Germination success of habitat specialists from the Succulent Karoo and Renosterveld on different soil types

Abstract

South African quartz fields in the Succulent Karoo and adjacent biomes are special habitats for a range of highly specialized dwarf succulents, many of which belong to the Aizoaceae. The soil chemistry of the quartz fields ranges from very acidic to slightly alkaline and from moderately to highly saline, and drives species turnover. The soil conditions are therefore critical for the vegetation composition. The aim of our study is to better understand the role of the regeneration niche of habitat specialists from different quartz-field habitats to inform conservation planning and ecological restoration. We tested germination responses to soil chemistry of 14 Aizoaceae species from quartz fields in the Succulent Karoo (Knersvlakte) and the Fynbos (Overberg) biome of South Africa. For each of the two sites we sampled species from two different quartz-field habitats. The seeds from 14 species were germinated under greenhouse conditions for 79 days on their home soils and on the natural soils of the three other habitat types as well as on salinized and acidified horticultural standard substrate and a standard substrate as control.Germination success in the control differed between the species from the two biomes. Of the seven species from the Knersvlakte, six species germinated rapidly, with 38–86% success. Only Dicrocaulon brevifolium germinated with merely 17% success. Of the seven species from Overberg only Gibbaeum hartmannianum germinated at a high percentage (83%). The other species germinated below 20% or not at all.Natural soils and manipulated substrates limited germination of all species to <20%. No seeds germinated on natural saline soils from the Knersvlakte until from day 46 to day 50, when an excess supply of rainwater diluted the ion content of the soil. The subsequent recovery of the seeds resulted in a germination percentage of 4–35% for seven species from different habitat types. Quartz fields provide a plant-adverse environment. Their soils contain a range of toxic ions at high concentrations. High volume and prolonged rainfall events that leach the soils temporarily seemed to be a requisite for successful germination.