Two new Oxalis species (Oxalidaceae) from the Ai-Ais / Richtersveld Transfrontier Park, South Africa

South Africa has a rich, but scantily surveyed, desert flora. Documenting annual and geophytic species in this biome is challenging, as they usually only flower after adequate precipitation, which is characteristically erratic and infrequent. Recent floristic surveys in the Ai-Ais / Richtersveld Transfrontier Park were conducted after abnormally high precipitation and revealed the presence of numerous potentially new plant taxa, including two new members of the genus Oxalis. These taxa are clearly morphologically distinct from any known species. Here we explore the relationships of these species to other southern African species based on analyses of Internal Transcribed Spacer (ITS) and trnS-trnG DNA sequence data. Molecular data and morphological comparisons show that these collections represent new species closely allied to Oxalis pes-caprae, O. compressa, O. copiosa and O. haedulipes. Accordingly we describe them here as O. petricola and O. rubricallosa. Oxalis petricola is known from one isolated population of fewer than 500 individuals and is probably of special conservation concern. Oxalis rubricallosa is known from two small populations. The belated discovery of a large and showy species such as O. rubricallosa highlights the urgent need for more extensive floristic surveys of southern African desert regions.