In dryland regions, where plants are sparse, the dominant cover is often biological soil crusts (biocrusts), a community of photosynthetic organisms (cyanobacteria, algae, lichens, and mosses), fungi, heterotrophic bacteria, and soil fauna. Because these communities cover most soil surfaces, they mediate almost all inputs and losses from dryland ecosystems. In addition, they fix atmospheric carbon and nitrogen, alter local to regional hydrologic cycles, stabilize soils, capture resources such as nutrient-rich dust and contribute to plant nutrition and growth. For these reasons, it is essential that their restoration be considered along with that of vascular plants. As the cover and species composition of biocrusts determines the degree to which they contribute to ecosystem processes, attention needs to be paid to this as well in restoration efforts.
The Earth’s Critical Zone is defined as the “heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources”. In most temperate and tropical ecosystems, tall plants and the large numbers of roots ramifying throughout deep soils result in a critical zone measured in many meters. Conversely, deserts have sparse short vegetation, soils that are shallow or non-existent, and large amounts of rock surfaces. Therefore, a different way of envisioning the critical zone in deserts is proposed: rather than encompassing meters of life above and below the soil and being defined mostly by where roots occur, the critical zone in deserts encompasses the top centimeter of rock or soil where biocrusts, not plant roots, dominate. Wherever possible, re-establishing the original species composition and cover of biocrusts to disturbed areas is important for restoring ecosystem health.