Events

Flowers from The Cape

Professor John Parker (Director of Cambridge University Botanic Garden): An evolutionary extravaganza: the Cape Floral Kingdom

Introduction

Climate

Within South Africa, plants can be found growing in habitats that range from subtropical to alpine; in soils that vary from strongly acid to alkaline; and with rainfall that can be high or very low and which can fall during different seasons. It would not be possible to adequately cover the whole of this diverse flora within a one-day symposium and therefore we are looking at just one climatic region: the winter-rainfall zone in the western part of the country. The boundary to this is essentially from Port Elizabeth in the east to Springbok and the Orange River in the north; Cape Town being the principal city within the area. Thus most of the Western Cape Province plus western parts of the Northern Cape and Eastern Cape Provinces are subject to winter rainfall. The western part of this zone experiences a strict Mediterranean climate with hot dry summers and wet winters, but the area east of about Swellendam also gets some summer rainfall. The remainder of South Africa, which covers a much larger area, has summer rainfall and dry winters.

During summer, this area is dominated by high pressure cells and dry south-easterly winds that can be very strong as a result of the juxtaposition of cold sea and warm land; the Cape of Good Hope experiences more than one hundred days of gales each year. During the winter, a series of cold fronts move in from the southern ocean and produce rain on an almost weekly basis between June and August interspersed by a few days of beautiful sunshine. Earlier and later, the fronts are more sporadic and less predictable. Cape Town and the Cape Peninsula experience the full effects of the cold fronts, so that the annual precipitation is in the order of 60-70cm. The frontal systems weaken as they move north and only a few of them deposit rain by the time they reach Namaqualand. So in the northern areas the rainfall is much lower and unpredictable. The extensive displays of annual wild flowers that can occur during spring (August-September), are extremely dependant on rains at the right time. Only in favourable years are they truly spectacular.

Summers can be quite hot, with maximum temperatures of 30 - 40C, sometimes higher in places. Minimum winter temperatures are effected by altitude and the distance from the sea. Cape Town inhabitants are unlikely to ever see frost in their gardens, but the high mountains around Cape Town experience subzero temperatures and snow. The coldest part of the Cape is inland on the Roggeveld Plateau around Sutherland, where winter night-time temperatures often fall to -10C. As a consequence of these rainfall and temperature patterns, the main growing season for many of the plants being discussed, is in the cool moist winter months, something which must be born in mind when cultivating them overseas.

Geology

The base rocks within the winter-rainfall area are quite old. The granites, which dominate the landscape in the north in Namaqualand, were extruded through the earth’s crust during the Precambrian era. The sedimentary rocks which are predominant in the southern part of the region were laid down 340 - 450 million years ago. The most significant layer is a 3km thick sandstone band of the Table Mountain Group which is very evident in the Cape mountains and the Cederbergs. This weathers to form an acidic sandy soil that is very nutrient deficient. The sedimentary layers also include shales and limestones which are exposed in places. The limestones are very rare and occur at a few places between Bredasdorp and Cape Agulhas in the south and on the Knersvlakte further north. These support some calcicole endemic species, but the vast majority of South African winter-rainfall plants grow on neutral to acidic soils.

Vegetation Types

Fynbos

This is the best-known of the South African vegetation types and is famous for its incredibly high biodiversity. It is best developed on the nutrient-deficient, acidic, sandy soils associated with Table Mountain Sandstone and is characterised by plants with four different growth forms. Most obvious are the tall broad-leafed shrubs which mainly belong to the Protea family and include Leucadendron and Leucospermum species as well as Protea. Below these are small heath-like shrubs which include hundreds of Erica species plus Aspalathus, Phylica, etc. Reed-like plants belonging to the Restio family are always present in fynbos and are a unique feature of the vegetation type. Finally, there are geophytic plants (bulbs, corms and tubers) which only appear above ground during the winter growing season. The proportions of the four different plant types vary according to rainfall pattern, soil depth and altitude.

Fire is an essential part of any fynbos community and typically occurs every 12 - 15 years; shrubs that are older than this tend to become senescent and even more prone to fire. Nutrients are recycled to the soil after a burn and many seeds need the influence of fire to germinate. It appears that chemical constituents of the smoke are more significant than heat in providing this stimulus. The flowering of geophytes can be particularly good in the season following a fire. A recent threat to fynbos has been the occurrence of fires at more frequent intervals, such that shrubs have not got to the flowering and fruiting stage before they are destroyed. The main threat however is the spread of alien plant species that are more successful than native species at recolonisation after a fire. The chief culprits are Australian wattles such as the Port Jackson Willow. Mankind is a lesser threat because fynbos soils are too poor for agriculture and often occur in hilly country that is unsuitable for ploughing or habitation.

