Introduction

For the purposes of this book, southern Africa is defined as the mainland region of the African continent south of the Cunene, Okavango and Zambezi rivers, a geopolitical region comprising Namibia, Botswana, Zimbabwe, South Africa, Swaziland, Lesotho and that part of Mozambique south of the Zumbe/t River.

The distinction between shrubs and trees is somewhat artificial and often breaks down in practice. Here, we define a tree, broadly, as any perennial woody plant growing to a height of at (east 2 m. Although a typical tree has a single trunk, it may be multistemmed. Our definition also embraces robust« woody climbers.

An estimated 1 7(X) tree species are native to southern Africa, and well over 100 more, introduced from other pans of the world, are now naturalized in the region. Many of these aliens have become invader weeds, penetrating and replacing indigenous vegetation.

Trees occur in a wide range of vegetation types, with many species found only in specific floristic regions and centres of endemism isec further on).

Climatic conditions (rainfall, temperature, the incidence of fire) largely determine the nature of the various principal vegetation types forest, grassland, savanna, desert, fynbos and so forth - which are so classified according to the general effect produced in a particular area by the growth form of Mime or all plant species in combination. Large areas which are relatively uniformly covered by any one of these broad vegetation types usually represent major biotic zones and are often referred to as bionics,

Biomes themselves can be divided into smaller, more homogeneous ecological units or vegetation types, based upon such criteria as the dominant plant species, plant density and height. Mopane woodland, thorn bush veld and miombo woodland are examples of Mich vegetation types found in parts of southern Africa, These types often reflect more localized conditions - mean annual rainfall for example, and the nature of the soil. A simplified vegetation map of southern Africa appears on page K. Note, however, I hat the vegetation depicted is that which would have been prevalent today had the destruction wrought by human encroachment never taken place. At present very little (no more than 10 per cent) remains of some vegetation types, particularly w ithin the Grassland and Fynbos biomes. As may be expected, vegetation types dominated by woody species are usually rich in tree diversity. In southern Africa, the greatest diversity of trees is found mainly within the Forest and Savanna ibush\eldl biomes.

The geographical distribution of individual plant species (which, taken together, comprise the flora of a region ) rarely covers precisely the same range as the vegetation type(s), Climate, though the main determinant of vegetation type, is clearly not the only phenomenon to be considered when trying to interpret present-day tree species distributions. A complex combination of man) other factors, such as the evolutionary history of the species* continental drift, pusi climatic change, geology» soil characters, topography and interaction with other plants and animals (including human beings v must also be taken into account

When the distribution of native plants is mapped* certain recurring patterns emerge within the great diversity. Particularly interesting are those areas that tend to embrace high concentrations of species w ith very restricted distributions. Commonly referred to as centres of endemism. or hoi spots'. these are pans of the region in which rare and unusual trees are most likely to be encountered. Some of the principal floristic centres of endemism in southern Africa are shown in the map on page 9. Hv far the three richest floristic areas in Africa are embraced within the southern subcontinent. These are: Cape Hloristic Region (about K ftfXi plant species: this is also one of the worlds st\ Floristic Kingdoms); Maputaland-Pondoland Region (about 7 0(H) speciesK and the Succulent Karoo Region {about 5 tHH) species)

Areas especially rich in endemic trees are the Pondoland, Maputaland, Albany and Kaoko\cld centres. Many southern African tree species atso appear to be confined to she Chi muni mani-Nyanga Centre In reality, however, most of these arc not endemic to that region, but are rnerel) tropical African species at the southernmost limit of their ranges. Because of their restricted distribution, most of these rare endemics and peripheral tropical species have not been included in this book.

Overleaf: The first map is a simplified representation of the biomes and vegetation types of southern Africa: adapted from White i 1983) ,md Low <St Rebelo i 19%). The second map shows the region's principal centres of endemism; based on unpublished data of one of the authors I BvW i- and on Davis el <il. (1994).

BiOMES AND VEGETATION TYPES

H >REST BtOME

AtriHiH>nt.inc jikI mLiftri fixrst iwiih ^rasUarkVTynhof) CoaMii! tiurw At swjinp iufi-\4 < witli \

Sand

SAVANNA (BUSHVELD) BIOME

MKvnbuftfft* to tfrifd^-JulberTumitihtu/brrftwni *ixxJlirid

Mupjnc uiHPdbml an J ^hruh WIKHJUIIJ Bmiutru piurtjtty(j Pltnx ofpit\ antiuwtti ut* JLind Thorn Mini* to) fattuhwkl

Aim ur(V^P^fnfxrrnitm Termttmltii kaljhjn Ku\h\cId

I Indiiiiiitntiatcd bushvekl .111J wtKKllanU

bst CtKist [hom bu%hvdd. vulky bushwcld & ihickiM

Watefberg mmjnuiti hushvcUl

Tart hiftktmHus rampiwratu^Grr*w flwti hushveJd

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