Cape Fynbos – an overview from a paleo history perspective
There are hundreds of articles out there that start off telling us how wonderful the flora of the broader Cape region is. But what is it all about and is it really that special? In order to get my own head around how it all came about and produce a readable précis on the topic, I've summarized the article “Explaining the uniqueness of the Cape flora: Incorporating geomorphic evolution as a factor for explaining its diversification” written by Richard M. Cowling, Serban Proches and Timothy C. Partridge and published in 2009 in the journal Molecular Phylogenetics and Evolution. It summarizes much work by Timothy Partridge, the co-author of 'Paleoclimate and Evolution, with emphasis on human origins'.
The Cape Floristic Region—hereafter ‘‘the Cape” —comprises a centre of plant diversity and endemism that is unparalleled globally. The Cape has twice the number of species predicted by a normally reliable model developed for global plant diversity patterns where water-energy emerged as a core predictor. It appears that the landscape and the history of the landscape development play a role in creating Fynbos biodiversity. From the birds point of view, it is also important to understand paleo climates. For instance, it is thought that the Cape Rockjumper has its ancestral roots dating back around 50 million years. In the face of anthropogenically driven global climate change, what interests me is whether this species has survived through warmer times.
Landscapes and soils
Today's Cape landscapes are dominated by the very old Cape Folded Belt. Sedimentation started 450 million years (Ma) and the tectonic forces that created the mountains date back 280 Ma, respectively. The mountains comprise a series of parallel ranges of folded quartzites and quartzitic sandstone of the Cape Supergroup’s Table Mountain and Witteberg Groups. They are of moderate altitude (1000–2000 m) but steep. These areas are defined as montane. The mountains are highest in the west: all 14 peaks reaching ca. 2000 m are located west of 23E. Relief in the west is also steeper than the east, where many of the ranges have concave flanks, and precipitous slopes are uncommon.
The coastal plain (<300 m) and intermontane basins (450–1000m) are underlain by softer sediments (usually shales of the Cape Supergroups’ Bokkeveld Group and the Precambrian Malmesbury Group). The coastal margin is characterized by calcareous marine sediments (aeolian sand and limestone). These areas are defined as lowlands.
Cape soils are mostly sandy, infertile and course on the upper slopes of the mountains, with somewhat more fertile and deeper soils washed down to the foothills. Lowland soils are mostly derived from shales and are moderately fertile and relatively fine-grained. Sandy soils of marine and wind-blown origin, including those derived from Quaternary limestone, mantle the coastal margin. There is a gradient from west to east of increasing soil fertility and loam and clay content
across all sediment types in Cape landscapes.
Most of the Cape receives an annual rainfall of between 300 and 2000 mm a year, although some montane sites in the west receive as much as 3000 mm/year (near Jonkershoek). In the west, rainfall, which is associated with cold fronts budded off from the circumpolar westerly system, is concentrated in the winter months (i.e. mediterranean-climate conditions prevail). In the east, rainfall is less seasonal; post-frontal events, especially the advection of moist air across a relatively warm Indian Ocean, produce rain throughout the year, but especially in the spring and autumn. Overall, orographic effects on rainfall are massive and the associated gradients are extremely steep. The winter-rainfall component is significantly more reliable in the west than the east; the opposite holds true for summer rainfall.
On the lowlands, temperatures are generally mild: frost is seldom recorded and summer maxima seldom exceed 30 C, except in the interior valleys. In montane landscapes, temperature minima are more extreme than the adjacent lowlands: frost is widespread on upper peaks where snow may lie for several weeks in the winter months. Summer maxima seldom exceed 25 C.
The Cape is an extraordinarily diverse biogeographical region covering a mere 90,000 km2. The flora, which is relatively well known compared to other species-rich regions, comprises about
9000 vascular plant species, 69% of which are endemic. The flora includes five endemic plant families and 160 endemic genera. A spectacular feature of the flora is the massive diversification of some genera: almost half of the species is associated with 33 ‘‘Cape” clades, these having initially diversified in the region and of which at least half of the component species grow in the Cape. Thirteen of these Cape clades comprise more than 100 species each.
Plant species diversity is high at all spatial scales, a consequence of the high turnover of species-rich assemblages along environmental and geographical gradients. This high turnover is a product of the speciation of many habitat specialists - local endemism is extraordinarily high. Turnover and local endemism decline eastwards: similar sized western landscapes support about twice as many species as eastern ones. However, phylogenetic diversity is higher in the east, where Cape clades coexist with a diverse array of lineages whose distributions extend northeastwards into tropical Africa.
