Habitat characteristics, genetic diversity and conservation concerns for the genus Lithops in Namibia

The dwarf succulent genus Lithops (Aizoaceae) is endemic to southern Africa, with 16 species in Namibia. Lithops are vulnerable to a variety of threats and assignment of accurate conservation status is difficult due to insufficient information about population parameters and influence of habitat variables. In addition, taxa are defined according to leaf characters that may be adapted to plant habitat, resulting in over-classification. Plant density, spatial arrangement of plants and habitat profile was investigated in a 1 ha study area in a population of L. pseudotruncatella. Five out of seven plant density estimation methods vastly over- or under-estimated plant number as determined in a census. Plant number in 100 10×10 m test plots was positively associated with a higher percentage cover of gravel and pebbles as opposed to sand or stones, and with a gentle rise as opposed to slopes and depressions. Plant abundance and habitat variables were also evaluated in 9 populations of L. ruschiorum. Evaluation of the 51 and 43 sites of the meticulously studied RUL population, and the remaining 8 populations, respectively, showed that plant density and/or plant number was associated with aspect, slope, soil texture, substrate and geographic distance from the coast. Results are concordant with a strong impact of fog-based precipitation on plant density in the coastal populations, whereas rain is probably more important at RUL, situated further inland. Plant number dropped by 50% in 21 10×10 m plots monitored at RUL over nine years, most likely due to a prolonged drought in 2012–2016. Plant number declined also in some of the 15 plots in the other populations. AFLP markers were employed to study 52 individuals from 7 populations of L. ruschiorum, and 223 individuals from 44 populations representing 15 Lithops species and 23 taxa in total. An AMOVA demonstrated low levels of genetic differentiation between populations of L. ruschiorum, and a significant but rather weak isolation-by-distance pattern. Analyses of genetic structuring and phylogenetic relationships identified several groups of closely related species, while subspecific taxa often overlapped considerably. Main reason for the overall low variability in Lithops is probably a rapid and recent diversification of succulents after the global expansion of arid environments in the late Miocene. Three nomenclatural changes were made, affecting: (1) L. optica and L. herrei, (2) L. amicorum and L. karasmontana, and (3) subspecies bella and eberlanzii of L. karasmontana.