Methodology based on volumetric biomass density to estimate solar radiation transmission through tree stands

Abstract

Solar radiation is a key factor which affects the hydrological balance of water exchange between a tree stand, the atmosphere and the ground. This paper presents a methodology of determining solar radiation transmission through tree stands based on vertical distribution of volumetric biomass density. Volumetric biomass density expresses the ratio of the biomass of all plants above the ground to the volume occupied by the whole tree stand (including the empty spaces between plants). It can be calculated based on empirical equations which describe the vertical biomass arrangement of every particular tree in a stand. Furthermore, the calculation scheme may be applied to any arbitrarily chosen layer in a given tree stand. Measurements of solar insolation were recorded at 4 levels for two mainly fir stands (Abies alba Mill.): a 135-year-old stand that is managed as a selection forest system, and a one-storied 115-year-old stand. The developed model, which is based on the Beer-Lambert law, estimates the solar radiation transmission through a tree stand with very high precision (R2>0.99). The model parameters largely depend on tree height, tree diameter measured at breast height (130 cm), and the height of the top surface of the investigated layer above ground level. The model and parameterization are proposed mainly for fir stands and depend solely on easily measurable biometric features; thus, it should be readily adaptable to stands composed of other tree species by using appropriate coefficients that differentiate these stands from fir stands. A thorough understanding of the factors determining solar radiation transmission through tree stands may considerably expand the knowledge of the water exchange balance within forest complexes, snow melting rate, as well as the estimation of site productivity and forest succession.