With more than 22,000 different higher plants, or about 9% of all the known species of plants on Earth, South Africa is particularly rich in plant diversity. The most prevalent biome in South Africa is the grassland, particularly on the Highveld, where the plant cover is dominated by different grasses, low shrubs, and acacia trees, mainly camel-thorn (''Vachellia erioloba''). Vegetation becomes even more sparse towards the northwest due to low rainfall. There are several species of water-storing succulents, like aloes and euphorbias, in the very hot and dry Namaqualand area. The grass and thorn savannah turns slowly into a bush savannah towards the north-east of the country, with denser growth. There are significant numbers of baobab trees in this area, near the northern end of Kruger National Park.