Soil crusts are ecologically important in arid and semi-arid regions worldwide. Soil crusts influence vascular plants by their effects on soils and by providing a physical barrier to seed establishment. In drier systems where lichens dominate soil crusts, crusts generally facilitate native plant recruitment while providing a protective barrier against exotic invaders. Once these crusts are disturbed, resistance to exotic invasion appears to decrease. In the Northern Rocky Mountain grasslands, interspaces between native bunchgrasses are covered by moss-dominated soil crusts. We know less about the role of moss-dominated soil crusts on vascular plant communities and how disturbance of these crusts might affect exotic invasion.
We investigated the influence of moss-dominated soil crusts, and damage to these crusts, on native and exotic plants. We experimentally added seeds from four native and four exotic plant species to damaged and intact soil crusts and to plots where crusts were experimentally removed at four sites in western Montana. We recorded germination and survival of all species for two years, with fresh seeds added each year, and compared recruitment among treatments. This experiment was also replicated in a greenhouse setting using transplanted soil crusts that were left intact, disturbed or removed and exposed to a daily or drought (every three days) water treatment.
Disturbance of moss-dominated soil crusts benefited seed germination for both native and exotic invasive species, but there was no difference in the number of germinants between native or exotic species on disturbed soil crusts or disturbed bare soil plots. While fewer seeds germinated on intact soil crusts, those that did germinate did so earlier and faster. However in drier conditions, differences in the speed of germination on intact and disturbed soil crusts were less pronounded and germinating on disturbed crusts appeared to benefit seedlings overall. Our results suggest that moss-dominated soil crusts may act as a physical barrier to the germination of native and exotic species in intermountain grasslands. Small scale disturbances allow seedlings to directly come in contact with soil providing access to much needed resources like nutrients and water. Thus, while natural disturbances in this system are necessary for native species establishment they also present an opportunity for exotic invaders.