Thermal Conductivity of Forest Floor from Temperate Beech Forests: Laboratory Measurements and In Situ Projections

The forest floor (FF) is the central place in forests where organic matter, nutrients, and water are stored, transformed, and transferred. The rate of these processes is influenced, among other factors, by the soil temperature regime. FF horizons, lying directly within the sphere of atmospheric influence, are frequently exposed to temperature fluctuations. To understand how heat is transported through the forest floor, values for heat conductivity and heat capacity are essential. We measured these parameters in various FF horizons from temperate beech forests under controlled laboratory conditions. Thermal conductivity was measured using a single-probe needle sensor, while water content dependence was assessed through an evaporation experiment with continuous measurements. Heat capacity was measured using a dual-probe needle sensor at various stages during the evaporation period. Our results show that volumetric water content is the most significant factor influencing both heat conductivity and heat capacity. We demonstrate that, at constant water content, increasing decomposition levels in FF horizons lead to higher thermal conductivity. We also found significantly lower thermal conductivities in FF horizons compared to underlying mineral soils at similar volumetric water contents. Moreover, unlike in mineral soil horizons, higher dry densities of FF material result in lower thermal conductivity when volumetric water content and the degree of decomposition remain constant. Our results support the hypothesis of an insulating effect of FF layers, which mitigates the impacts of temperature extremes on the underlying mineral soil. We recommend incorporating the thermal conductivity–water content relationships into heat balance modeling of forest soils.