Late Cenozoic geomorphological evolution of the Cape
While much of the Cape landscapes are uniformly ancient, many of the geomorphic features that today support specialist floras are relatively young, dating from the early Pliocene. Climate-wise, during the Cenozoic the Earth had begun a drying and cooling trend, culminating in the glaciations of the Pleistocene Epoch. The Cenozoic can be subdivided into seven epochs, three of which are of evolutionay relevance to the Cape in terms of geomorphology – the Oligocene, Miocene and Pliocene. This account of the geomorphological evolution of the Cape starts in the Oligocene, a time when Cape clades began radiating and before tectonic uplift rejuvenated Cape landscapes.
Oligocene (33.7–23.8 Ma)
At the start of the Oligocene, the African cycle of erosion had been in operation for some 100 million years, ever since the emergence of Africa as a separate continent. The Great Escarpment is a product of the backwards erosion of a scarp from the coast inland, which cut into the elevated (1800–2200 m) Gondwanan surface. It separated a high, interior plateau from a coastal plain. This coastal plain included the Cape region as we know it today.
The Cape Fold Mountains (hereafter Cape Mountains) were largely reduced to the eroded cores of resistant quartzites; softer rocks—Precambrian sediments, Cape granites, Folded Belt shales, and Cretaceous sediments—underlaid the gently, coastwards sloping African Surface. However, the Cape Mountains were nonetheless significant topographical features in the Oligocene. Residual peaks and ridges comprised quartzites of the Peninsula Formation of the Table Mountain Group. As is the case today, mountain elevations were probably highest in the west; in the east, especially east of the Swartberg–Kamanassie Mountains, the Folded Belt was much subdued relative to today.
The greater erosion of the Folded Belt eastwards probably resulted from the greater extent of finer-grained sediments there, a consequence of deposition in an eastwards sloping basin. Eastern mountains would have had a more subdued relief than those in the west, and possibly supported deeper soils and less exposed bedrock. In other words, as a template for plant diversification, the west may have had a considerably larger extent of mountain scenery.
The lowlands (coastal plain and intermontane basins) of Oligocene times formed the African Surface, which was very likely uniformly capped by silcrete duricrusts. These duricrusts were a global phenomenon deposited in the earliest Palaeocene over hundreds of thousands, if not millions of years, probably as a result of climatic changes induced by the enormous quantities of dust and gases released by massive volcanism centred in north India. The silcretes, which were underlain by deeply weathered, kaolinised soils, were probably mantled by deepish, infertile, sands (podzols), derived principally from the weathering of the mountains.
The African Surface was drained by sluggish, meandering rivers that had cut winding paths seawards. Oxbow lakes, vast wetlands and extensive areas of nutrient poor alluvial sand and gravel, would have characterised the broad and shallow river valleys. The dry and cool Oligocene climate may well have resulted in the mobilization of alluvial sands to produce extensive sand plains. The initiation of the glaciation of East Antarctica would have caused a drop in sea level, perhaps as much as 500 m below today’s shoreline. However, rising sea levels associated with warmer (and wetter) climates in the late Oligocene (ca. 25 Ma) may have produced a broader dune belt.
In summary, Oligocene scenery in the Cape was considerably more subdued than today, with much lower topographic and edaphic diversity. Of great relevance for the diversification of the Cape flora is the complete prevalence in the region of infertile, sandy substrata, both on the lowlands and the mountains; the widespread occurrence of deep, leached sands on the lowlands; less mountainous terrain relative to today; a greater extent of deep, colluvial soils along the flanks of mountains; higher and more extensive montane habitat in the west; large areas of oligotrophic wetlands in riparian zones; and a relatively narrow cordon of coastal dunes.
Owing to a lower eastern escarpment, moist air from the Indian Ocean may have penetrated deep within the Cape, and summer thunderstorms may have been a common and widespread occurrence.
It seems likely that in the early Oligocene, the predominant vegetation in the Cape resembled contemporary subtropical thicket, a closed rainforest-like formation that was widespread globally at that time. Cape elements would have been restricted to wetlands (the home of ancestral Restionaceae) and on the small patches of skeletal soils in the mountains, these being too shallow to support thicket or forest. They may also have coexisted with thicket elements on the sandplains—forming ancient thicket-fynbos mosaics. Vegetation dominated by Cape elements may well have carried lightning-induced fires that in synergy with aridification began to erode the extent of thicket, as intense fires do today, creating habitat for the radiation of Cape lineages. The widespread occurrence in the Cape of nutrient-poor soils, which retard thicket colonization, would have promoted this biome switch.
