Biophysical controls on the directional spectral reflectance properties of bracken (Pteridium aquilinum) canopies:: results of a field experiment

Bracken presents a serious environmental problem in many regions of the world and remote sensing offers the potential of monitoring the invasion and management of this plant. This study evaluates this potential by reporting the results of an investigation of the relationships between the biophysical properties of a bracken canopy and its spectral reflectance and how such relationships are effected by view angle. An experiment was carried out through the bracken growing season at sites in West Sussex, UK. In the principal plane of the Sun, red reflectance becomes lower and relatively less variable with view angle as the canopy develops, while near-infrared reflectance increases and becomes relatively more variable with view angle. As Leaf Area Index (LAI) increases, the variability of the Normalized Difference Vegetation Index (NDVI) with view angle decreases, while the simple ratio vegetation index (SR) behaves in almost the opposite manner. Polar plots, illustrating full directional reflectance distributions, reveal that as the canopy develops the hot spot in red reflectance at the retro-solar angle diminishes, to be replaced by enhanced red reflectance at high zenith angles in the forward-scatter direction. Near-infrared reflectance behaves very differently, as a hot spot at the retro-solar angle becomes more pronounced as the canopy grows. The angular distribution of NDVI and SR varies markedly over the growing season, however both indices are well correlated with canopy biophysical properties for most view angles sampled, apart from the extreme off-nadir. Red edge position was largely insensitive to view angle, but had a good correlation with LAI, as did percenage reflectance at the red edge position.