Contemporary models of light conditions on the forest floor can be divided into two categories: undercanopy models that allow the light conditions in a stand under the canopy to be simulated, and models that take into account shielding from the side. Under-canopy models precisely estimate the availability of wavelengths of light spatially distributed under the canopy of stands: however, these models require a large amount of data on the spatial structure of forest stands. The other class of models describe the light conditions on a particular open surface. These incorporate shielding from the side and are easier to use as they require less data than under-canopy models. In practice, in forest conditions, such models require data on the size, shape and geographical location of surveyed surfaces (e.g. gaps and cut areas) and on the height of the surrounding stand. Often, these data are available in databases, such as the State Forest Information System (SKP), orcan otherwise be obtained relatively easily (and inexpensively). Compared to under-canopy models, these models provide a cheap way to obtain useful information on variation in the light environment that affects the microclimate for regenerating plants on clearcuts and canopy gaps.
The aim of the study was to characterize latitudinal changes in the radiative microclimate of small forest openings (artificial gaps) under Polish conditions. The global, direct and diffuse radiation on the forest floor in gaps was modelled using the Solar Radiation tool in ArcGiS 10.2 Esri. The gaps were modelled as holes of elliptical shape (60 m × 40 m diameters) in flat terrain and with a depth of 20 to 30 meters to mimic the height of a surrounding tree stand. The range of global radiation diversity on an open and flat surface predicted by our model was comparable with findings of empirical studies. Theoretically, the investigated gaps in the northern-most part of Poland receive only 82% of global solar radiation, 74% direct and 90% of diffuse radiation compared to gaps in the most southern part of the country. The comparison with empirical data indicates that local values of the transmittance parameter of the atmosphere may have a large influence on the actual values of solar radiation and may partially mask the latitudinal impact. Nevertheless, the model constitutes a valuable tool for characterising solar radiation diversity in a gap and supports silvicultural decision-making.