The study area in southern Namibia presents an approximate triangle around the Namus and Huns mountains and the southern reaches of the Fish River. It is bordered by a section of the lower Orange River in the south (Figure 1). This includes the state-protected Ai-Ais Hotsprings Game Park and adjacent private farmland. Termed the Greater Fish River Canyon Landscape (GFRCL), the area covers approximately 11 500 km² and falls floristically into the southern Namib and west Gariep districts (Jürgens 1991) or Gariep ecogeographic unit in the Extra Cape flora (Snijman 2013).

The climate is arid, with a mean annual rainfall between 50 mm and 150 mm, increasing along a south-west to north-east gradient. Summer and winter rains can occur. Mean annual temperatures range between 16 °C and 20 °C, increasing along a west–east gradient. The south-eastern corner of the study area borders one of the hottest areas in Namibia, where temperatures over 36 °C are recorded during the hottest months (Mendelsohn et al. 2002). These broad bioclimatic gradients are modified locally by mountainous terrain. The higher reaches receive more rainfall and temperatures are lower. Occasional fog influences the western part of the study area. Rainfall during the field survey period at Keetmanshoop, some 30 km to the north of the northern boundary of the study area, was above average in 2011–2012 (July–June: 195 mm, average: 152 mm), but below average in 2012–2013 (July–June: 58 mm) (Meteorological Services of Namibia; www.meteona.com).

The topography is rugged and altitudes range from less than 100 m (amsl) at the deeply incised Orange River to 1654 m at the Namuskluft peak. The underlying geology is complex and ranges from the oldest rocks in Namibia to recent Quaternary sediments. In large parts of the study area some sedimentary rocks of the Nama Group (600 Ma–550 Ma) are exposed but there are also gneisses, granites and other metasedimentary rocks of the Namaqua Metamorphic Complex (1800 Ma–1000 Ma) and the Gariep Group (800 Ma–500 Ma), as well as sedimentary rocks and volcanic intrusions of the much younger Karoo Group (345 Ma–140 Ma) (Swart 2008). The Fish River and Orange River dissect these rock formations and created a heterogeneous landscape of plains, slopes, hills, inselbergs, plateaus, valleys, washes and rivers. The north eastern part of the GFRCL is largely plains with isolated mountains and hills, but the majority of the landscape to the south and west comprises rugged mountain terrain with deeply incised rivers. Soils are shallow and poorly developed regosols. Arenosols are present in rivers and washes.

The vegetation is dominated by succulent dwarf shrubs (mostly in the west) and dwarf shrubs. Grasses become more dominant towards the north and east, as in the Gariep Desert in the Richtersveld (Jürgens et al. 2006). Tall stem-succulents such as Aloidendron dichotomum, A. pillansii (formerly Aloe, Grace et al. 2013) and Pachypodium namaquanum, and occasional trees (e.g. Ozoroa concolor and O. dispar) occur on mountain slopes in the west and more widespread, arid-adapted trees such as Boscia albitrunca and Maerua schinzii on slopes in the east. The permanently flowing Orange River locally supports riparian thicket with Vachellia karroo (formerly Acacia), Maerua gilgii, Salix capensis, Searsia pendulina and Tamarix usneoides. Perennial plant cover is generally low and hardly reaches 20% canopy cover, except locally in washes and along the Orange River, where plant cover can reach 100%. The flora of the Huns Mountains is suggested to be ancient, as indicated by the oldest Indigofera clades, evidenced by I. nudicaulis and I. merxmuelleri in the study area (Schrire et al. 2009).

Field surveys were undertaken during the period 2011–2013, with three surveys in the winter season (September–October) and one in the summer season (April–May 2013). These specifically targeted under-collected areas within the study area, which were determined by plotting the collecting status of the national plant specimens database in Namibia and reviewing published sources (Irish 2008) (Figure 1). Plant collections focused on species that were unknown to us and on groups of plants that are difficult to identify or under revision. All specimens were lodged at the National Botanical Research Institute of Namibia (NBRI). Doubtful and potential new species were sent to relevant specialists: Prof. Shakkie Kativu (University of Zimbabwe) for Chlorophytum, Dr John Manning (Compton Herbarium) and Dr Peter Goldblatt (Missouri Botanical Garden) for Moraea, and Prof. Leanne Dreyer (University of Stellenbosch) for Oxalis.

Range extension was determined by reviewing information from the NBRI’s specimens database, which was consolidated and cleaned, BRAHMS online (Craven & Kolberg 2017), published sources (Jürgens et al. 2010; Klaassen & Kwembeya 2013; Mannheimer & Curtis 2009; Merxmüller 1966; Snijman 2013; Swanepoel 2011) and SANBI’s web-based threatened species programme (Raimondo et al. 2009).

