Biological soil crusts and their microenvironment: Impact on emergence, survival and establishment of seedlings

To elucidate the impact of biological soil crusts (BSCs) on the establishment of habitat-typical vascular plant species, we studied the effects of seed location (surface versus sub-surface), age of crusts (initial versus stable), long rainy periods (continuous versus discontinuous watering) and microenvironment (cracks versus no cracks). In addition, we investigated growth height, phytomass and N-content of one vascular plant species (Phleum arenarium). Initial crusts were compared with older, stable crusts using seven habitat-typical plant species representing different life forms (annuals versus perennials). Our model ecosystem, situated in the temperate zone (but edaphically dry), is characterised by calcareous sand with threatened pioneer vegetation (Koelerion glaucae). We carefully translocated soil monoliths of these crusts and analysed the effects under common garden conditions.The results reveal a great importance of crust age and of the microenvironment: the inhibitory effects of BSCs are species-dependent; all investigated perennials were inhibited by BSCs, while habitat-typical annuals were not or were beneficially affected. The location of seeds is important for emergence. Fewer seedlings appeared below the surface than emerged on the surface. Furthermore, emergence through the crust itself was less likely for vascular plants than emergence through cracks in the crust. Continuous watering resulted in more seedlings after winter than discontinuous watering; furthermore, the establishment rate was higher in one perennial species.Although the emergence, survival and establishment were inhibited, successful plant individuals could profit from crusts by acquiring a higher N-content and increasing their growth height and phytomass.