natural distribution of plants and animals has led to the establishment of phytochoria and zoogeographical choria of various ranks. A ‘phytochorion’ is defined as a chorological vegetation unit of any rank, e.g. region, domain, etc. Phytochoria are natural floristic areas that broadly correspond to climatic types, and are based on the total number of species with the same concentric distribution pattern. A phytochorion may embrace a range of different ecosystems, and some extend from mesic to arid environments. Thus, in the higher phytochorial divisions the range may be considerable, while in the lower divisions it is likely to be corre-spondingly smaller (Renvoize et al. 1992). For a more detailed discussion see Clayton and Cope (1979).

1.1 Continental Drift and Other Theories

Gondwana is a name used by the great Austrian geologist Eduard Suess (1831–1914) for fossiliferous beds in the Gondwana region of central India (from Gond, an ancient Indian tribe, and wana, meaning land), a term now applied to the former

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southern supercontinent, which later broke up to form South America, Africa, India, Australia and Antarctica. The land in Gondwanaland is clearly tautological (Fortey 2005).

An understanding of past and present views on Gondwana and its break up are essential to our understanding of the differing views held by phytogeographers in interpreting plant distribution in relation to continental drift. Our present-day concepts are based on the pioneer work of the German meteorologist and geophysicist Alfred Wegener and his theory of continental drift (Wegener 1915, English translation 1924). Wegener believed that the outer layers of the earth’s crust had the consistency of a viscous liquid in which the semi-rigid continents or sial ‘floated’, partially submerged in the underlying sima. Weaknesses in his hypothesis were later countered by evidence of sea floor spreading, and finally the two concepts were combined into one theory of plate tectonics.

At first, the continental drift theory obtained little or no support from those phytogeographers who believed in the steady state principle, according to which the position of continents were fixed in the distant past, before the evolution of Angiosperms during the Jurassic, in 179–158 MaBP. The then current knowledge of polar shift and palaeomagnetism was discounted. Many were the theories to explain the disjunct distributions of flora and fauna. Croizat (1952, 1964), for example, favoured the idea of an ‘African Gate’ with the Bombacaceae, now known as the Bombacoideae, migrating northwards from ‘Antarctica’ through Madagascar, across into Africa and via the East Indies to Australia. He assumed a distribution for the Bombacoideae prior to the break up of Gondwana, an idea that cannot now be supported by floral evolution and geological evidence.

Simpson (1940), a critic of the various theories produced by phytogeographers, wrote:

So multiple and varied are the facts and conjectures that have been published in this field that judicious selection and emphasis of them can be made to support almost any opinion not completely irrational. No one person can hope to know at first hand all the pertinent data, and a general review of the literature leaves one feeling that it can be taken to prove any of a dozen conflicting theories and that it therefore proves nothing.

Faute de mieux, the botanist van Steenis (1962) formulated the theory of transoceanic land bridges formed by geological changes in ocean floor levels, an idea no doubt fos-tered by his wide experience in Southeast Asia and the floras of its numerous islands. Even then, van Steenis was unable to suggest a direct migration pathway between Africa/Madagascar and Australia for such shared genera as Adansonia, Caesia, Hibbertia, Keraudrenia and Triraphis. As Herbert (1950) pointed out, to be effective, land bridges must have some reasonable continuity of habitats with suitable edaphic and climatic conditions. For non-wind pollinated plants, suitable pollen vectors must also be available. In the absence of land bridges, dispersal between the African and Australian continents would entail sea transport over distances in excess of 8,000 km, even in a direct west to east direction. While sea dispersal is a more likely alternative to a land bridge for some of the above-mentioned species, it is doubtful whether the seed of the grass Triraphis, for example, could survive such a journey. A high standard of taxonomy is critical for anyone studying allegedly disjunct distributions.

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The phytogeographer Good (1964) declared both the steady state principle and the land-bridge theory to be geologically untenable as well as incapable of explaining the incompatibility of a number of distribution patterns. Since the petrography of the ocean floors is markedly different from that of the continental areas, the likelihood of the seabed ever having been land is clearly low, except in a very few cases (Hawkes and Smith 1965). Professor Good concluded that only continental drift could provide a satisfactory explanation for the past and present distribution of plants.

Hess (1960) first proposed the idea of sea floor spreading, which was followed by the theory that the earth’s crust is composed of a series of rigid plates. These ideas were brought together by the Isaaks et al. (1968) in the theory of plate tectonics to show how the movement of these plates in relation to each other is responsible for the major features of the earth’s surface. The description of such a mechanism supports Wegener’s original theory, providing a convincing explanation of continental drift and the distribution of the world’s fauna and flora. As the plates drifted apart to form the present continents, each carried a different selection of the fauna and flora.

Palaeomagnetism had now become accepted as a geological fact, and was used to plot changes in the relative positions of the continents. Although the position of the earth’s magnetic poles changes over time, certain igneous and sedimentary rocks acquire a permanent record of the earth’s magnetic field at the time of their formation by virtue of their iron content. By plotting over a sufficiently wide area the direction of the preserved magnetism from a particular geological horizon in relation to the north or south magnetic poles, it became possible to plot the movement of the poles in geological time (Tarling and Tarling 1972; Whitten and Brooks 1972). The positioning of the southern continents prior to the break up of Gondwana has been determined by the use of geological and geophysical data to find the best geometrical fit at the depth 500 fathoms (914.5 m).

The earliest recognised angiosperms first appeared in the fossil pollen record from the Valanginian–Hauterivian of Israel and Italy in the Jurassic, 179–158 MaBP, shortly (in geological time) before West Gondwana (South America and Africa) began to split apart in 130–125 MaBP. The last direct land connection was severed at the end of the Albian, 96 MaBP, with the Rio Grande Rise/Walvis Ridge and Sierra Leone Ridge forming more or less continuous stepping stones between the two land masses until 88–85 MaBP. There may have been an island-hopping migra-tion route in the South Atlantic during the Eocene until 45 MaBP. Certainly Morley (2003) claims that continental connections between South America, Antarctica and Australia were possible from before the origin of angiosperms until the close of the Eocene, c. 40 MaBP and that this route may have been possible for certain meso-thermal taxa (e.g. Bombacoideae, Polygonaceae, Restionaceae and Sapindaceae).

Throughout most of the Cretaceous, from 148–80 MaBP until the Paleocene, 63 MaBP, Africa drifted some 2,800 km towards Europe, when, as a result of continuing compression, the Atlas and Alpine systems were formed. During the following 10 Ma Africa drifted away from Europe, thereby breaking the only direct connection between the northern and southern hemispheres allowing angiosperm


movement. The connection was re-established during the Early Miocene, c. 18 MaBP, when Africa again converged on Europe.

During the late Mesozoic and Tertiary successive uplifts occurred in eastern and southern Africa, resulting in the Great African Plateau, most of which is more than 900 m above sea level, and in watershed areas throughout the continent; peneplanation appears to have accompanied each plateau uplift. In the mid Miocene, c.15 MaBP, the development of the Great Rift Valley resulted in the separation, still in progress, of Arabia from Africa to form the Red Sea and the Gulf of Aden.

During the Early Cretaceous East Gondwana (India, Australia and its bordering lands and Antarctica) provided direct migration routes to West Gondwana via both Antarctica and India–Madagascar. The Indian Plate separated from this land mass in the Aptian, 125–119 MaBP, but still provided a fairly direct but interrupted dispersal route across the Indian Ocean between Africa and Australia. From c. 80 MaBP India moved rapidly northwards until it collided with Asia in the Middle Eocene, 50–39 MaBP, resulting in the upthrust of the Himalayas and the Tibetan Plateau (Raven 1983; White 1983; Morley 2003; Fortey 2005).

