Characterizing plant canopies with hemispherical photographs.

Hemispherical (fisheye) canopy photography is a technique for characterizing plant canopies using photographs taken through an extreme wide-angle lens (Anderson, 1964). Typically, the viewing angle approaches or equals 180°. Hemispherical photographs can be taken either looking upward from within a canopy or looking downward from above a canopy. The resulting photographs serve as permanent records of the geometry of canopy openings. The geometric distribution of openings can be measured precisely and used to estimate potential solar radiation penetration through openings and to determine aspects of canopy architecture such as ground cover, leaf area index (LAI), and leaf angle distribution. Hemispherical photographs can be analyzed by hand using sampling grids; however, hand analysis is extremely tedious and generally impractical for large numbers of photographs. Digital image analysis techniques have recently been developed that allow for efficient analysis of large numbers of photographs (Chazdon and Field, 1987b: Rich, 1988, 1989; Becker et al., 1989).

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