Assumptions for Fourier-based modelling of diurnal temperature variations in the top soil layer under Istebna spruce stands

Abstract

Abstract Soil temperature is a key factor which affects both soil-forming processes and the hydrological balance of water exchange between the atmosphere and the ground. The measured temperature waveforms in the top layer of forest soils demonstrate very characteristic diurnal variations. This paper presents a method of determining the amplitude of the diurnal temperature variation using the Fourier transform. Temperature measurements were carried out in the top layer of soil at a depth of approx. 0.08 m within 4 homogeneous, even-aged stands of different age-classes (from a 12-year-old thicket to a 117-year-old mature stand) and at 3 inter-forest meadows. The main aim of this paper was to relate all parameters in a simple empirical model of diurnal temperature variations in the top layer of soil under a stand to biometric features of stands. The parameters describing the model depend to a large extent on the soil exposure factor and the total biomass of above-ground tree parts. The soil exposure factor and the total biomass of a stand depend solely on the number of trees per hectare, the mean height of a stand, and the mean tree diameter measured at breast height (i.e. at a height of 130 cm), all of which are very easy to measure from the ground. The analyses also show that the amplitude values, particularly in an open field, also depend on the mass of organic matter accumulated in soil. The presented model was verified using measurement results in spruce stands of the Istebna ecotype (the Silesian Beskid Mts.). The model and parametrisation proposed for spruce stands depends solely on easily measurable biometric features and should be readily adaptable to stands composed of other tree species, using appropriate coefficients that differentiate these stands from spruce stands. The necessary improvements to the presented empirical model will be possible after a series of measurements taken within various types of stands. A thorough understanding of the factors determining temperature variation in woodlands may considerably expand the knowledge of the water exchange balance within forest complexes, as well as the estimation of site productivity.