The location of Madagascar against Africa determines the assumed positions of India, Australia and Antarctica during the break up of Gondwana (King 1973), and is therefore of considerable phytogeographical importance. The paucity of geophysical evidence from Antarctica has meant that the positioning of Australia has been largely determined by geological evidence. According to most geologists and geophysicists today, Madagascar broke away from Africa during the late Cretaceous from a more northerly position between Somalia, Kenya and Tanzania in the west and India in the east, and then slid southwards to its present position (Smith and Hallam 1970). Others provided evidence suggesting a more southerly derivation for Madagascar, from the mouth of the Zambezi River (Tarling 1972a, b; King 1978), but their claims failed to find support. Recent studies have shown that Madagascar began to break away even earlier. The Mozambique Channel was apparently formed c. 250–220 MaBP during the period from the Middle Permian to the Lower Triassic, Madagascar drifting away in the Middle Jurassic (at about the same time as the initial break up of Gondwana) and reaching its present position in the early Cretaceous, 121 MaBP (Brenon 1972; Rabinowitz et al. 1983). Seafloor anomalies in the Indian Ocean do not indicate any large movement of Madagascar during the past 75 Ma (Tarling 1972a). Furthermore, the Mozambique Channel separating Madagascar from Africa consists of two identical basins lying one on either side of a longitudinal, median, sandstone ridge. The sediments of these two basins were laid down with no discontinuities since the Cretaceous. From this evidence it follows that Madagascar is part of the African continental plate, and must have occupied its present position from very early times (Paulian 1984).

Wild (1975) considered the distribution of some 80 species common to Madagascar and mainland Africa, and followed Smith and Hallam (1970) in the northerly position for Madagascar. Such a view assumed an Upper Cretaceous movement of Madagascar away from Africa, wheras the now accepted Middle Jurassic break occurred very early in the evolution of the angiosperms. Indeed, it is probable that angiosperms were absent from Madagascar until shortly before its

arrival at its present position. Despite the absence of data from the Late Cretaceous, it is reasonable to assume that by the close of the Mesozoic numerous angiosperm lines had arrived in Madagascar, either overland through India or by short distance sea dispersal (Schatz 1996).

Antarctica has remained in its present position for at least 100–90 Ma, possibly with some rotational movement. The Tasman Sea formed during the Late Cretaceous–Paleocene, 82–60 MaBP, separating Australia–Antarctica from New Caledonia and New Zealand. Rifting between Australia and Antarctica started during the Cretaceous, was initially very slow and was not completed until the Late Oligocene, c. 32 MaBP. Australia and New Guinea then began to migrate rapidly northwards. As Australia began its drift northward, all of New Guinea and much of northern Australia were below the sea, with the northernmost tip of Australia at about 38°S. New Guinea began to appear above the sea during the Late Eocene, c. 40 MaBP, and moved steadily northward with the Australian plate, making contact with the proto-Indonesian island arc and coalescing into an extensive land area in the Early Miocene, 20–15 MaBP. As the Australian plate moved northwards it rotated in an anticlockwise direction to its Mesozoic latitude position, so that the south-western ‘corner’ occupied a more northerly position relative to the rest of the continent (Beadle 1981; Raven 1983; Crisp et al. 2004).

Marine intrusion from the Great Australian Bight c. 30 MaBP, perhaps with aridification inland, isolated the south-west. The subsequent uplift of the Nullarbor aridity barrier separates the south-west from the south-east.

2 Phytogeography of Adansonia

Of the three sections of Adansonia recognised by Baum (1995b), section Adansonia contains one species, A. digitata, native to tropical Africa and south-east Arabia, section Brevitubae has two endemic species in Madagascar, and section Longitibae has five species, four in Madagascar and one in north-western Australia. Bingham (1994) argued that Madagascar may not have been the centre of origin for the genus since plant species that are dispersed by wide-ranging herbivores never share their range with congenic species having the same method of dispersal. From this it follows that the baobab could not speciate until it reached Madagascar, where the lack of wide-ranging dispersal agents provided the necessary genetic isolation for speciation. However, A. digitata is a tetraploid (2n = 160), with a far higher number of chromosomes than the other species (2n = 88), indicating that it is the most recent species to have evolved (Baum and Oginuma 1994).

Early interpretations of geological history assumed Madagascar to be the centre of diversity and origin for the genus Adansonia (Aubréville 1975b; Armstrong 1983). Although the monophyly of the Madagascan species makes such a sugges-tion feasible, a centre of origin in Australia or Africa should not be discounted (Baum et al. 1998b). There is always the possibility of finding new fossil evidence that will overturn the current theory.

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Gerber (1895) was the first to suggest that Adansonia originated in Australia. He hypothesised from his study of the leaves and floral organs of A. digitata, A. madagascariensis (which is possibly a misidentification of A. za) and A. gregorii that the progressive anatomical complexity from A. gregorii to A. digitata suggested that the genus originated in Australia and migrated eastwards across a now extinct land mass to Madagascar, and later to Africa. Assuming the baobab fruits were sea dispersed from Australia to Madagascar and currents flowed in similar directions to the present currents, a route via the Java and North Equatorial Currents would appear possible but improbable. The phylogenic trees (Figs. 36 and 37) of Baum et al. (1998) and Baum (2003) certainly indicate an early origin of A. gregorii.

Aubréville (1975a) postulated a south-westwards migration of the Eurasian equatorial belt due to the general north-eastward movement of the original super-continent Pangaea. As a result, most of the African continent was invaded by the Laurasian flora (Laurasia being the northern counterpart of Gondwana after the break up of Pangaea) and thereby provided the original stock of the Sudano–Zambezian flora. By assigning A. digitata to the Kalahari domain in southern Africa Aubréville was then able to assume a direct migration route for Adansonia between Madagascar and Africa, and Madagascar and Australia via India, before the break up of western Gondwana (Aubréville 1975a, b, 1976). Later evidence (see below), suggests that Australia and Antarctica became isolated from all the other land masses during the Triassic to the Late Oligocene. Raven and Axelrod (1972) pointed out that the break up of Gondwana and the separation of Africa from both South America and Antarctica took place before Adansonia could have evolved.

Whatever the geological history, the Gondwana break up occurred very early in the evolution of the flowering plants. The earliest pollen records place the origins of the angiosperms in the Valanginian–Hauterivian of the Early Cretaceous, 141–132 MaBP, while the earliest fossil pollen record that can definitely be referred to living angiosperm genera is from the Senonian in the Upper Cretaceous, 88.5–73 MaBP. However, DNA sequence data now place the origin of the crown group of extant angiosperms in the Early to Middle Jurassic, 179–158 MaBP, and that of the eudicots (a monophyletic clade of the dicotyledons) in the Late Jurassic to mid Cretaceaous, 147–131 MaBP. DNA sequence data differ from the fossil record and place the origins of the Malvaceae in the Early Eocene, 54 MaBP, and that of Bombax in the Early Miocene, 17 MaBP (Wikström et al. 2001). Such age discrepancies between fossil records and DNA sequencing may be attributed to the incompleteness of the fossil record, taxonomic misidentifications and errors in the geological ages of fossil-bearing rocks (Near and Sanderson 2004).

