Flora of the Sand Sea by Herta Kolberg & Patricia Craven, [email protected]

Five flowering species are endemic to the sand sea. They are Hermannia minimifolia Friedr.- Holzh. (Sterculiaceae), Stipagrostis seelyae De Winter, Stipagrostis pellytronis De Winter (Poaceae), Sesamum abbreviatum Merxm. (Pedaliaceae) and Monsonia ignorata Merxm. & A.Schreib. (Geraniaceae). A few other species also occur here as well as into sandy habitats further north e.g. Trianthema hereroensis Schinz. Others have disjunct distributions on other sandy habitats, e.g. some legumes, nara ( horridus Welw. ex Hook. f.) and Hexacyrtis dickiana Dinter.

The sand sea area has been identified as one of the Namibian floristic groups (Craven 2009). The floristic area is delineated by the ranges of a small, but unique group of endemic species. It stretches from the sand dunes south of the Kuiseb River to the Koichab River (at about the latitude of Lüderitz), and includes Sossusvlei. The area is 100 to 150 km broad and has a maximum elevation of 900 m. The eastern border is a narrow section of Namib plains leading up to the southern escarpment. Features of the area that affect the are the linear dunes that run roughly south to north and dominate the inland areas, the very high reflectivity of the soil, and limited rain recorded as being during summer. The area is homogenous and there is therefore no subdivision of species composition.

The diversity of plant species is limited, but the populations may be large in good seasons. Only highly adapted species are able to persist in this environment as perennials, either herbaceous or as dwarf shrubs. Especially on the Namib plains within the sand sea, ephemeral annual plants and bulbs can appear in huge numbers after good rainfall. Only in exceptional years will there be new plant growth on the sand dunes themselves, as witnessed at Sossusvlei in 2006. The most representative family is the Poaceae (grasses) and single species of the Aizoaceae, Geraniaceae and Pedaliaceae also occur. While most floristic groups show relationships with other areas of the world, these narrow endemics do not. Only some species that are found in the south of the area can be found in the Northern Cape Province of South Africa.

The area identified floristically excludes a very narrow coastal strip because this is occupied by species from the Southern Namib Succulent Desert (Craven 2009). Similarly some of the species found on the few isolated mountains and hills that occur in the main Namib sand sea, for example the Hauchab and Uri-Hauchab, are associated with Highland Group further east in Namibia (Craven 2009), where the altitude increases and more habitats are available to the plants. This differs from Burke et al . (1998) in that endemics for that ecosystem were differentiated from the others found there because of other habitats within the sand sea. The Tsauchab and Tsondab Rivers extend a part of the Namib plains westwards into the sand sea. These rivers and the Tsaris River disappear under the sand and never reach the sea. A number of plant species are associated with these water courses, but are not limited to the sand sea.

Reasons why plants occur where they do and clues to their origins are sought by assessing all available phylogenies, palaeoclimate data and vegetation analyses. There is very little evidence for this area in the fragmentary fossil record for Namibia and no molecular studies have been carried out on the species here. This group of plants occurs between a floristic group in the north of considerable age with molecular evidence showing links to arid NE Africa, and a young floristic group in the south consisting mainly of succulent plants that also occur down to the Orange River and marginally into the Northern Cape. Two of the families represented on the sand sea also have the most endemic taxa in other sandy areas, like the Kalahari, but they are represented by different 1 taxa that must have evolved separately to become adapted to the lower solar radiation levels, cloudiness, humidity, wind and lower altitudes. The importance of an area cannot be defined by numbers, but rather in the uniqueness of the taxa as explained above.

The actual number of species that occur in the Namib sand sea is not easy to calculate because most are associated with other habitat types like rocky outcrops, washes etc. within the sand sea and this is not always clearly indicated on the data available. A simple search on plant distributions will therefore not give the desired result. The Namib Sand Sea is also an under collected area from a botanical point of view. The few collections that have been made generally come from the rocky outcrops. Also because of the highly variable rainfall, many species may have been missed by collectors because they were not at the right place at the right time. In the past, plant collections were mapped (and databased) on a quarter-degree grid system. This is too coarse a system to distinguish between species that really occur on sand and those that occur in the small pockets of other habitats (washes, rocky outcrops) within the sand sea. The available data therefore are insufficient to provide a list of species from which other statistics could be derived Kolberg & Craven, ongoing).

Burke, A., Jürgens, N. and Seely, M.K. 1998. Floristic affinities of an inselberg archipelago in the southern Namib desert - relic of the past, centre of endemism or nothing special? Journal of Biogeography 25:311–317. Craven, P. 2009. Phytogeographic study of the Kaokoveld Centre of Endemism. Unpublished PhD, University of Stellenbosch http://scholar.sun.ac.za/bitstream/handle/10019.1/1325/Craven,%20P.pdf?sequence=1 Kolberg, H. & Craven, P. Ongoing. Computerised Herbarium of Plants in Namibia (CHOPIN).

2