Horizontal transport induced by upwelling in a canyon-shaped reservoir

P.N. Okely, Jorg Imberger

Research output: Contribution to journalArticle

18 Citations (Scopus)

Abstract

This study compares circular fisheye photography and destructive leaf area index (L) estimation with two alternative indirect methods for estimating L in broad-leaved forest: fullframe fisheye photography and cover photography. Fullframe fisheye photography differs from circular fisheye photography in that the images have a reduced field of view; the zenithal range of 0°–90° extends to the corners of the rectangular image, roughly doubling image resolution compared to circular fisheye images. Cover images are obtained by pointing a 70 mm equivalent focal length lens (in 35 mm format) straight upwards. Cover and L were measured in twelve stands of a 17 years old Eucalyptus marginata forest that had been planted at four initial densities: 625, 1250, 2500 and 10,000 trees per hectare. L, from destructive sampling, averaged between 2 and 2.4 for stands planted at between 1250 and 10,000 trees ha−1 but was only 1.3 for the stands planted at 625 trees ha−1. Cover photography provided good indirect estimates of L assuming a spherical leaf distribution, except in the stands with 10,000 trees ha−1. These trees appeared to have a more horizontal leaf angle based on the calculated zenithal extinction coefficient for those stands (not, vert, similar0.7). Rapid and automated analysis of cover images using WinSCANOPY 2006 yielded similar results to manual image analysis using Adobe Photoshop. Estimates of L from fullframe fisheye photography were better correlated with L from destructive sampling than L estimated from circular fisheye photography, but neither performed as well as cover photography. Photographic methods that use a single threshold to separate sky and foliage appear less sensitive to the camera's gamma function than methods that use a two-value threshold. Higher resolution (>8 megapixel) cameras and better lenses, may further improve L estimation using fullframe fisheye photography. We recommend that cover photography be used for routine L estimation in broadleaf forest until it is demonstrated that fisheye methods can provide similar accuracy.
Original languageEnglish
Pages (from-to)343-355
JournalHydrobiologia
Volume586
Issue number1
DOIs
Publication statusPublished - 2007

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