The earliest fossil of the Bombacoideae was recorded in the North Tethyan flora, from the Campanian–Santonian in the Upper Cretaceous, 84–80 MaBP, and the earliest fossil pollen of the genus Bombacacidites was found in the Maastrichtian in south-east USA, c.69 MaBP (Wolfe 1975, 1976; Krutzsch 1989; Wikström et al. 2001). The Bombacacidites pollen type is assumed to have affinities with Bombax and related genera of the Adansonieae, around which the generic limits are contro-versial (Kemp 1978; Macphail et al. 1994; Mabberley 1997). However, Bombax is not Adansonia’s closest relative. Some data sets put Adansonia close to the South

American genera Catostemma, Cavanillesia, or Scleronema, or even in a sister-group relationship with Rhodognaphalon (Baum 2004, personal communication).

During the Paleocene and Eocene, 60–40 MaBP, the North Tethyan Bomba-cacidites spread eastwards into Central Europe and also expanded southwards via Central America into South America and across to Central Africa, forming a South America–tropical Africa centre of evolution, and probably reached Madagascar during the Tertiary. There was a further spread during the Eocene into Antarctica and the Antipodes, where Bombacacidites failed to become established. Following the Middle Eocene and the onset of cooler climates, Bombacacidites disappeared from Central Europe, retreating eastwards into northern India, where it spread rapidly through Southeast Asia to become part of a new Asian centre of evolution (Muller 1981; Nilsson and Robyns 1986; Krutzsch 1989).

Wolfe (1975) noted the sudden entry of advanced Bombacoideae pollen types in the South American pollen record, which he contrasted with the apparently ancestral types from eastern North America, and postulated that the South American Bombacoideae may represent three separate invasions by different groups of the subfamily.

Of the 23 genera in the Bombacoideae recognised by Kubitzki and Bayer (2002), 21 are found in the New World and only Adansonia (8 species in tropical Africa, Madagascar and north-west Australia) and Bombax (8 species in tropical Africa, Asia and Australia) are found in the Old World. The concentration of the Bombacoideae in tropical America lends support to a West Gondwana origin for the subfamily.

Axelrod (1970) argued that the adaptation of Adansonia (and other genera) to a dry environment created the impression that they were slightly modified survivors from the seasonally dry climate of the Early Cretaceous.

From their rigorous examination of the biogeography and floral evolution of Adansonia using multiple data sets, Baum et al. (1998) concluded that the dispersal of the progenitors of A. digitata and A. gregorii must have occurred well after the fragmentation of Gondwana and therefore must have been transoceanic. Such a means of dispersal is not unlikely given that the fruits of many of the species are sometimes water dispersed (Wickens 1982a; Baum 1995b).

No fossil Adansonia pollen has ever been found. This should not be regarded as evidence for a late evolution and dispersal of Adansonia since the chances of pollen preservation, discovery and identification, and the time factor are uncertain: ‘a taxon is only as old as its youngest fossil’ and ‘absence of evidence is not evidence of absence’. The argument by Raven and Axelrod (1972) that the absence of the very distinctive Bombacoideae pollen earlier than the Palaeocene, 65 MaBP, indicated a late evolution of the family has been overtaken by discoveries of earlier pollen. Nevertheless, long-distance dispersal by sea during the Tertiary remains the most likely scenario based on the available evidence.

Baum et al. (1998) further argued from fossil pollen evidence from the Upper Cretaceous that the earliest possible period for the origin of the stem lineage of Adansonia is during the Lower Campanian, c. 90 MaBP, with the advent of a smaller and unsculptured forerunner of Cavanillesia. By the end of the Cretaceous



Bombacoideae pollen types (probably Tribe Adansonieae) were present in the Upper Cretaceous from the Maastrichtian, 73 MaBP.

One species of the Old World genus Bombax, B. ceiba, syn. B. malabaricum, is found in northern Australia, India and the drier parts of Asia, suggesting a distribution before the Gondwana break up. However, present evidence suggests that Adansonia represents a separate migration to the Old World from that of Bombax, syn. Rhodognaphalon, and that Adansonia used the boreotropical route through Europe to enter Africa. Its arrival time in Madagascar and Australia is unclear but was probably substantially later, through long-distance fruit dispersal (Baum 2003, personal communication). Since the chromosome counts for A. digitata demonstrate that it evolved later than its Madagascan counterparts, it follows that the original migrant species has disappeared, apparently without trace.

The reconstruction of the baobabs’ floral evolution (Fig. 40) suggests that the ancestral pollination system was by hawk moth with two parallel switches to mammal pollination by section Adansonia in Africa and section Brevitubae in Madagascar (Baum et al. 1998). A phylogenetic analysis by Baum (2003) suggests that the six Malagasy species form a monophyletic group of uncertain relationship to the African and Australian species (Fig. 41).

Fig. 40 Hypothesis for pollination and floral evolution in Adansonia under the assumption that hawk moth pollination is plesiomorphic, i.e. a primative state whose origin can be traced back to a remote ancestor. The relative length of the staminal column (white) and filaments (grey) and the bud shape are shown schematically for each extant species. The ancestral state for these characters under parsimony is shown at the root. Some floral characters are mapped onto this phylogeny based on parsimony. (From Baum et al. 1998.)

Speciation is the division of populations into evolutionarily independent units involving genetic separation and phenotyic differentiation. Allopatric speciation, i.e. species divergence resulting from geographical isolation, is well documented. However, mathematical models have shown that sympatric speciation, i.e. speciation without geographical isolation, is possible. It is regarded as highly controversal and is only known from a few zoological examples Savolainen et al. (2006) have now successfully demonstrated that sympatric speciation is possible in the plant kingdom. Their researches on the two endemic species of Howea, on the remote Lord Howe Island, have revealed that substantial disjunction in flowering time was correlated with soil textures, while a genome scan indicated that a few genetic loci were more divergent between the two species than would be expected under neutrality, a finding consistent with models of sympatric speciation involving disruptive/divergent selection.

These findings are relevant to furthering our understanding of speciation in Adansonia. Table 24 shows a number of sympatric members of Longitubae in Malagasy conservation areas. Their presence at these locations does not necessarily imply that that is where they evolved but it does suggest that section Longitubae speciated in Madagascar. Note the early speciation of A. gregorii, which is believed to have evolved from proto-boab after it arrived in Australia, a result of its need to adapt to the new environment. The evolution of species in sections Brevitubae and Adansonia is not yet understood.

Baum et al. (1998) postulated from molecular clock inferences and fossil pollen records for closely related Neotropical taxa, that the earliest date for dispersal of

9.4 -10.5 Ma

3.6 Ma

Mammal pollination

Hawkmoth pollination

3.6 Ma

1.3 Ma0.9 Ma

1.4 Ma

A. gregorii

A. digitata

A. grandidieri

A. suarezens

A. rubrostipa

A. za

A. madagascariensis

Madagascar

A. perrieri

Fig. 41 Current estimate of Adansonia phylogeny based on DNA sequence data and morpholgy. One of the equally parsimonious reconstructions of pollinaton data is mapped into the tree. Branching times are estimated, based on internal transcribed spacer sequences using a molecular clock combined with the assumption that the divergence of Adansonia from other Bombacoideae occurred at 58 MaBP. These dates are very approximate but they do provide a useful working hypothesis. (From Baum 2003.)



