Assessing sensor effects and effects of leaf-off and leaf-on canopy conditions on biophysical stand properties derived from small-footprint airborne laser data

Canopy height distributions were created from small-footprint airborne laser scanner data collected over 51 georeferenced field sample plots with a size of 232.9 m2 and 27 large test plots with an average size of 3435 m2. Laser data were acquired under leaf-on and leaf-off canopy conditions. The plots covered stand conditions from young forest to mature forest. The plots were divided into two categories, i.e., coniferous forest dominated by spruce and pine, and mixed forest with an average proportion of deciduous species of 31-42%. Height percentiles, mean and maximum height values, coefficients of variation of the heights, and canopy density at different height intervals above the ground were computed from the laser-derived canopy height distributions. In the mixed forest, corresponding metrics derived from the two laser data acquisitions were compared. In general, canopy metrics derived from the last returns were more affected by canopy conditions than the first return data. Furthermore, canopy height measures of the lower and intermediate parts of the canopy were more affected than maximum canopy height, and the variability of the height distribution tended to increase from leaf-on to leaf-off conditions. The coniferous plots were used to demonstrate to what extent canopy properties derived from airborne lasers may be affected by sensor-specific characteristics. The same laser system was used during the two acquisitions, but the repetition frequency was upgraded from 10 to 33 kHz in between the two missions. Comparison of the two acquisitions showed that the first return measurements of canopy height tended to be unaffected or shifted somewhat upwards by system upgrade and ground penetration was reduced, whereas the last return data indicated unaffected or downwards shifted canopy heights and increased penetration.