Here’s some info I found on Spekboom’s distribution in South Africa.
Addo Elephant National Park
Spekboom (Portulacaria Afra) grows in abundance in the drier parts of the Eastern Cape, especially on the high plateau Karoo hill slopes or flats (c. 400 to 1,060 meters above sea level) and is particularly prominent in the Addo bush to the south where there is significant summer heat. A vast section of Spekboom grows in the Addo Elephant National Park, situated in the Eastern Cape Province, near Port Elizabeth. The park was created to protect the once numerous Eastern Cape elephants. It consists of 12,126 hectares (30,315 acres) of bushveld, dominated mostly by Spekboom, which covers approximately 80% of the park’s area. Some 500 species of plants are to be found in the park. Elephants eat P. Afra from the top downwards allowing the plant to spread itself vegetatively by spreading horizontal branches at ground level. Outside the park the same plants are eaten by goats who eat it from ground level upwards preventing the plant from spreading vegetatively. Consequently, these plants must rely solely on seed to proliferate the species which often proves difficult in such a dry climate. As a result, it was observed that inside the park, where the plant is subjected to browsing by elephants, it survives and spreads successfully, whereas outside the park the plant is becoming sparse as a result of overgrazing and poor regeneration.
Spekboom Succulent Thicket
P. Afra is also prominent in the Spekboom Succulent Thicket (aka Spekboomveld), an area of some 5,011 sq.km., 1.76% of which is conserved in some reserves such as at Graaff Reinet and on higher altitude slopes. The steep mountain slopes in the Eastern Cape and the eastern parts of the Western Cape receive perhaps 250 to 300 mm of rainfall per year, mainly in the autumn and spring. Temperatures are moderate, although extremes may be experienced for short periods. The thicket occurs on sandstone, quartzitic and shale substrata, which gives rise to shallow soils. Spekboom can form pure stands, but usually dominates a dense scrub which includes woody shrubs, succulent herbs and grasses.
Spekboom also grows on the eastern areas of the country from the Eastern Cape northwards into KwaZulu-Natal, Swaziland and the Northern Province in rocky areas of dry succulent Karoo scrub, thicket and bushveld. It also occurs in dry hot river valleys of the old eastern Transvaal (Gauteng, Limpopo and Mpumalanga provinces most of the North West) and north into Mozambique.