Biological soil crusts of the Succulent Karoo: a review^§

§ This article is from the ‘Karoo Special Issue: Trajectories of Change in the Anthropocene’.

Abstract

The Succulent Karoo is characterised by a dense coverage of biological soil crusts (biocrusts) belonging to different types and successional stages. Whereas the Soebatsfontein region hosts cyanobacteria-dominated and minor amounts of lichen- and bryophyte-dominated biocrusts, the Knersvlakte comprises a rich cover of hypolithic crusts growing on the sides and undersides of quartz pebbles. Apart from dominating photosynthesizers used to classify biocrusts, each crust type hosts a rich and specific fungal and bacterial community and also diverse protists. In a remote-sensing mapping approach, soil-inhabiting biocrusts of the Soebatsfontein region covered ∼27% of the surface area, whereas in the Knersvlakte soil-inhabiting biocrusts covered ∼16% and hypolithic biocrusts ∼42% of the region. Combining these data with biomass contents, results suggested that the Knersvlakte, despite somewhat harsher environmental conditions, harboured about 65% more biocrust biomass per surface area. In physiological measurements we observed that biocrusts emit the reactive nitrogen compounds nitric oxide and nitrous acid, showing water pulse-dependent emission patterns. In addition, CO₂ gas fixation showed characteristic type-specific patterns depending on climatic conditions. Long-term microclimate measurements along a gradient revealed that forecasted climate and land-use change may be detrimental for biocrusts with potentially adverse effects on soil stability and overall fertility of the Succulent Karoo.

Keywords:

• biodiversity
• climate change
• CO₂ gas exchange
• reactive nitrogen compounds
• remote sensing

Notes

§ This article is from the ‘Karoo Special Issue: Trajectories of Change in the Anthropocene’.