Adansonia from Africa/Madagascar to Australia would be not later than 17 ± 6 MaBP, either directly by sea or by no longer extant populations across the northern Indian Ocean. By that time Australia would nearly have attained its current position, encompassing an extensive tropical zone (Quilty 1994) with suitable habitats available for baobabs. However, circumstantial evidence points to a later date, with fossil Bombacoideae pollen (Bombacacidites) appearing in Africa by the Paleocene–Eocene boundary (Germeraad et al. 1968).

It must not be assumed that the names of fossil wood and pollen necessarily represent species close to modern genera. For example, the fossil genera Bombacoxylon and Bombacacidites are unlikely to be more closely related to present-day Bombax than to any other Bombacoideae genera. Bombacacidites pollen from the Lower Eocene was found in Cameroon. In North Africa Bombacoideae wood from Bombacoxylon galetti and B. owenii were found in Late Senonian–Eocene deposits in Ethiopia; B. owenii was also found in Oligocene deposits of France and Tunisia, and during the Miocene, in Germany, Sardinia, Libya, Somalia and Pakistan. B. owenii syn. Dombeyoxylon owenii sensu Koeniguer (1967) was present in the Rio de Oro and Algeria in the Neogene. (Beauchamp and Lemoigne 1973; Boureau et al. 1983; Dupéron et al. 1996; Hinsley 2005).

Bombacoideae pollen, represented by Bombacacidites bombaxoides, appeared in south-eastern Australia by the early Eocene, reached New Zealand in the Oligo–Miocene, and then became extinct (Stover and Partridge 1973; Kemp 1978; Macphail et al. 1994).

In general, angiosperm taxa appeared in south-eastern Australia before New Zealand, notable exceptions being the Bombacoideae and, less certainly, the Tilioideae from within the Malvaceae (Macphail et al. 1994). Macphail et al. also reported the surprising occurrence of Camptostemon (subfamily Malvoideae) pollen in south-eastern Australia during the Late Eocene–Early Oligocene, a genus now confined to northern Australian and parts of Malesia, a biogeographical region that includes the Malay Peninsula, Indonesia, Borneo, New Guinea and the Philippines (van Welzen et al. 2005). The presence of both Bombacacidites and Camptostemon suggests a migration pathway through Antarctica. Thus, Krutzsch (1989) concluded that the main diversification of the Adansonieae in Africa probably occurred during the Eocene, 58–35 MaBP, from which Baum et al. (1998) argued that the deep splits within the genus Adansonia took place 17 ± 6 MaBP.

2.1 Adansonia digitata

2.1.1 African Palaeoenvironment

Originally part of the great southern land mass known as Gondwana, the African continent was created by the break up of Gondwana during the Late Jurassic and Early Cretaceous. Since separation the African land surfaces have undergone a number of changes, with episodic uplifting during the Cretaceous, Tertiary and

Quaternary, and each uplift has been accompanied by notable differential warping and tilting. Thus, all pre-existing landscapes were subjected to episodes of denudation and deposition (King 1978).

The separation of Madagascar from Africa began with initial fracturing in the Jurassic and was completed by the Middle Cretaceous c. 100 MaBP (Tarling 1972a). Assuming a Malagasy origin for Adansonia, this separation suggest that A. digitata arrived in Africa by sea dispersal. The presence of A. digitata in south-west Arabia accompanied by a number of East African species in the Dhofar coastal strip of Oman, is recognised by White and Léonard (1991) as indicating an extension of the Somalia–Masai regional centre of endemism of White (1983). This extension into Arabia is probably coeval with the invasion of Sahelian elements into Israel via the Nile valley during the Miocene, and its partial retreat during the Pliocene (Zohary 1962). Assuming a former contiguous distribution, A. digitata’s presence in Arabia preceded the separation of the African and Arabian plates by the rifting that gave rise to the Red Sea and the Gulf of Aden during the Miocene–Early Pliocene (Barbour 1961; Gass and Gibson 1969; Roberts 1969; Grove 1980).

It is very doubtful that the former distribution of A. digitata could have extended into India. The absence of early baobab pollen and of any mention in early Sanskrit writings is considered evidence for an introduction in fairly recent historical times (Burton-Page 1969). Although the Somali Current at times flows strongly past the Dhofar coast and then weakly to India, the sea transport of baobab pods to Dhofar or India is considered unlikely.

During the past 2 Ma of the Pleistogene, sometimes referred to as the Quaternary (an unstable term based on the appearance of hominid fossils), the global changes in temperature had a profound effect on the palaeoclimates. The thermal climate was displaced southwards during a glacial due to an extension of the northern ice sheet. However, this displacement did not result in a southern extension of the intertropical convergence zone (ITC) in southern Africa because the westerly winds moved simultaneously northwards. Thus, the summer rainfall area not only diminished in size but received less rainfall. At higher latitudes in southern Africa the northerly shift of the westerlies during a glacial maximum brought cyclonic rain inland, possibly as far as 26° S, and a regime of winter rains to a vast area that today receives summer rains. This northward shift of cyclonic rains resulted in a marked lowering of the temperature.

The opposite development occurred during warm glacial and interstadial periods when the climatic belts moved southwards. The present savanna and woodland regions of southern Africa then received more summer rainfall. The zone of the westerlies shifted to higher latitudes so that the rain-bearing depressions could only reach a limited area of the south-west Cape in winter. This shift in the rainfall belt was accompanied by a considerable amelioration in temperature (Van Zinderen Bakker 1978).

Throughout the last two million years of the Quaternary the climate has fluctuated between wet phases (pluvials) and dry phases (interpluvials). During the drier phases aeolian sand from the north was deposited over the Sahelian land surface as free-draining dunes and deep sand sheets. During the dry phase, c. 20,000 BP,



savanna covered much of lowland Africa and the rain forest had contracted to such an extent that it offered no barrier to an east to west migration of the baobab. The period 20,000–12,000 BP also witnessed the spectacular advance of mobile dunes, which stretched from the Atlantic coast to as far east as the Nile, with their southern margins more or less parallel to the present-day limit of active dunes but 600–400 km further south.

Over large areas of Sudan’s Northern Kordofan Province the now stabilised dunes are up to 20–30 m high and the sand sheet as deep as 20–30 m. The baobab is unable to grow in such deep, unconsolidated sands. In opposition to the belief that glacial periods in the high latitudes were coeval with pluvial periods in the low latitudes, Williams (1975) reported sound evidence for late Pleistocene aridity in many parts of the tropics, with late glacial, tropical African aridity associated with the formation of the Sahel’s 5,000 km long belt of fixed dunes. In the Sudan these dunes were associated with a southward shift of the isohyets by around 200–450 km (Warren 1979).

2.1.2 Palaeoenvironments and Adansonia digitata

The present-day distribution of A. digitata within the chorological divisions of White (1983) is discussed in Chapter 13 and shown in Fig. 32. The relatively flexible chorological divisions correspond approximately to the more traditional hierarchial classification by kingdoms, regions and domains.

The distribution of the baobab in Kordofan along fossil drainage lines is dis-cussed in Chapter 13. This is the only evidence in western Sahel of the baobab’s association with a riverine system, whereas in West Africa and south of the equator the baobab is largely found in the major river catchment areas. Further study of the pre-Pleistocene drainage systems of the Sahel should tell us more about the baobab’s association with water.

The present disjunct distribution of the baobab in West Africa requires an expla-nation. Schweinfurth in Chevalier (1906) was of the opinion that the baobab may have been continuously distributed from the Atlantic to the Indian Ocean and sug-gested that the link had been destroyed by grass fires. However, while a previous continuous distribution may have been the case, fire is unlikely to have been the main cause of the disjunction. It may, however, have contributed to the maintenance of a disjunct distribution resulting from the vicissitudes of the Pleistocene sand invasions following the regression of Mega-Chad.

