Decisions affecting estimations of understory light environments during photograph acquisition, storage, and analysis using hemispherical photography

Hemispherical photography has been used to study forest canopy geometry and understory light environments for approximately half a century. Several studies have examined some potential problems and limitations of hemispherical photography from image acquisition and storage to analysis. General users of hemispherical photography should be aware of situations when these problems might have major impact on their studies, and of solutions to many of the problems, which rely on advances in techniques and associated computer technology. In this study, we examined the influences of camera type, resolution, compression, and the image analysis package on the estimation of canopy light transmittance using hemispherical photography. The results indicate that compression did not affect the estimation of canopy light transmittance in our studied forest but resolution did; higher resolutions yielded higher estimates of canopy light transmittance. We found that storing uncompressed images is much more time consuming, which can be a disadvantage in the field when the time-period suitable for hemispherical photograph acquisition is limited. We recommend the use of compressed high-resolution images in forests with dense canopies because they provide better distinction between small plant components and gaps. Digital and film camera systems differed in estimates of canopy light transmittance by more than 5%, as did different analytical packages. Any cross-study comparison must look into the resolution, software, and camera system being used. If canopy light transmittance differs by less than 10%, researchers should interpret their results with great caution. We also found that estimates of indirect light transmittance were less robust than direct light transmittance. This is because there is less uncertainty in the calculation of transmittance for the center of a hemispherical photograph which at subtropical latitudes is mostly located in the sun path and contributes more to direct than indirect light transmittance.