Surveys in border regions potentially generate new distribution records, and two new distribution records for Namibia in this study were Ornithogalum decus-montium G.Will. (Hyacinthaceae) (Figure 2a) and Salvia dentata Aiton (Lamiaceae) (Figure 2b). The geophyte O. decus-montium was described from the Richtersveld on arid mountain slopes approximately 20 km east of Sendelingsdrif near the Orange River at about 200 m altitude (Williamson 1998). We found it on slopes of the southern Huns Mountains on the Namibian side, approximately 25 km east of Sendelingsdrif, thus very close to the record in the Richtersveld (voucher: AB12027). The delicate geophyte is easily overlooked, appears for a short time after rains, and is only known from a few localities in Namibia and South Africa. More prominent, but nevertheless new to Namibia is the shrub S. dentata (voucher: AB13038). It is fairly widespread in the mountains of the Northern Cape (Snijman 2013) and grows on rocky hills, particularly granites, and in water courses from Namaqualand to Clanwilliam (Hedge 1974). In Namibia, it was found on a granite outcrop on the farm Zebrafontein, some 15 km north of Rosh Pinah. It is an attractive shrub when in flower, but easy to overlook during the dry season.

Five new or recently described plant species were recorded during these surveys, one a completely new discovery, and the others either not previously recorded in Namibia or without adequate material for taxonomic description.

Oxalis canaliculata ined. (Oxalidaceae) (voucher: LN576) had been found on the Springbokvlakte in the Richtersveld before and is due to be described (Oberlander, Roets & Dreyer 2014). It is now also recorded on the Namibian side of the Orange River on mountain slopes very close to the river, some 10 km south of Aussenkehr (Figure 3a). The small, delicate plant with distinctive, linear leaves is short-lived and only appears after adequate rains. Its known distribution is limited to the Richtersveld and now one locality near Aussenkehr in Namibia. A second species, Oxalis hunsbergensis ined. (Oxalidaceae) (voucher: LN598), had been found at a few localities in the Huns Mountains previously (L. Dreyer, University of Stellenbosch, pers. comm., 2013). More localities were added during the recent surveys (Figure 3b). Possibly a third new species of Oxalis was recorded (voucher: AB13001). It had also been found previously (L. Dreyer, University of Stellenbosch, pers. comm., 2013). More localities were added during these surveys, one notably in the Naute Game Park, much further north than these Oxalis species had been recorded previously.

A small species of Iridaceae, growing beneath a granite rock overhang on slopes facing the Orange River, was identified as the new species, Moraea thermarum Goldblatt and Manning (2013) (voucher: AB12035). Vegetative material had been collected before in the Aurus Mountains in the Sperrgebiet to the west of the study area, but because of the lack of reproductive structures, the plant could not be described at the time. This new Moraea is a Namibian endemic and only known from these two localities (Figure 3c).

A geophyte that had never been collected before was also discovered during the field surveys growing on mountain slopes near the Orange River and has now been described as Chlorophytum boomense Kativu (Kativu & Bjora 2016) (Asparagaceae), named after the Boom River where it was found (vouchers: AB12032, LN380). Although locally common, it is only known from this one locality (Figure 3d).

In addition to the cross-border range extensions, the study also generated some range extensions within Namibia. The tall shrub Euryops lateriflorus (L.f.) DC (Asteraceae) is a rare plant in Namibia, although more widespread in South Africa (Foden & Potter 2005; Snijman 2013). The shrub grows on the higher slopes of the western Namus Mountains (Figure 4a and b). It was known from the Orange River Mountains and few localities in the south eastern Sperrgebiet, but this survey extended its range further north east and south than previously recorded in Namibia (Figure 5a). The tall shrub Euclea asperrima Holzh. (Ebenaceae) (Figure 4d) is endemic to Namibia, but has a disjunct distribution with the largest population in the Naukluft and Zaris mountains in the central escarpment, a small population in the Tsaus Mountains and associated hills in the Sperrgebiet, and a few populations in the western Huns Mountains (Mannheimer & Curtis 2009). The shrub is always associated with limestone, and the recent surveys resulted in a range extension to the east for the southern-most population (Figure 5b). The rare, stem-succulent shrublet Othonna cyclophylla Merxm. (Figure 4c) had only been recorded twice in Namibia previously (Craven & Kolberg 2017), but also grows in the south eastern Richtersveld, east of Vioolsdrif and in the Kamiesberge area (Snijman 2013). This study added an additional record on the north bank of the Orange River much further west than previously recorded in Namibia (Figure 5c).