Abstract
Artificial canopy gaps (forest openings) are frequently used as an element of regeneration cutting. The development of regeneration in gaps can be controlled by selecting a relevant size and shape for the gap, which will regulate the radiation microclimate inside it. Based on the size and shape of a gap computer models can assess where solar radiation is decreased or eliminated by the surrounding canopy. The accuracy of such models to a large extent depends on how the modeled shape of a gap matches the actual shape of the gap. The aim of this study was to compare the results of modeling solar radiation availability by applying Solar Radiation Tools (SRT) that use a different digital surface model (DSM) for a description of the shape of a studied gap, with the results of the analysis of 27 hemispherical photographs. The three-dimensional gap shape was approximated with the use of simple geometrical prisms or airborne laser scanning (LiDAR) data. The impact of two variations of exposure (automatic and manual underexposure) and two variations of automatic thresholding on the congruence of SRT and Gap Light Analyzer (GLA) results were studied. Taking into account information on differences in height between trees surrounding the gap enhanced the results of modeling. The best results were obtained when the boundary of the gap base estimated from LiDAR was expanded in all directions by a value close to a mean radius of the crowns of surrounding trees. Modeling of radiation conditions on the gap floor based on LiDAR data by an SRT program is efficient and more time effective than taking hemispherical photographs. The proposed solution can be successfully applied as a trustworthy source of information about light conditions in gaps, which is needed for management decision-making in silviculture.
Highlights
Small oval-shaped forest openings with an area of less than 1 ha have for a long time [1] been an important element of regeneration cuttings used for the creation of stands with a diverse age and species structure [2,3,4,5,6,7]
Impact of Exposure and Photograph Thresholding Method on Assessment of Congruency of Gap Light Analyzer (GLA) and Canopy openness was chosen as a synthetic indicator of the influence of different hemispherical photography exposure and thresholding scenarios on the assessment of forest canopy properties
The application of underexposure (−2 EV) in the same method of thresholding resulted in a further decrease of openness and in better visibility of tree crowns surrounding the artificial gap. These results indicate that underexposure of photographs in artificial gaps by a −2 EV value is a better solution than automatic exposure
Summary
Small oval-shaped forest openings (artificial canopy gaps) with an area of less than 1 ha have for a long time [1] been an important element of regeneration cuttings used for the creation of stands with a diverse age and species structure [2,3,4,5,6,7]. Relative gap size (the ratio of the gap diameter D to the height of surrounding canopy H) and its shape and orientation can impact the availability of solar radiation at the forest floor [8,9,10]. The gap heat balance has an impact on spring thawing of snow, soil temperature and moisture, the temperature and humidity of the air, and the occurrence of ground frost. By adjusting gap sizes and shapes it is possible to provide conditions appropriate to the preferences of desired tree species by minimizing the risk of occurrence of ground frost [14] or by creating optimal moisture and light conditions [6,7,15,16,17].
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