Abstract
Solar radiation is one of the most significant environmental factors that regulates the rate of photosynthesis, and consequently, growth. Light intensity in the forest can vary both spatially and temporally, so precise assessment of canopy and potential solar radiation can significantly influence the success of forest management actions, for example, the establishment of natural regeneration. In this case study, we investigated the possibilities and perspectives of close-range photogrammetric approaches for modeling the amount of potential direct and diffuse solar radiation during the growing seasons (spring–summer), by comparing the performance of low-cost Unmanned Aerial Vehicle (UAV) RGB imagery vs. Hemispherical Photography (HP). Characterization of the solar environment based on hemispherical photography has already been widely used in botany and ecology for a few decades, while the UAV method is relatively new. Also, we compared the importance of several components of potential solar irradiation and their impact on the regeneration of Pinus sylvestris L. For this purpose, a circular fisheye objective was used to obtain hemispherical images to assess sky openness and direct/diffuse photosynthetically active flux density under canopy average for the growing season. Concerning the UAV, a Canopy Height Model (CHM) was constructed based on Structure from Motion (SfM) algorithms using Photoscan professional. Different layers such as potential direct and diffuse radiation, direct duration, etc., were extracted from CHM using ArcGIS 10.3.1 (Esri: California, CA, USA). A zonal statistics tool was used in order to extract the digital data in tree positions and, subsequently, the correlation between potential solar radiation layers and the number of seedlings was evaluated. The results of this study showed that there is a high relation between the two used approaches (HP and UAV) with R2 = 0.74. Finally, potential diffuse solar radiation derived from both methods had the highest significant relation (−8.06% bias) and highest impact in the modeling of pine regeneration.
Highlights
Incoming global solar radiation is a key factor which influences energy and water balance and is fundamental to most biophysical and physical processes [1,2]
We evaluated the correlation between the crown openness as a structural parameter of our study area and potential solar radiation components (Figure 10)
This paper has demonstrated the ability of a low-cost image-based Unmanned Aerial Vehicle (UAV) for the assessment of potential solar radiation as a potential replacement method for Hemispherical Photography (HP)
Summary
Incoming global solar radiation is a key factor which influences energy and water balance and is fundamental to most biophysical and physical processes [1,2]. Characterization of the solar radiation regime and forest canopy structural architecture has undergone considerable evolution since [5,6] first reported using Hemispherical Photography (HP). Hemispherical or fisheye photography is a field-based Remote Sensing (RS) technique to characterize tree canopies from analysis of wide angle (usually between 100 to 180◦) photographs [6]. Taking advantage of fisheye lens polar projection, Coombe and Evans [8], who pioneered the method to characterize light conditions, super-imposed sun path diagrams on HP to predict periods of direct sunlight and the fraction of light that was transmitted to the ground using manual analysis of the photographs. Chazdon and Field [13] were the first to describe a computerized method to estimate canopy openings, diffuse and direct sunlight, as well as Photosynthetic Photon Flux Density (PPFD), which could be used as key descriptors of the light environment
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