Threshold determination by reference to open sky overcomes photographic exposure error in indirect leaf area index estimation

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  • 全天空写真を用いた葉面積指数推定における撮影時の露出値の重要性と全天条件基準法の提唱

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Abstract

The hemispherical photography method is widely used to estimate the leaf area index (LAI) of forest stands. Both photographic exposure and threshold determination in producing binary images critically affect LAI estimates. To remedy LAI estimation error, we propose a series of protocols that we term the Open-sky Reference Method (ORM). ORM uses reference photographs of an open-sky area and photographs within the canopy at one-stop overexposure, relative to the reference photos. In threshold determination, the reference photograph is used as the maximum digital number classifying the open-sky area into white. In total, 14 stands of multi- or single-layered broad-leaved forests and sparse conifer forests located in the Japanese temperate zone were used to investigate the effects of photographic exposure error (or shift from an optimum value) on LAI estimated using ORM. We also compared our LAI estimates with those obtained using other photographic protocols that are more sensitive to exposure error because LAI estimates change largely in proportion to exposure shifts. ORM stabilized LAI estimates regardless of exposure shifts, indicating that ORM is tolerant of unpredictable field measurement errors and is, thus, a more practical LAI estimation method.

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