Abstract
Hemispherical photography is a widely used tool for the indirect estimation of forest light environment and quantification of canopy openness, LAI, and direct and diffuse canopy light transmittance. Several comparisons have been made in existing literature between digital and film systems and their performance under different stand structures and light intensity levels, but the conclusions are not always in agreement. The aim of this research is to compare the light estimates coming from analysis of digital- and film-based hemispherical photographs taken in a mountain Norway spruce (Picea abies (L.) Karst.) forest in the Valbona forest reserve (Paneveggio - Pale di S. Martino Regional Park, Trent, Italy). Two 100x100m permanent plots were established in a previous study ([44]), one each in a single- and a multi-storied stand. The images were taken from grid points 12.5 m apart using a digital camera equipped with a Nikon FC-E8 fisheye adapter, immediately followed by a film camera shot (Sigma 8mm F4 fisheye lens). Images were digitized and analyzed using Gap Light Analyzer ([17]). The output variables (canopy openness, LAI, total, direct and diffuse transmitted PAR) were compared by statistical means across the full sample (48 photos) and for the two sites independently. The ratio between digital and film output was analyzed in its relationship to canopy cover, local tree density, spatial pattern of canopy gaps and zenith angle. Under the dense, even-aged cover the results were similar, while the digital images provided significantly lower transmittance estimates (up to 32% less than film-based estimates) in the uneven-aged stand. We addressed different sources of error for the two techniques; some corrective strategies are proposed herein.