Airborne Estimation of Boreal Forest LAI in Winter Conditions: A Test Using Summer and Winter Ground Truth

A recently developed airborne method for estimation of leaf area index (LAI) in coniferous forests is used for comparing the LAI values in summer and winter conditions. The airborne measurements based on a wide-optic camera are carried out in winter when the forest floor is completely snow covered and thus acts as a light background for the image analysis. The photographs are taken automatically and stored on a laptop during the flights. The R² value of the linear regression between the airborne and ground-based LAI measurements was 0.97 for all plots. Despite the unfavorable weather conditions, the average difference between the ground-based and airborne regression-based LAI estimates was 0.08, and in 90% of the cases, it was smaller than 0.13. The corresponding relative differences were 14% and 23%. The standard deviation of the ground-based LAI values measured within a plot was, on the average, of the same order. The winter-time values of the LAI of coniferous trees were estimated to be 24% smaller than the preceding summer-time values.