Assuming a pre-Quaternary trans-Sahelian distribution for the baobab, its isolation in West Africa could be attributed to the destructive effects of flooding in the Chad basin during the Quaternary pluvials and the formation of the Mega-Chad lake (Fig. 42).

The last major flooding, c.10,000 BP, covered an area of 40,000–300,000 km2, and was followed by several minor fluctuations of water level. This last major flooding is important phytogeographically since it would have resulted in the

destruction of all baobabs in the basin. The subsequent failure of the baobab to re-establish itself across the Chad basin may have been due mainly to the deep, unconsolidated, sand deposits formed during the pre-12,000 BP dune incursions (Hunting Technical Services 1964; Grove and Pullan 1964; Grove and Warren 1968; Warren 1970; Hamilton 1976; Dumont 1978; 1982; Wickens 1976; Grove 1977; Talbot 1980). The distribution maps of Wickens (1976) show a number of species with similar disjunct distribution patterns.

The baobab is absent from the northward salient of Anogeissus leiocarpus savanna woodland on the stabilised sand sheets of the Qozes Salsilgo and Dango described by Parry and Wickens (1981) to the south of Jebel Marra.

The fixed dunes of these two sandsheets demonstrate that the Sahara at one time advanced over 500 km to the south of Jebel Marra. The fossil oil palms (Elaeis guineensis) in the southern foothills (Wickens 1975) and diatomite evidence for a large lake at Barbis during the early Pleistocene, 50,000–1,000 BP, strongly suggest that rain forest reached Jebel Marra via gallery forests, and that the northern limits of the savanna formerly lay well to the north of Jebel Marra (Williams et al. 1980). The Jebel Marra massif complex and its northern outlier, Jebel Gurgeil, together with the Qozes Salsilgo and Dango to the south, now form an effective barrier to any westward migration of the baobab. It is possible that the joint effects of a very high saturation deficit and the low relative humidity of the Sahel inland (Le Houérou et al. 1993) negated the effectiveness of what would otherwise be an adequate rainfall for germination and establishment.

Chevalier (1906) provided an interesting alternative theory, that the baobab was native to the Atlantic coastal regions and had been gradually introduced further and further eastwards into the interior of Africa by man. This implies that the genus originated in tropical America and migrated to western Africa, whereas it is now

Altitude above 1000 m Northern limit of savanna duringthe PleistoceneSouthern limit of Pleistocene sandinvasion

Lake Mega-Chad

Alluvial and lacustrine deposits ofthe White Nile and its tributaries

Fig. 42 The Sahel environment during the Pleistocene. (From Wickens 1974; Talbot 1980.)



believed that Madagascar was the putative centre of origin. Nevertheless, Chevalier’s proposal does suggest an alternative explanation for the isolation of the West African baobabs. After the baobab had spread across southern Africa, the northward flowing Benguela Current could have transported fruit from Angola to the Gulf of Guinea. This intriguing idea suggests that the baobab may not have been introduced by man to islands in the Gulf of Guinea and the Cape Verde Islands. Mega-Chad and the deep sands of the Sahel ensured that the baobab and other Sahelian species did not spread eastwards across Chad to the Sudan.

Although the baobab is present in the broad-leaved woodlands of the Sudanian region of West Africa, there are no records of it in the botanically little-explored southern Chad, northern Central African Republic and southern Sudan, suggesting a disjunct distribution between the northern Sudan and the Somalia Masai region, which requires an explanation.

During the early Holocene, c.11,000–7,000 BP, the climate was warm and wet. There were high water levels in both the White and Blue Niles, with overbank flooding of the Blue Nile. The flow of the White Nile was impeded, suggesting extensive swamps along the White Nile and its tributaries (Adamson et al. 1982; Wickens 1982b; Williams et al. 1982). It is possible that the alluvial clays of the White Nile and Bahr el Arab flood plains effectively isolated the baobabs from the Anogeissus–Khaya–Isoberlinia woodlands of southern Sudan, causing the disjunct distribtion between the Sudan and Kenya. Fire and overgrazing would have prevented any recent recolonisation of suitable areas within the edaphic grasslands of the White Nile and Bahr el Arab flood plains.

Fluctuations in equatorial rainfall during the Miocene–Pleistocene must have brought about expansions and contractions of the rain forest and other communi-ties. Contractions of the rain forest would have enabled the baobab to spread from eastern Africa to Angola, or vice versa.

Maley (1991) believed that from 9,000 BP the rain forest expanded during a period of high rainfall to cover the Dahomey Gap. The Gap reappeared during the Late Holocene, c. 3,700 BP. This was associated with the upwelling of cold water in the Benin Gulf and the reappearance of the ‘little dry season’ inland. The ‘little dry season’ was usually during August and September, as opposed to the ‘great dry season’ from December to February. Assogbadjo et al. (2006) believe that this was the period when the baobab probably arrived in the Dahomey Gap.

An increase in rainfall would have resulted in a considerable decrease in mopane-baobab woodlands (or the equivalent community) of the Limpopo, Zambezi and Cunene valleys. A decrease would have produced a northerly shift in the vegetation, accompanied by an increase in mopane–baobab woodland at the expense of Brachstegia–Acacia–Isoberlinia open woodland in the east, while mopane–baobab woodland would have extended northwards along the coastal plain of Angola (Cooke 1964; Axelrod and Raven 1978; White 1993b).

If the West African population originated in Angola or East Africa via the Sudan, there may still be some taxonomic or DNA evidence to confirm this. More fieldwork and herbarium collections are clearly required.

2.2 Malagasy Species of Adansonia

2.2.1 Malagasy Palaeoenvironments

At present there is insufficient information to establish an accurate picture of the early climatic history of Madagascar and of biome development for the whole island during the Tertiary period, 65–2 MaBP. Today, the regional plant formations correspond well with the climatic zones which, in turn, appear to conform to topography, plate tectonics, and regional patterns of air and water circulation. By considering the origins of the climatic zones Wells (2003) was able to speculate on changes in vegetation patterns.

From mid- to late Cretaceous, Madagascar may have had a moist, temperate climate, particularly along its coasts. It may have been during this period that the present-day cool, dry, interior highland floras and cool, moist montane floras originated in the lowlands and retreated to the highlands. As Madagascar drifted southwards during the Paleocene and Eocene much of the island passed through the subtropical arid zone with, starting in the north, widespread arid environments probably preceding humid forests. Since Madagascar is smaller than the arid zone through which it passed, it is possible that its passage was accompanied by a widespread loss of biota. As it emerged into the trade-wind belt, moister conditions prevailed in the east and north, while the west and south-west, being in the rain shadow of the central highlands, remained dry. From this conceptual picture Wells (2003) deduced that spiny deciduous bush thicket is older than the rain forests of the east and is now much reduced in area.

In view of the intense post-Eocene lateritic weathering in the east and the position of Madagascar throughout the Eocene astride the subtropical arid belt, it is likely that rain forest conditions started on the eastern watershed during the Oligocene. The contraction of the arid zone into the south-west, as Madagascar drifted into somewhat moister tropics during the Paleocene to Eocene, would have been accom-panied by an increase in deciduous semi-arid to subhumid forests from what is now Toliara to north-east of Ankarafantsika. The far north-west would have become significantly wetter with the onset of the Indian monsoons about 8 Ma ago or later making the Sambirano forest the most recent biome (Wells 2003).