Miocene (23.8–5.3 Ma)
The protracted period of tectonic stability that had prevailed in the Cape came to an end in the early Miocene, about 22 Ma, with the initiation of the Post-African I erosion cycle. This uplift caused by a hot plume of mantle material some 3000 km to 1000 km below southern Africa’s continental crust and persisting for about four million years, rejuvenated the subcontinent’s scenery. Uplift was greatest in the eastern part of southern Africa (250–300 m); the Cape, however, experienced less uplift, amounting to about 200 m in the east and 150 m in the west. This asymmetry greatly accentuated escarpment elevation in the east as well as the westward tilt of the interior land surface, resulting in renewed erosion of the upland plateau, where large volumes of Karoo sediments were removed.
The impacts of the Post-African I cycle on Cape scenery were not overly dramatic. The elevation of the Cape Mountains, including the hitherto buried sediments in the east, would have exposed fresh quartzite and sandstone bedrock as well as areas of Cape granites at the base of the north-trending mountains in the west. Rejuvenated drainage would have produced modest incision of the major rivers, exposing small areas of softer rocks on the lowlands and intermontane valleys. Unlike on the upper plateau, the silcrete duricrusts in the Cape were largely preserved during the Post-African I cycle; however, as result of the erosion of the overlying sandy mantle, silcrete bedrock was probably the most extensive substratum on the lowlands for most of the Miocene. River incision would have drained wetlands and exposed alluvial gravels. Given the low and variable winds that prevailed as a consequence of the hothouse climates that coincided with the Post-African I event, the coastal dunes would have been limited in extent. Sea levels as much as 150 m above the contemporary one were the norm; consequently, some half of the Cape coastal plain would have been inundated.
Despite its relatively small amplitude, the Post-African I event significantly altered the mix of habitats and substrata available for plant colonization in the early Miocene Cape. First, silcrete bedrock dominated the lowlands; second, for the first time in 40 million years, there appeared in the Cape, on a limited scale, exposures of fine-grained rocks and their associated clay-rich soils; third, relatively larger areas of montane quartzite and sandstone were exposed, especially in the east; fourth, gravels and shallow loamy sands would have replaced the deep, hydromorphic soils and associated wetlands of the major river valleys. Overall, the Post-African I erosion cycle increased habitat diversity in the Cape but landscapes were still considerably less heterogeneous than they are
The late Oligocene to middle Miocene (ca. 25–15 Ma) saw the return to the Cape of warm and wet climates, albeit with brief reversals to moderate icey conditions. The widespread establishment of subtropical forest formations in the Cape may have occurred at this time. The early Miocene saw the radiation of 4 Cape clades. It is likely this radiation was triggered by the Post-African I uplift that produced large areas of shallow, rocky soils, both on the lowlands (where silcrete was exposed) and in the mountains (where quartzite was exposed).
At about the middle Miocene, the climate began to deteriorate once again and the Benguela upwelling system and associated summer-dry climates of the western reaches of the Cape were established by about 10 Ma. Three Cape lineages diversified at this time, probably in response to the shift to drier, more seasonal conditions, which, in conjunction with fire, would have created habitat for open vegetation components.
Another three clades began radiating towards the end of the Miocene, at a time more-or-less coincident with initiation of the Post-African II cycle. This was a period of rapid cladogenesis
for many lineages, namely Phylica, subclades of Moraea , Ehrharta, the core Ruschioideae, Pelargonium Hoarea clade, and subclades of Muraltia. The explosion of environmental heterogeneity in the Cape caused by this uplift, acting in synergy with rapid cooling and drying, could explain the exceptionally high levels of diversification observed over this period.
Pliocene (5.3–2.6 Ma)
In the late Miocene, about 5 Ma, southern Africa experienced another uplift event, associated with the similar tectonic forces as the previous one. This Post-African II event, which lasted about two million years, was considerably more dramatic than the previous one, amounting 600–900 m uplift in eastern southern Africa. However, uplift in the Cape was less intense: 200–300 m in the east and 150 m in the west.
