Specular reflection in the signal of LAI-2000 plant canopy analyzer

•Specularly reflected sky radiation affects the LAI-2000 results.•More than 60% of leaves reflect sky radiation specularly to the PCA sensor.•Bias in the estimated LAI may be more than 20% in case of overcast sky.

In a vegetation canopy part of leaves reflect sky radiation specularly to the LAI-2000 sensor. This fraction is smaller in a planophile canopy and higher in an erectophile canopy. As the specularly reflected radiation does not enter the leaf nor it is absorbed by leaf pigments and depending on the incidence angle on the leaf the reflection coefficient may be very high. Depending on the leaf area index (LAI) and leaf angle distribution the scattered sky radiation may cause an overestimation of the gap fraction and consequently an underestimation of LAI up to 20–25% if the LAI measurements with plant canopy analyzer LAI-2000 are carried out under ideal perfectly overcast conditions. Similar biases are present in measurements of gap fraction and LAI with hemispherical photos if these measurement procedures are calibrated against LAI-2000. Considering the scattered sky radiation in optical techniques (LAI-2000, hemispherical photos) of measuring gap fraction in vegetation canopies explains at least part of the observed discrepancies in gap fraction estimated with terrestrial laser scanner and hemispherical photos. The scattered sky radiation in the LAI-2000 signal introduces bias both in the estimated LAI and foliage clumping index.