In the extreme south, the end of the Neogene, 2 MaBP, corresponded to a period of high seasonal rainfall, followed by the establishment of coastal dunes and periods of fixation and rubefaction, i.e. pluvials and inter-pluvials. The Holocene witnessed a progressive decrease in total annual rainfall to the present, when the conditions are semi-arid with irregular rainfall. In contrast to the extreme south, the central uplands were predominantly dry during the Neogene. In the Pleistocene there followed three pluvials. The second pluvial was followed by an inter-pluvial from 35,000 to c.10,000 BP, and a third pluvial from 10,000 BP (Battistini 1996).

The Malagasy environment is usually constructed as a continuous climax forest prior to the arrival of humans (Perrier de la Bâthie 1921; Humbert 1927), a picture



implying a catastrophic destruction of the forest following colonisation. However, pollen samples from near a carbon-dated vertebrate subfossil site at Ampasambazimba in central Madagascar indicated that a mosaic of woodlands, bushlands and savanna existed in c. 8,000–7,000 BP. Although most of Madagascar’s extinct ‘subfossil’ fauna are believed to have been forest dwellers, several are plausibly placed in more open habitats, such as that inferred for the Ampasambazimba area. The bizarre and highly endemic xerophytic flora of southern Madagascar does not tally with the idea that the island had previously been covered with a continuous rain forest.

Because dry savanna woodland burns more readily than dense forest, colonisation and fire would have transformed the savanna environment. Once cleared, the original forest would have rapidly been replaced by secondary growth and then by grassland. Continuous burning thus soon brought about an irreversible environmen-tal destruction (Burney 2003).

2.2.2 The Phytogeography

The phytogeographical divisions of Madagascar (Fig. 34) recognised by White (1983) follow the major divisions of Humbert (1955).

The East Malagasy region embraces the central mountains and eastern plains. Apart from the Sambirano domain, a western coastal outlier of the region, it is of no further interest to this present study.

The West Malagasy region, in which two domains are recognised, occupies the western side of the island from sea level to about 800 m.

The Southern domain of White (1983) is represented in south-western Madagascar by a limestone escarpment up to around 400 m high, descending to a narrow coastal plain. The region lies in the rain shadow of the mountains; the south-east monsoon provides an annual rainfall of 300–500 mm, which comes mostly in the summer as local heavy showers, although some rain may fall during every month the year. A cold current parallel to the coast causes atmospheric moisture to condense over the sea, thereby increasing the aridity of the winds reaching the land. Even so, nightly dew during the dry season months of March to November may be a useful source of moisture for the plants. The dry season normally lasts at least 8 months, although droughts may last 12–18 months.

The vegetation consists of deciduous thicket dominated by the endemic family Didiereaceae, especially species of Didierea and Alluaudia, with arborescent species of Euphobia codominant. In addition to the above the pachycaul habit (see Chapter 12) is represented by Adansonia and Pachypodium (UNESCO 1979; White 1983; Rabesandratana 1984; Walter and Breckle 1986).

The genus Adansonia is well represented in Madagascar by six indigenous species, compared with one (A. gregorii) in Australia and one (A. digitata) in Africa. The latter is also found around Mahajanga (Majunga) in Madagascar, where it is believed to have been introduced by Arab traders.

The Madagascan baobabs are restricted to the West Malagasy region and the Sambirano domain. A. suarezensis occurs in the Northern sector of the Western

Domain and A. grandidieri in the Middle West sector; A. perrieri is endemic to the Sambirano domain and A. madagascariensis is found in the Northern and Ambongo–Boina sectors of the Western domain. A. rubrostipa (syn. A. fony) and A. za occur throughout the Western domain except for the Northern sector; both penetrate the Southern domain, where they are constituents of the scrub vegetation.

2.3 Adansonia gregorii in Australia

2.3.1 Australian Palaeoenvironments

The interpretation of Australian palaeoenvironments is rather speculative since the bulk of the evidence is from south-east Australia, while the north-west has been poorly investigated. A. gregorii only occurs in the north-west, where there is no fossil plant evidence for past environments.

Severe aridity set in during the Pliocene, 5–2 MaBP, resulting from a steepening of the latitudinal temperature gradient, an intensification of the subtropical high-pressure ridge, and an expansion of the Antarctic ice cap. The extreme wet–dry glacial cycles of the present climatic system set in around the Mid-Pliocene, 2.0 MaBP, replacing the mild wet climate of the Early Pliocene. Araucariaceae, Nothofagus and Podocarpus disappeared from the southern latitude; thee was a rapid increase in shrubby and herbaceous members of the Compositae, Chenopodiaceae and Gramineae during the arid periods, and of a sclerophyllous flora dominated by Acacia and Eucalyptus during the wetter periods (Crisp et al. 2004).

During the Late Cretaceous there was a general retreat of marine conditions from central Australia. The drainage divide moved north, with run-off from most of Australia generally to the south-central coast. In south-east Australia cool conditions continued until the Santonian, 87.5–83.0 MaBP, when the area became much warmer and more humid, remaining so until the end of the Cretaceous. Along the west coast sediments are predominantly of biogenic-carbonates containing fossils, indicating warm and, at times, subtropical waters. Drainage was localised around the cratonic blocks of the region and was minor, except in the north.

During the Paleocene–Early Eocene, southern Australia had a temperate climate with high and probably seasonal rainfall, while along the north-west coast the climate graduated from wetter, warm temperate to subtropical conditions in the Carnarvon Basin to hotter, drier or arid conditions with seasonal rainfall to the north-east. Central Australia drained inland to the newly formed Birdsville Basin. These conditions persisted until the Middle–Late Eocene, when a general cooling began. This temperature decline was most noticeable around the Eocene–Oligocene boundary and persisted through the Oligocene, probably accompanied by more marked seasonality of precipitation and greater seasonal and diurnal ranges of temperature. Associated with the accumulation of polar ice, this trend continued into the Miocene, superimposed on a general trend towards falling temperatures and increasing aridity, with periods of major and frequent climatic fluctuations



during the Pliocene and Pleistocene. In the Late Pliocene, probably c. 2.4–2.4 MaBP, there was a period of glaciation in southern Australia, producing drier conditions in central and northern Australia (Kemp 1978; Truswell 1993; Carpenter et al. 1994; Quilty 1994; Frakes 1999).

Quaternary climatic studies by Rognon and Williams (1977) found a curious parallelism between the Late Quaternary climatic history of the African Sahel and that of southern Australia. The authors suggest heavy rainfall and lower temperatures in southern Australia between 40,000 and 20,000 BP, dry conditions and less mesophytic vegetation in the tropical margins, and aridity in north-east Australia. From 17,000–12,000 BP these conditions were followed by aridity, dune building, low temperatures and less mesophytic vegetation along the tropical margins, and desiccation in semi-arid New South Wales. There was increased precipitation and more mesophytic vegetation in both northern and southern Australia from 11,000 to 5,000 BP, followed by decreased rainfall. In north-west Australia shorter trees of narrower girth, better able to tolerate lower rainfall and longer dry seasons, replaced the tall mangrove forest of 7,500–6,000 BP. To the north of Darwin, on Melville Island, nests of jungle fowl (Megapoides sp.) have recently been radiocarbon-dated; the age and distribution of these nests indicates that the rain forests have progres-sively retreated between 8,000 and 2,000 BP. In north-east Queensland the pollen spectra from several sites on the Atherton Tablelands showed an increase in sclerophyllous woodland and a decrease in mesophyll forest elements, suggesting a decrease in effective rainfall.