The Post-African II erosion cycle had a profound impact on Cape scenery. Renewed erosion incised the river gorges and resulted in rampant river capture. Almost all of the silcrete duricrusts and their kaolinised soils beneath them were stripped off the lowlands, revealing for the first time since the earliest Cenozoic extensive areas of shales and Cretaceous sediments (mudstones and conglomerate), all yielding soils more fertile and clay-rich than the adjacent mountains. Small areas of intact silcretes are preserved today on remnant African Surface landscapes throughout the Cape lowlands. These appear as flat-topped koppies on the coastal forelands or as residual plateaux along the flanks of eastern mountains. Lowered African Surfaces are well preserved throughout the Cape Mountains, but especially in the east where they prevail on relatively fine-grained sandstones (e.g. Kouga–Baviaanskloof, Zuurberg) and are associated with relatively fertile gravels and loamy sands. The interior rift basins of the east (e.g. Baviaanskloof–Gamtoos, Oudtshoorn) have been extensively dissected by the Post-African II event; here Cretaceous sediments have been stripped away, exposing deeply dissected and edaphically heterogeneous valleys.
|This photo, taken from the Kouga Mountains looking south, shows the dissected landscape of the Ancient African Landsurface in the foreground, with the resistent Tsitsikamma (Fomosa) quartzite mountains in the background.|
Starting in the late Miocene, renewed glaciation of Antarctica led to rapid cooling and a drop in sea level. The maximum sea level reached only 35 m higher than present. It seems likely that calcareous substrata would have accumulated along the coast, especially towards the terminal Pliocene as the regional climate grew increasingly cooler and drier.
By the end of the Pliocene, landscapes resembled very closely the ones of today. The elevated Cape Mountains achieved their rugged and dissected appearance, with large areas of exposed cliff and steep, rocky slopes, and numerous deeply incised gorges. The level, silcrete-capped lowlands were replaced on the coastal forelands by gently undulating, lowered African Surface landscapes underlain by relatively soft rocks that yielded clay-rich soils. Along drainage lines, these surfaces were incised (deeply in the east but only moderately in the west) exposing fresh bedrock. Dissection of interior rift basins produced finely sculptured landscapes of high habitat heterogeneity.
The Post-African II cycle created the relatively rare habitats that today support distinctive and endemic plant assemblages. These include the walls of deeply incised, narrow gorges; the lag gravels of quartz (quartz fields) of the Little Karoo region; the granite inselbergs of the west coast forelands; sheer walls of rock throughout the Cape Mountains; and rocky shale ridges (rante) on low-lying ground in the Little Karoo region.
While the radiation of Cape clades occurred throughout the late Cenozoic, speciation was most prolific during the Pliocene. By the end of the Pliocene, contemporary Cape climates, with their steep moisture and thermal gradients, had become established. It was during Pliocene times that climatic and edaphic conditions enabled the zonal development of now widespread Cape vegetation
types such as renosterveld, succulent karoo, limestone fynbos, dune fynbos and grassy fynbos
Quaternary (2.6 Ma-present)
The major geomorphic phenomena of the Quaternary were the deposition along the coast of large areas of calcareous aeolianites (including cemented and mobile dunes), calcarenites, and calcretes;
the deposition on the coastal forelands of reworked, alluvial sand as acid sand plains; and the exposure of the Plio-Pleistocene Bredasdorp Formation. These processes were a consequence
of the sawtooth icehouse–hothouse cycles of the Pleistocene. Large areas of coastal foreland were exposed during glacial times, when the seashore was ca. 60 km distant from the present one on the west coast, 200 km on the Agulhas Bank, and 100 km along the south–east coast. During transgressive phases, the strong Pleistocene wind regimes would have promoted the development of aeolianites. Thus, the distinctive Quaternary geomorphic feature for Cape plant diversification was the occurrence, probably for the first time in the Cenozoic, of large areas of calcareous sandy
substrata along the coast.
Relative to other mediterranean-climate regions, Quaternary temperature fluctuations in the Cape
were mild. This may have enabled the regional persistence during the extensive cold phases of the Quaternary of vegetation types and their associated floras in frost-free habitats such as the walls of incised valleys in the east, where the ancient, succulent thicket lineages would have retreated. The important point is that speciation in the Cape has probably continued throughout the Quaternary while extinction rates have been relatively low, and these processes have contributed to producing the extremely species rich flora of today. Generally, areas with stable contemporary climates, such as the Cape, have also had stable climates historically; owing to high rates of diversification and low rates of extinction consequent upon this, these regions all support rich biotas today.