Renosterveld

This vegetation develops on clay-rich soils that are more fertile than fynbos soils. It superficially resembles fynbos and is similarly fire-prone, but restios are absent and proteas are rare. The marker plant for this community is renosterbos (rhinoceros bush), which is a member of the daisy family. Because of its higher fertility, renosterveld can be ploughed up for agriculture and more than 70% has already been lost in this way. Some geophytic species that used to grow in this vegetation type have become extinct or extremely rare in the wild.

Succulent Karoo

This is restricted to areas that have less than 25cm of rainfall per annum. Thus in the south it is found only in rain-shadow areas such as the Little Karoo, but in the dryer north from about Vanrhynsdorp to the Orange River it is the dominant vegetation type. At first sight, the vegetation appears quite sparse; restios and proteas are absent. The most obvious plants are low shrubs that mainly belong to the Mesembryanthemum family, but close inspection reveals a wealth of small plants too. A high proportion of these are succulent, but there is also a rich geophytic flora that grows and flowers during the winter months. The colourful spring-flowering annuals are normally associated with areas that have been disturbed and are in effect weeds. Biodiversity within the succulent karoo is very high and fortunately much of it is unsuitable for cultivation; the main threats being from overgrazing and mining activities.

The dry area north of Vanrhynsdorp is called Namaqualand, but there are distinct areas within it. Immediately north of Vanrhynsdorp is a flat, low-lying plain known as the Knersvlakte that during the Miocene era was the estuary of the Orange river. The quartz-strewn patches are very rich in dwarf succulents. North of this is Namaqualand proper with its rounded granite hills interspersed with sandy plains. These hills contain a myriad of crevices and detritus-filled pans than are home to succulents and geophytes. The far north-west is a mountainous, geologically-complex area known as the Richtersveld, which is a wilderness area that has great attractions for the plant enthusiast.

Afromontane Forest

As the name implies, this is found on mountains in other parts of Africa but a large area has developed near the southern coast around Knysna on the Garden Route. It requires deep fertile soils and relatively high year-round rainfall. In addition, because it is real forest with tall, long-lived trees, it can only grow in areas that are not prone to burning. In comparison to the other vegetation types in the Cape, it is relatively species-poor.

The Cape Floral Kingdom

There are six floral kingdoms in the world, of which five cover enormous areas. For example, the Boreal Kingdom, of which Britain forms a tiny part, stretches across all the northern hemisphere continents and covers 40% of the Earth’s surface. In contrast the Cape Floral Kingdom, which consists of much of the South African winter-rainfall area but excluding Namaqualand in the broad sense, covers just 0.04% of our planet’s surface; a mere 9000 sq km. It includes elements of all the vegetation types described above and is home to about 8500 different plant species of which approximately 5800 are endemic and not found outside the Kingdom. In contrast, the British Isles can only boast 20 species that are endemic. The fynbos provides the richest biodiversity and contributes 80% of species to the total number.

Suggested Reading

Fynbos, South Africa’s Unique Floral Kingdom by R Cowling & D Richardson, Fernwood Press, 1995

Namaqualand, a Succulent Desert by R Cowling & S Pierce, Fernwood Press, 1999

Richtersveld, the Enchanted Wilderness by G Williamson, Umdaus Press, 2000

Internet and Other Sources

Botanical Society of South Africa (http://www.botanicalsociety.org.za/): Quarterly journal Veld & Flora with articles on flora, habitats and conservation, advertisements for SA nurseries & accommodation, seed list, book list. Postal address: Botanical Society of South Africa, Kirstenbosch, Private Bag X10, Claremont 7735, Republic of South Africa.

National Botanical Institute (http://www.nbi.ac.za/): South African Botanical Gardens (such as Kirstenbosch), herbaria and publications (e.g. Veld & Flora, South African J. Bot. & Bothalia).

South African Environmental Atlas ENPAT (www.environment.gov.za/Enviro-Info/intro.htm): Excellent site of the Dept. of Environmental Affairs & Tourism. Provides a wide range of national and provincial maps, including topography, geology, biomes, vegetation & rainfall. Also lists Tourism Board contacts and tourist web sites.

Topographical Maps (w3sli.wcape.gov.za/): Excellent 1:250,000 and 1:500,000 topographical sheet maps of South Africa are published by the Chief Directorate of Surveys and Mapping; post: Private Bag X10, Mowbray 7705, South Africa; telephone: 0027-21-658-4300; mailto:cdsm@sli.wcape.gov.za