Stable seif dunes now extend onto the continental shelf of north-west Australia. Holocene shallow-water marine and estuarine clays rest unconformably on these dune sands, which lack a soil profile and were probably active until the Holocene (Jennings 1975; Williams 1975).

Information is lacking about how tropical anticyclones behaved in the southern hemisphere between 12,000–10,000 and 6,000–5,000 BP, when both the temperate and tropical borders of Australia were more humid than at present. There is also a dearth of accurate information on the climate of central Australia prior to European explorations in the 1850s.

The Aborigines arrived in Australia from Asia at least 40,000 and possibly 60,000–100,000 years ago. It was a time when sea levels were very much lower and Australia was still connected to Papua New Guinea and Tasmania; the extended continent was separated by sea from Asia by a distance of only 100 km (Johnson and Hempstead 2000).

Following Australia’s separation from Antarctica, its flora developed in isolation, apart from those taxa capable of dispersal and establishment across the oceanic gap. Prior to separation the dominant vegetation of the south-east was rain forest dominated by Nothofagus, and included many taxa present in northern Queensland today. In central Australia the floras resembled the present-day, seasonally dry sclerophyll biome of Eucalyptus woodland, forest and heath, and the monsoonal tropic biome of Eucalyptus and Acacia savannas. The sclerophyll flora had probably existed on swampy oligotrophic soils at forest fringes from the Late Cretaceous onwards.

Global climatic change resulting from a steepening of the latitudinal temperature gradient led to the onset of glaciation in Antarctica and a significant cooling in the Australian Oligocene. Between 25 and 10 MaBP the Australian climate became drier and more seasonal as the continent moved north into the subtropical high-pressure ridge, which blocks the rain-bearing low-pressure ridges (currently about 30° S). The fossil pollen record from this period shows a steep decline in the domi-nance of Nothofagus and its replacement by a sclerophyll flora dominated by members of the Bataceae and Casuarinaceae. According to Herbert (1950), taxa forming Australia’s xeric vegetation can readily be linked with ancestral forms found in more mesic sites bordering present-day arid and semi-arid areas.

The hypothetical dispersal of Adansonia from Madagascar to Australia must not be considered in isolation from that of other biotas with similar distribution patterns. According to Beadle (1981) only six genera are shared between Africa/Madagascar and Australia. This is a poor tally in comparison with about 40 shared between Australia and South America, an assemblage representing a former Gondwana migration via Antarctica. In addition to Adansonia and the composite Helipterum there are four monocot genera common to Africa and Australia: Bulbine, Caesia, Iphigenia and Wurmbea. Iphigenia in Australia is an extension of one of the six species in India. A taxonomic revision has shown Helipterum to be represented by Helichrysum in Africa, and by Rhodanthe and Syncarpha in Australia (Mabberley 1997). The above taxa probably represent a much earlier land migration around the Indian Ocean. None is common to Madagascar and Western Australia. It therefore seems that Adansonia was either a much later accidental marine migrant to Australia or the sole survivor of a number of such migrants. Although pollen evidence is lacking, the accidental migrant hypothesis is the most probable.

2.3.2 Origins and Dispersal of Adansonia gregorii

A. gregorii, like its closest relatives in Madagascar, is pollinated by hawk moths. The distribution of hawk moths suggests that they only arrived in Australia during the Early Miocene, after Australia had collided with the Asian plate c. 27 MyBP. As it is unlikely that hawk moth pollination could have been lost and regained, it can be assumed that the earliest that proto-boab (ancestral A. gregorii) could have arrived in Australia was during the Miocene. The palaeoclimates of northern Australia are poorly known but since the continent had crossed into the tropics shortly before the Miocene, it can be assumed that when proto-boab arrived in northern Australia the climate was similar to that of Madagascar (Baum and Handasyde 1990).

Proto-boab is believed to have arrived by dispersal of fruits over the sea from Madagascar (Beard 1981). There is no clear evidence as to when this event occurred or of the distances between the continents as they drifted apart. From molecular clock inferences and palaeopalynological records for closely related neotropical species dispersal from Africa and Madagascar to Australia, Baum et al. (1998)



postulated that Adansonia arrived in Australia in the early Miocene to Pleistocene, 17–7 MaBP. The dispersal could have been by sea (5,400 km currently separates Australia from Madagascar) or indirectly via extinct baobab populations that may have been growing along the northern rim of the Indian Ocean. Despite the close proximity of Africa, the Madagascan flora exhibits a remarkably high affinity with the Asian floras (Schatz 1996).

Blench (2006b) speculated that linguistic and botanical evidence for contact between Austronesians and the north coast of Australia raises the possibility that pods of proto-boab may have been carried for food by coastal movements of early modern humans or by Austronesian seafarers en route from Madagascar. The findings of Baum et al. (1998) for an early Miocene introduction do not support this idea.

The dispersal of seeds or fruits by sea over large distances is almost entirely determined by currents; the palaeocurrents may not have flowed in the same direction as they do today. Speculation that Macassans or other visitors introduced the baobab can be discounted: speciation from an unknown parentage would have been impossibly rapid.

For sea dispersal the propagules must have access to the sea, either directly or by means of a convenient river. They have to be capable of floating, even when encrusted with barnacles and marine algae, as well as resistant to the toxicity of saline water and the activity of marine borers. Finally, they have to arrive in a viable state in a favourable environment, where the seeds can germinate and the seedlings compete with the existing vegetation and become established. A freak tidal wave could deposit the propagule far enough inshore for establishment. The Kimberley coastline varies from wide beaches with a gentle gradient to high cliffs. It experiences tidal waves up to 11 m high, as at Derby. Cyclones during the wet season cause tidal surges, which could easily carry floating baobab pods inland and deposit them on fertile ground (Lowe 2003, personal communication). Successful establishment may also depend on the plant being self- or cross-pollinated; suitable pollinators and resistance to local pests and diseases are also necessary.

The currents of the Indian Ocean are shown in Fig. 43.During the north-west monsoon (February and March), the North Equatorial

Current and the South Equatorial Current flow towards the west and on reaching East Africa swerve south, the weaker Equatorial Counter Current flowing eastwards between them.

During the south-west monsoon (August and September), the Equatorial Counter Current seems to disappear and the North Equatorial Current reverses its flow and becomes the Monsoon Current, flowing eastward and south-eastward across the Arabian Sea and the Bay of Bengal. Near Sumatra this current curves westward clockwise and augments the South Equatorial Current, which circulates clockwise in the northern Indian Ocean. The Somali Current also reverses direction during August and September, and northern flowing currents attain speeds of 5 knots or more.

As the South Equatorial Current approaches Africa it curves south-west and divides, a part flowing through the Mozambique Channel and part along the east coast of Madagascar. The two branches then unite to form the swiftly flowing

Agulhas Current. To the south of South Africa the Agulhas Current joins the eastward flowing Antarctic Circumpolar Current, also known as the West Wind Drift. A small part of the Agulhas Current rounds the Cape of Good Hope and augments the Benguela Current. The cold Antarctic Circumpolar Current flows eastwards across the southern Indian Ocean to south-western Australia, where it splits. One branch continues along the southern coast while the other branch flows northwards along the western coast.

In the eastern Indian Ocean the seasonal flow of the South Java Current is affected by the monsoon winds so that its strongest westward flow during August coincides with the easterly monsoon winds.

