Mosses are often overlooked; however, they are important for soil-atmosphere interfaces with regard to water exchange. This study investigated the influence of moss structural traits on maximum water storage capacities (WSCmax) and evaporation rates, and species-specific effects on water absorption and evaporation patterns in moss layers, moss-soil-interfaces and soil substrates using biocrust wetness probes. Five moss species typical for Central European temperate forests were selected: field-collected Brachythecium rutabulum, Eurhynchium striatum, Oxyrrhynchium hians and Plagiomnium undulatum; and laboratory-cultivated Amblystegium serpens and Oxyrrhynchium hians. WSCmax ranged from 14.10 g g−1 for Amblystegium serpens (Lab) to 7.31 g g−1 for Plagiomnium undulatum when immersed in water, and 11.04 g g−1 for Oxyrrhynchium hians (Lab) to 7.90 g g−1 for Oxyrrhynchium hians when sprayed, due to different morphologies depending on the growing location. Structural traits such as high leaf frequencies and small leaf areas increased WSCmax. In terms of evaporation, leaf frequency displayed a positive correlation with evaporation, while leaf area index showed a negative correlation. Moisture alterations during watering and desiccation were largely controlled by species/substrate-specific patterns. Generally, moss cover prevented desiccation of soil surfaces and was not a barrier to infiltration. To understand water’s path from moss to soil, this study made a first contribution.

Abstract. Soil erosion continues to be one of the most serious environmental problems of our time and is exacerbated by progressive climate change. Until now, forests have been considered an ideal erosion control. However, even minor disturbances of the forest floor, for example, from heavy vehicles used for timber harvesting, can cause substantial sediment transport. An important countermeasure is the quick restoration of the uncovered soil surface by vegetation. To date, very little attention has been paid to the development of nonvascular plants, such as bryophytes, in disturbed areas of temperate forests and their impact on soil erosion. This study examined the natural succession of pioneer vegetation in skid trails on four soil substrates in a central European temperate forest and investigated their influence on soil erosion. For this purpose, rainfall simulations were conducted on small-scale runoff plots, and vegetation was continuously surveyed during the same period, primarily to map the development of bryophytes and the occurrence of biological soil crusts (biocrusts). Biocrusts appeared immediately after disturbance, consisting primarily of bryophyte protonemata and cyanobacteria as well as coccoid and filamentous algae that lost their biocrust characteristics as succession progressed. They were present from April to July 2019, with a particular expression in the skid trail that was on shale clay (Psilonotenton Formation) and silty clay loam substrate. In general, skid trails on clayey substrates showed considerably higher bryophyte cover and species richness. Although bryophytes were subsequently overtopped by vascular plants, they managed to coexist until their growth was restricted due to leaf litter fall. Brachythecium rutabulum and Oxyrrhynchium hians were the most important and persistent pioneer bryophyte species, while Dicranella schreberiana and Pohlia lutescens were volatile and quickly disappeared after spreading in the summer. Sediment discharge was 22 times higher on disturbed bare soil compared with undisturbed forest soil and showed the largest sediment removal in the wheel tracks. Counteracting this, soil erosion decreased with the recovery of surface vegetation and was particularly reduced with growing pioneer biocrusts in summer, but it again increased in winter, when vascular vegetation became dominant. This leads to the conclusion that the role of bryophyte-dominated biocrusts in forests has been underestimated so far, and they can contribute more to soil conservation at specific times of succession than vascular plants.

Abstract. Soil erosion continues to be one of the most serious environmental problems of our time, which is exacerbated by progressive climate change. Until now, forests have been considered an ideal erosion control in this regard. However, even minor disturbances of the forest floor for example from heavy vehicle used for timber harvesting can cause substantial sediment transport. An important countermeasure is the quick restoration of the uncovered soil surface by vegetation. In this context, biological soil crusts (biocrusts) can play a vital role, as they are known for their soil-protective effect. This study examined the natural succession of pioneer vegetation in skid trails on four soil substrates in a central European temperate forest and investigated their influence on surface runoff and sediment discharge. We applied rainfall simulation experiments on small-scale runoff plots and continuously surveyed vegetation during the same period, primarily to map biocrust development. Skid trails on clayey substrates showed considerably higher biocrust cover and species richness. Biocrust cover was higher in center tracks than in wheel tracks, while there was no clear difference for biocrust species richness with regard to track position. Although biocrusts were quickly overtopped by vascular plants, they managed to coexist until their growth was restricted due to leaf litter fall. Brachythecium rutabulum and Oxyrrhynchium hians were the most important and persistent pioneer biocrust species, while Dicranella schreberiana and Pohlia lutescens were volatile and quickly disappeared after spreading in summer. Soil erosion was reduced with pioneer biocrust vegetation in summer, and again increased in winter. Total amount of sediment discharge was clearly site-dependent, indicating a high relevance of underlying substrates. Sediment discharge was 13.2 times higher in wheel tracks compared to undisturbed forest soil, and bare soil runoff plots produced 22-fold sediment discharge compared to undisturbed forest soil. Overall, bryophyte-dominated biocrusts contributed more to mitigating soil erosion than vascular plants. When soil coverage exceeded 50 %, biocrusts resulted in an average of 18 times less sediment loss compared to vascular plants.

Biological soil crusts, or “biocrusts”, are biogeochemical hotspots that can significantly influence ecosystem processes in arid environments. Although they can cover large areas, particularly in managed sites with frequent anthropogenic disturbance, their importance in mesic environments is not well understood. As in arid regions, biocrusts in mesic environments can significantly influence nutrient cycling, soil stabilization, and water balance; however, their persistence may differ. We call for interdisciplinary physical, biological, microbiological, chemical, and applied soil science research with a special focus on biocrusts of managed soils from mesic environments, to better understand their impact on overall ecosystem health and resilience, particularly with regard to climate change.

<p>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<sub>max</sub>) and its interactions with soil water content was conducted on the species level.</p> <p>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<sub>max</sub> of the studied moss species varied widely, which could not be explained by their total surface area or leaf area index, and higher WSC<sub>max </sub>values were correlated with low leaf area and high leaf frequency.</p> <p>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.</p>