Increasing the precision of canopy closure estimates from hemispherical photography: Blue channel analysis and under-exposure

• We tested improvements to hemispherical photography estimates of canopy closure.
• Blue channel analysis significantly decreased variance of canopy closure estimates.
• Overexposed images had higher variance than auto- or underexposed images.
• Hemispherical photography performed similarly to Li-Cor LAI2200.
• Hemispherical photography is a cost effective alternative for quantifying canopies.

Accurate measurement of canopy structure is fundamental to the fields of ecological modeling and restoration. A large number of methods exist for estimating the structure of forest canopies, with widely varying costs and effectiveness. Hemispherical photography has been in use for several decades, and the rise of lower-cost consumer grade digital SLR cameras has expanded the availability of this technique. We examine two improvements to the hemispherical photography technique for estimating canopy closure: computer-based blue channel analysis and under-exposing images. Photographs taken in the field (without a filter) showed much lower variation in the blue channel than in red or green channel of the same images. We found a higher variance in canopy closure measurements due to over-exposure of images, while images with automatic light metering and under-exposed images remained consistent. We conclude that under- or normal exposure combined with blue channel analysis together minimize variability and maximize the precision of canopy closure estimates. Results from hemispherical photography were comparable to the widely used LAI-2200, supporting hemispherical photography as a viable, low-cost alternative.

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