Retrieval of leaf area index from top-of-canopy digital photography over agricultural crops

Leaf area index (LAI) was estimated from vertical gap fraction measurements obtained using top-of-canopy digital colour photography over corn, soybean and wheat canopies. A histogram-based threshold technique was used to separate green vegetation tissues from background soil and residue materials in order to derive the canopy vertical gap fraction from the digital photos. The results show that the gap fraction obtained using digital photography was linearly related with the diffuse non-interceptance obtained with a LAI-2000 plant canopy analyzer (R2 = 0.78). LAI derived from the photographic method was comparable to the LAI measured using LAI-2000 (R2 = 0.83) in the absence of senescence. We recommend using digital photography in addition to conventional equipment for acquiring LAI and gap fraction of agricultural crops, because the approach is less limited by radiation conditions, and the protocol can easily be implemented for extensive sampling at low cost.