Kombinierte Methoden aus Modellierung, Messung und Geländearbeit Waldökologie, Landschaftsforschung und Naturschutz 11 (2011) 19 AFSV Waldökologie, Landschaftsforschung und Naturschutz Heft 11 (2011) S. 19–33 5 Fig., 11 Tab. urn:nbn:de:0041-afsv-01117 Abstract There have been many studies involving the use of hemi- spherical photographs to indirectly estimate canopy struc- tures and forest light environments. A variety of commercial and free software packages are available for the analysis of hemispherical photographs. The costs of investment might represent an advantage of the free programmes over the commercial, but as yet little has been documented about the differences in their outputs and in the technical applications from a user (ecologist and forester) perspective. The objective of the study was to compare the canopy structure variables (canopy openness and effective plant area index) and solar radiation transmission estimates (direct, diffuse and global solar radiation transmittances) from digital hemispherical photographs taken under two forest canopy conditions (gap and closed canopy) in three different broadleaf forest regions (Chile, Germany, Venezuela) and calculated using four dif- ferent programmes. The hemispherical photographs were analysed using one commercial (HemiView) and three free programmes (Gap Light Analyzer, hemIMAGE and Winphot). The results obtained revealed that all of the programmes computed similar estimates of both canopy structures and below-canopy solar radiation. Only the results relating to the effective plant area index with an ellipsoidal leaf angle distribution made with HemiView and Winphot deviated signiicantly. Other user aspects are also discussed, such as costs, image formats, computer system requirements, etc. Keywords: hemispherical photography, solar radiation transmittances, canopy openness, plant area index, software packages Zusammenfassung In vielen Studien werden Hemisphärenphotos genutzt um indirekt die Kronenstruktur und die Belichtungsverhältnisse zu schätzen. Verschiedene kommerzielle und kostenfreie Softwarepakete sind zu Analyse von Hemisphärenphotos verfügbar. Es gibt bisher keine umfassende Vergleichsstudie zu Ergebnissen oder technischer Handhabung aus Sicht der Nutzer dieser Programme (Ökologen und Forstwissenschaft- ler). Das Ziel dieser Studie war der Vergleich der Schätzungen von Kronenstrukturvariablen (Kronenöffnung und effektiver Planzenlächenindex) Solartransmission (direkte, diffuse und Global-Strahlung) aus digitalen Hemisphärenphotos berechnet mit vier verschiedenen Programmen (kostenplichtig: Hemi- View und frei: Gap Light Analyzer, hemIMAGE and Winphot). Die verwendeten Photos stammen aus drei verschiedenen Laubwaldregionen (Chile, Deutschland und Venezuela) und repräsentieren jeweils Verhältnisse unter geschlossenem Kronendach und in Lücken. Die ermittelten Schätzungen für die verschiedenen Struktur- variablen und Einstrahlungsverhältnisse zeigten eine sehr hohe Übereinstimmung. Einzig der effektive Planzenlächenindex basierend auf ellipsoider Blattwinkelverteilung unterschied sich signiikant zwischen den Programmen. Weitere für Nutzer interessante Aspekte wie Kosten, Bildformate, Systemvoraus- setzungen und mehr wurden verglichen und diskutiert. Stichwörter: Hemisphärenphotos, Solarstrahlung, Kronenöff- nung, Planzenlächenindex, Lichtschätzungsssoftware 1 Introduction The greatest importance of solar radiation for plant life lies in the plants’ dependence upon photosynthesis for growth and development, and the dependence in turn of photosynthesis on light (BArNES et al. 1998). Several instruments have been developed to estimate either directly or indirectly the forest understorey light environment. Many comparisons of direct and indirect methods for the estimation of below-canopy irradia- tion have been conducted in order to determine the best way to estimate the light environment (CHAZDoN & FIELD 1987, rICH et al. 1993, WAGNEr 1996, CoMEAu et al. 1998, ENGELBrECHT & HErZ 2001, Ferment et al. 2001, BELLoW & NAIr 2003). How- ever, many ecologists and foresters prefer indirect means of light estimation due to the dif iculties inherent in measuring light directly (JENNINGS et al. 1999), and because the amounts of light greatly changes in the microenvironments across the forests. Since its introduction (EVANS & CooMBE 1959), hemispherical photography (Fig. 1) has become a widely applied means of calculating the forest light environments, but also to the estimation of canopy structure variables. A number of studies have demonstrated a high level of agreement between both estimates (rICH et al. 1993, CoMEAu et al. 1998, GENDroN et al. 1998, CLEArWATEr et al. 1999, ENGELBrECHT & HErZ 2001). However, in deeply shaded environments, the applicability of hemispherical photographs for the calculation of understo- rey light environments still needs to be veriied conclusively (roxBurGH & KELLy 1995, MACHADo & rEICH 1999). The theoretical basis for estimating the various components of solar radiation using hemispherical photography were developed by ANDErSoN (1964, 1966). Then, a variety of semi- automated and computerised techniques have been developed (CHAN et al. 1986, CHAZDoN & FIELD 1987, BECKEr et al. 1989, BArrIE et al. 1990, SMITH & SoMErS 1993, WALTEr & T orquEBIAu 2000). A range of software packages are currently available for the analysis of hemispherical photographs (CoMEAu 2000). These include both commercial and free versions, with the latter available for download from the internet. The question that arises now is whether there are differences in the usability and the results provided by the different soft- ware solutions. FrAZEr et al. (1997) compared two canopy characteristics (percent open sky and effective leaf area Comparison of four different programs for the analysis of hemispherical photographs using parameters of canopy structure and solar radiation transmittance Alvaro Promis, Stefanie Gärtner, David Butler-Manning, Cristabel Durán-rangel, Albert reif, Gustavo Cruz and Lionel Hernández