Mapping defoliation during a severe insect attack on Scots pine using airborne laser scanning

In a balanced experiment based on 20 field plots located in a 21 km2 Scots pine forest in southeast Norway covering age classes from newly regenerated to old forest, leaf area index (LAI) was determined in field by a LAI-2000 instrument and hemispheric photography. Based on a formalized framework, i.e., the so-called Beer-Lambert law, gap fraction derived from small-footprint airborne laser scanner data was regressed against field-measured LAI. LAI was strongly (R2 = 0.87–0.93), positively, and linearly related to the log-transformed inverse of the gap fraction derived from the laser scanner data. This was as expected according to the Beer-Lambert law, as was the absence of an intercept, producing a directly proportionality of the two variables. We estimated an extinction coefficient for the first return echoes to be 0.7, fortunately fairly stable across age classes, and this is likely to be a parameter specific for the applied laser scanner system under the given flight and system settings, such as flying altitude and laser pulse repetition frequency. It was demonstrated that airborne laser was able to detect defoliation in terms of estimated changes in LAI, by three repeated laser data acquisitions over the area where severe insect attacks were going on in between the acquisitions.