Water-related soil-moss interactions at different scales

Despite being small in size, mosses fulfill vital roles in ecosystem functioning, especially in temperate ecosystems. Due to their unique ecology and physiology, they affect water and nutrient cycles, even at larger scales. This study investigated water-related interactions between soil and moss from the site scale of skid trails in temperate forests to the microscopic scale of individual structural moss traits. First, the natural succession of mosses in skid trails was surveyed, together with their effect on soil erosion using a rainfall simulator. Second, different soil-moss combinations and their impact on runoff formation, percolation, and sediment discharge were investigated. In addition, the temporal dynamics of soil water content were recorded during erosion measurements as well as during watering and subsequent desiccation. Third, a detailed study on how structural traits affect maximum water storage capacity (WSC_max) and its interactions with soil water content was conducted on the species level.

Mosses appeared in our temperate forests as biocrusts during the first few weeks after disturbance and developed for four months until they formed a mature moss cover and biocrust characteristics steadily disappeared. Soil erosion was most reduced when moss-dominated biocrusts were abundant. In general, mosses made a major contribution to erosion control in skid trails after disturbance, showing stronger impacts than vascular plants. The different soil-moss combinations showed clear variations among bare & dry, bare & wet, moss & dry and moss & wet treatments in terms of surface runoff, percolated water volume and sediment discharge. Surface runoff and soil erosion were significantly decreased in the moss treatments, while the amount of percolated water was increased; however, these processes were superimposed by desiccation cracks and water repellency. Moss treatments exhibited lower water contents over time compared to bare treatments, highlighting the strong influence of moss covers and desiccation cracks on the soil water balance. During watering of soil-moss combinations, no clear relationships between water absorption and moss structural traits could be found, which suggests capillary spaces as important influencing factor. In general, mosses were no barrier for infiltration in case of high precipitation rates and they did not store much of the applied water themselves, but passed it on to the soil. During desiccation, mosses with high leaf area index had lower evaporation rates and they prevented desiccation of the substrate, although even dense moss covers did not completely seal the surface. WSC_max of the studied moss species varied widely, which could not be explained by their total surface area or leaf area index, and higher WSC_max values were correlated with low leaf area and high leaf frequency.

Our results underlined the importance of mosses for the soil water balance and protection of soil against erosion in disturbed forest ecosystems. However, it became simultaneously apparent that the role of mosses in forest ecosystems is not yet fully understood and that there is still great potential for further research on soil-water relations and erosion control.