Different management strategies of disturbed forest areas in Central Germany: Effects on soil nutrient distribution, forest soil structure and microclimate

It has become increasingly challenging to maintain "regular" forest management, as the pace and magnitude of climate-related changes exceeds the response of many forest ecosystems. This has led to large-scale disturbances in recent years. To limit impacts on soil structure, the loss of soil organic matter and soil nutrients and to restore forest climate and functions, the development of suitable reforestation strategies on disturbed areas must be prioritised. Alternative management measures are needed to point out further options for the extensive clearing. To draw conclusions about the site-specific selection of suitable management options for disturbed areas, this study monitored mid- to long-term effects on soil structural and soil chemical parameters of the humus layer and topsoil as well as microclimatic properties. Studies were carried out on large-scale Norway spruce (Picea abies (L.) Karst.) disturbance areas in low mountain regions in Thuringia. Different management variants were compared to vital spruce stands, ranging from standing deadwood to "high stumps", mulched and completely cleared areas. In addition to basic soil chemical and physical parameters, soil moisture and temperature of the topsoil and the organic layer as well as the surface temperature were recorded continuously over 30 months. The results revealed significant differences in surface and soil temperature between the variants. Clear-cut areas showed higher soil temperatures and greater variability during the vegetation period. Standing deadwood lowered the average temperature and, in particular, mitigated high and low temperature peaks. Soil moisture showed temporal changes depending on the status of decline of the spruce forests. Even within the first few months, effects on the soil structure of the cleared disturbed areas can be detected, as macroaggregates degrade and the size fraction of microaggregates increases. With a narrowing C/N ratio in the mineral soil along with increasing thinning on the plots and intensity of management, mid-term (>6 months) losses in plant-available and total phosphorus content in the organic layer and the upper mineral soil were observed. While mulching led to an increase in soil organic matter (SOM) in the surface layer, a significant loss of SOM was determined over time in all other variants compared to vital spruce stands. Overall, we conclude that the aggregate stability and nutrient availability of the soil decreased faster with more intensive management intervention so that characteristics of the microclimate moved away from forest interior climate. Prolonged monitoring is required to verify whether the results are mid-term occasional effects or on the way to an equilibrium.