Can the plant area index of a submerged vegetation canopy be estimated using digital hemispherical photography?

• We tested digital hemispherical photography (DHP) on submerged vegetation.
• An exposure setting of 4–8 times the sky reference exposure was identified.
• A photographic distance of 60–90 cm is recommended for PAI estimation by DHP.
• We proved the feasibility of DHP for PAI estimation for submerged vegetation.

Digital hemispherical photography (DHP) has been widely used to estimate the plant area index (PAI, a surrogate for leaf area index, LAI) for terrestrial vegetation. However, DHP has not been applied in aquatic environment fields for submerged vegetation, where only a few methods are available for LAI measurement. The objective of this study was to identify whether DHP could be used to estimate the PAI of submerged vegetation (the total projected area of leaf and stem per unit of ground surface area) using the submerged plant species Potamogeton malainus. The results suggested that the exposure setting and photographic distance from the water surface significantly influenced the gap fraction derivation of upward photographs taken from beneath the water surface. To derive the gap fraction accurately from upward photographs, we recommend an exposure setting of 4–8 times the sky reference exposure and photographic distances of 60–90 cm from the water surface to take upward photographs. A simple model was developed to calculate the percentage of projected stem (or leaf) area deeper than the camera lens to the entire projected area of aquatic vegetation; the model was proven effective by field-truth data and could thus be used to compensate for the projected area that is undetected by the camera. Using our recommended exposure settings, photographic distance and adjusted model, a significant linear relationship was observed between the measured PAI and PAI estimated using DHP (p < 0.001), and only a 5.1% deviation was found between the measured and estimated PAI. Therefore, DHP can be used to estimate the PAI for certain submerged vegetation species, such as Potamogeton malainus, using our recommended optimum procedure.

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