Ecophysiological properties of three biological soil crust types and their photoautotrophs from the Succulent Karoo, South Africa

Abstract

Background and Aims: Biological soil crusts cover about one third of the terrestrial soil surfaces in drylands, fulfilling highly important ecosystem services. Their relevance to global carbon cycling, however, is still under debate. Methods: We utilized CO2 gas exchange measurements to investigate the net photosynthetic response of combined cyanobacteria/cyanolichen-, chlorolichen- and moss-dominated biocrusts and their isolated photoautotrophic components to light, temperature, and water. The results were compared with field studies to evaluate their compatibility. Results: Different biocrust types responded similarly, being inhibited by limited and excess water, saturated by increasing light intensities, and having optimum temperatures. Cyanobacteria/cyanolichen-dominated biocrusts reached their water optimum at lowest contents (0.52–0.78 mm H2O), were saturated at highest light intensities, and had a comparably high temperature optimum at 37 °C. Chlorolichen-dominated crusts had a medium water optimum (0.75–1.15 mm H2O), medium saturating light intensities and a moderate temperature optimum of 22 °C. Moss-dominated biocrusts had the highest water optimum (1.76–2.38 mm H2O), lowest saturating light intensities, and a similar temperature optimum at 22 °C. Isolated photoautotrophs responded similar to complete crusts, only isolated moss stems revealed much lower respiration rates compared to complete crusts. Conclusions: In addition to their overall functional similarities, cyanobacteria/cyanolichen-dominated biocrusts appeared to be best adapted to predicted climate change of increasing temperatures and smaller precipitation events, followed by chlorolichen-dominated biocrusts. Moss-dominated biocrusts needed by far the largest amounts of water, thus likely being prone to anticipated climate change.