Throughout much of the southern Indian Ocean the currents are highly variable in seasonal strength and direction. The Equatorial Counter Current has a definite

Fig. 43 Currents in the Indian Ocean based on Bowditch (1984). (1) Agulhas Current; (2) Somali Current; (3) south-west Monsoon Current; (4) North Equatorial Current; (5) Equatorial Counter Current; (6) South Equatorial Current; (7) Java Current; (8) Leeuwin Current; (9) Western Australian Current; (10) Antarctic Circumpolar Current.



eastward flow from late April to early June, when the direction changes to a westward flow which again reverses in late August to late November to flow eastwards again. The eastward flow extends from the coast of Africa to 90–95° E, with surface speeds of between 1.85 and 5.55 km hr−1 (1 and 3 knots). Between 85–95° E the current weakens to 1 knot or less and, as the South Java Current, flows south-east past the coasts of Java and Sumatra and then southward and westward across the westward-flowing South Equatorial Current. The latter has its origins in the Pacific, and prevents the South Java Current from entering the Timor Sea. The shallow, eastward flow, thereafter known as the East Gyral Current, heads towards Australia, feeding the Leeuwin Current off the west coast of Australia. The warm offshore Leeuwin Current flows south for only half the year; during the summer months prevailing southerly winds halt the southern flow, which is partly replaced by the northward flowing West Australian Current, which in turn joins the South Equatorial Current.

Ocean currents do not follow rigid pathways; they vary annually, seasonally and interseasonally. Although most of the shallow water along the Kimberley coast comes from the Pacific via Indonesia, it is possible that some drift material milling around the Timor Sea approaches the Kimberley coast (Bowditch 1984; Cutler and Swallow 1984; Fox 1999; Craig 2004; Wijffels 2005, personal communication).

A tsunami generated by an earthquake along the Java Trench could carry drift mate-rial in the shallow waters bordering the Kimberley coast and deposit it well inland.

The speed of a tsunami is reduced by the rapid braking effect of shallow water, which causes an increase in wave height and run-up, i.e. the distance the wave travels inland to where the ground reaches wave height; in August 1977, a tsunami at Cape Lévêque had a run-up of 6 m.

A tsumani is also influenced by the state of the tide. For example, between Broome and Darwin, tides are semi-diurnal (two high and two low waters every day) to semi-diurnal with diurnal inequalities, and with an average range of 2.1–7.6 m. In the early 1980s, a major tsunami near Darwin came at low tide, which cancelled out most of its force. Three hours earlier and there would have been considerable damage (Murty 1994).

There is slight evidence that drift seeds/fruits reached northern Australia by the ocean currents described above. The Aldabra archipelago lies in the path of the predominantly westward-flowing Equatorial Counter Current but, at various periods of the year, may also be acted on by currents flowing north from the east coast of Madagascar or north-east from Africa. Drift material known to have been trans-ported to Aldabra include a baobab pod with viable seeds, possibly A. digitata from Africa (Wickens 1979a, 1982a). The fruits of coco de mer (Lodoicea maldivica), the endemic palm of the Seychelles, have also been found on Western Australian shores, supporting the hypothesis of a direct route from the west. Such evidence is inconclusive since these fruits are often collected by tourists and the ones found in Western Australia may not have come directly from the Seychelles. An ostrich (Struthio camelus) egg, origin unknown, was retrieved from a dredge in the Timor Sea. The egg, heavily weighed down with algae, was not floating but suspended in mid-water (Long, 1993; Long et al. 1998). Brown (1985) suggested that Adansonia

fruit, seed or even whole trees might have drifted like that to the Kimberly coast, although it is doubtful whether baobab trunks or unprotected seeds could have survived such a long sea journey.

The alternative route from Africa and Madagascar is via the Mozambique and Aiguihas Currents, connecting with the eastward-flowing Antarctic Circumpolar Current. The mean drift speed of the Aiguihas Current is around 7–8 cm s−1, so proto-boab would have taken about 200 days to drift southwards down the east coast of Africa, to reach the Antarctic Circumpolar Current which has a mean surface speed 20–25 cm s−1 (Rapley 2002, personal communication). The tree, pod or seed would have taken another 120 days to reach south-west Australia, giving an estimated total flotation time of at least 320 days.

Whether the proto-boab then drifted northward to the Kimberly region or migrated overland is uncertain. An arid belt had created a migration barrier preventing any spread of plants to either east or north-west Australia (Herbert 1950), making an overland migration of the proto-boab very unlikely. However, it must also be remembered that, during the Mesozoic, Australia was closer to Madagascar and the south-western corner was in a more northerly position relative to the rest of Australia. The Austalian plate has since rotated in an anti-clockwise direction relative to the Mesozoic longitudinal positions (Beadle 1981). Evidence from present-day currents is inconclusive as to the route taken by proto-boab to reach north-west Australia, although the northern and more direct route is more probable.

It is not uncommon to find African plant material washed up on the south-west coast of western Australia (Kenneally 1972). During the summer months, the West Australian Current flows north to the Timor Sea and Gulf of Capentaria at an aver-age surface speed of less than 1 km hr−1 (Hydrographer to the Navy 1973). The current reverses direction during the winter months, flowing south to the North West Cape, after which it becomes somewhat erratic. Whether the eastward Southern Equatorial Current joins the West Australian Current past the Kimberley coast is not certain.

More interesting than the finds so far mentioned is the discovery of a 2,000-year-old fossilised egg of the extinct Madagascan elephant bird or flying dragon (Aepyornis maximus) in Holocene dunes overlying lagoonal deposits, near Cervantes, 150 km to the north of Perth. Did the Southern Equatorial Current carry it cross the Indian Ocean and transport it south, or did the West Australian Current carry it north?

In the south-west, another elephant bird egg of unknown age was found near Augusta, at the mouth of Scott River, as were two eggs of the king penguin (Aptenodytes patagonicus), whose nearest known habitat is the Kerguelen Islands, some 2,000 km away in the southern Indian Ocean (Long 1993; Long et al. 1998; Lowe 1998). The close proximity of the three eggs suggests they were transported by the Antarctic Circumpolar Current.

It should be borne in mind that proto-boab arrived in an enviroment where it had to adapt and compete with an existing flora. The substantial taxonomic differences between A. gregorii and other members of section Longitubae suggest that A. gregorii evolved over a lengthy period and is not a recent arrival in Australia. In the absence of any pollen record, the possibility of extant boab populations, with a much wider



distribution that has since contracted, through fires or human activities, cannot be ruled out. Pre-industrial humans arrived in Australia approximately 56,000 ± 4,000 years ago and, while their impact on the vegetation is controversial, we do know that in most regions they used fire to manage it. Certainly within 16,000 years following the arrival of man, all Australian land mammals, reptiles and birds weighing more than 100 kg had become extinct, but whether man was indirectly responsible for these extinctions through changes he made to the environmental is debatable (Hope 1994; Roberts et al. 2001). The role of extinct megafauna in boab seed dispersal is unknown.

There are no Tertiary fossil records for north-west Australia; indeed, there are no pollen records of Adansonia for anywhere in Australia. Bowman and Connors (1996) have put forward the hypothesis that the eastward distribution of the boab on the Sturt Plateau was stopped by large tracts of land occupied by Acacia shirleyi, a relict species from the dry, cold climates of the last ice age c.17,000–15,000 BP. Nevertheless, in view of the wide ecological amplitude exhibited by A. gregorii, Bowman (1997) is puzzled by its limited distribution in north-west Australia.

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The Baobabs: Pachycauls of Africa, Madagascar and Australia || Phytogeography