Forest floor property controls and consequences for forest soils’ thermal-hydrological functioning

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Climatic conditions, airborne nutrient inputs, and tree species selection - among other factors - control forest floor (FF) properties and thus the ecological functions of our forests. Available inventory data enable the identification of relations between basic FF properties (e.g., thickness, mass, morphology) and ecological site conditions. Although it is widely assumed that such close relations exist this remains to be proven in a setting covering a wide spatial range. Testing relations between FF properties, precipitation, and air temperature is a special challenge, because FF properties themselves control the relation between mean air temperature (MAT) and mean annual precipitation (MAP) and the temperature and water tension within FFs.In general, very little is known about the link between basic FF properties and physical properties (pore-size distribution, water retention, hydraulic conductivity, thermal conductivity) that control essential FF services such as the fluxes of water, energy, air, and matter; within and through the FF. Moreover, little is known about how these properties alter under climate change, N-eutrophication and changes in tree species composition. Changes in water retention of FF may impact not only the water cycle in forests, but also their thermal regime, because the heat flow in soils depends on the geometry and connectivity of pores and the degree of water saturation. While this has been shown for the boreal zone, similar investigations for temperate forests are lacking. It hampers our understanding of how the combined soil moisture and soil temperature regime under forests could alter in response to changes in abiotic and biotic drivers.Existing data on FF properties and dynamic site conditions from a number of monitoring sites and inventories (e.g., ICP-Forests Level I and II sites) in combination with a model-based assessment of the impact of changes in thermic and hydraulic properties of FF on the water and heat balance of FF and underlying mineral soil bear huge potential for closing these knowledge gaps. However, a systematic and combined approach to identify the controls of FF properties and resulting FF services has not yet been carried out. The aim of our project is thus two-fold: (1) Answer the question of how changes in the structure of FF as a result of changing nutrient availability and air temperature can affect the heat and water balance of forests, using coupled modelling of the water and heat balance of FF and the underlying mineral soil horizon, and (2) based on an improved understanding of the relationships between FF properties and ecological site conditions at the research unit (RU) and other monitoring/inventory sites, these new findings will be used to transfer basic FF properties from the stand level to the regional and national level.