Effect of microbiotic soil surface crusts on emergence of vascular plants

Microbiotic crusts are a common and widespread feature of arid and semi-arid landscapes. Their effect on vascular plant success has been discussed controversially. However, only very few field studies have yet tried to experimentally investigate the effect of microbiotic crusts on vascular plant establishment. In this study we investigate the influence of cyanobacteria dominated microbiotic soil surface crust on vascular plants in a desert sand dune area by using a series of manipulative experiments. Crusts were disturbed and removed and density response to these treatments was monitored during the two following years. Emergence densities of vascular plants were considerably higher when crusts were removed and destroyed. This effect was particularly pronounced when the disturbances were applied before the main seed dispersal period. By experimentally preventing seed dispersal into disturbed areas we could show that undisturbed crusts reduce the probability for seeds to come to rest. This indicates that soil surface roughness is a major determinant for the establishment of vascular plants in the study area. We conclude that small-scale and well-timed disturbances of microbiotic crusts in arid lands may increase vascular plant establishment probabilities. A potential feedback process of long-term vegetation dynamics is suggested: disturbance of microbiotic crusts results in higher numbers of emerging plants, which in turn represent a rough seed-trapping element leading to a further increase in densities in consecutive years.