Estimating tree crown dimensions using digital analysis of vertical photographs

The aim of this study was to develop a relatively rapid, simple and repeata ble method to characterize the crown architecture of Queensland maple (Flin dersia brayleyana F. Muell.) by vertical hemispherical photography. Calibra tion scales were developed to provide horizontal planes of measurement at h eights from 15 to 40 m above the camera. Hemispherical images of tree crown s were merged with the specific calibration scale appropriate for the mid-h eight of the crown in order to adjust for image distortion during measureme nt. Merged images were analyzed by three procedures to yield measurements o f crown diameter and projected area. The most precise measurement of projec ted crown dimensions was assumed to be provided by a complete analysis of a digital image of the crown, after removal of the trunk image. Crown diamet ers estimated by tape measurements on the ground were relatively imprecise (R-2 = 0.54), Crowns classified as symmetrical were described much more pre cisely (R-2 = 0.72) than those classified as asymmetrical (R-2 = 0.37). The precision of estimates of crown dimensions by tape did not alter significa ntly over the range of crown sizes sampled (2-10 m diameter in trees 15-40 m tall). This novel image analysis method can yield relatively precise and accurate measurements on the relatively shallow crowns of tall trees, often found in rainforests, regardless of the degree of asymmetry in crown shape s. However, the method is slower to apply than conventional tape techniques and additional equipment is required. The principal advantage of the photo graphic technique is the opportunity for precise measurements of changes in crown size and foliage projected area over time. (C) 2000 Published by Els evier Science B.V. All rights reserved.