Comparison of basal area and stem frequency diameter distribution modelling using airborne laser scanner data and calibration estimation

The diameter distribution of a forest stand is of primary importance for deriving various stand characteristics, but measuring the diameters of a tree stock is rather time-consuming, so theoretical diameter distribution models are usually employed. We examine here the prediction of diameter distribution using information obtained from airborne laser scanning (ALS). The aim is to compare the accuracy of ALS-based stem frequency and basal area diameter distribution models and the usability of calibration to adjust the predicted distributions to be compatible with the ALS based estimated stand volume. A network of 472 sample plots of 9 m radius was established in a typical managed boreal forest area in eastern Finland. ALS data were acquired from an altitude of 1500 m above ground level, which produced a sampling density of about 0.7 pulses per square metre. A two-parameter Weibull function was employed to determine field-measured stem frequency and basal area diameter distributions using maximum likelihood estimation, and these parameters together with the number of stems, basal area and stand volume were then regressed simultaneously using ALS-based characteristics as explanatory variables. Predicted basal area diameter distribution from each plot was scaled using the ALS-based estimate for the basal area, and the stem frequency diameter distribution was scaled using the ALS-based estimate for the stem number. Finally, the distributions were calibrated against volume, basal area and number of stems which were ALS data based model estimates. Without calibration estimation the accuracy of the volume prediction was almost equal for both distribution types (basal area and stem frequency). However, calibration estimation improved the accuracy of the volume estimates of both distribution types and also of the stem number estimates in the case of the basal area distribution.