USING DISCRETE LASER PULSE RETURN INTENSITY TO MODEL CANOPY TRANSMITTANCE

Five comparable airborne lidar datasets were collected over a mixed wood site on five separate occasions throughout a single growing season to capture changing canopy transmittance conditions. Using the small footprint discrete pulse intensity return data, the vertical pulse power distribution was reconstructed for 30 plots each containing 5 digital hemispherical photo (DHP) stations. Canopy gap fraction was calculated for the 150 DHP images collected coincident with the lidar data and used as validation for overhead canopy transmittance. By modifying a Beer-Lambert approach, we relate the ratio of lidar intensity-based ground return power / total return power to the canopy gap fraction. The results are compared to the commonly cited and utilised ground-tototal returns ratio. It is found that for the mixed wood environment studied, a lidar intensity-based power distribution ratio provides a slightly higher coefficient of determination with DHP gap fraction (r 2 = 0.92) than does the often used ground-to-total return ratio approach (r 2 = 0.86). Moreover, if the intensity power distribution ratio is modified to account for two-way pulse transmission losses within the canopy, the model requires no calibration and provides a 1:1 estimate of the overhead (solar zenith) gap fraction. The premise of the study is that the interaction between forest canopy and laser pulses emitted from an airborne lidar (light detection and ranging) mapping system can be considered in some ways analogous to the interaction of direct beam solar radiation with canopy covered environments. We examine the reconstructed vertical pulse power distribution returned from a commercial small footprint discrete pulse airborne laser scanning system and relate properties of the distribution to canopy structural and radiative transfer characteristics. In particular, we compare published gap fraction (P) algorithms to new algorithms that utilize the return intensity information. From the algorithms tested we develop a non-parameterized quasi-physical model of the spatiotemporal variation in canopy gap fraction for a mixed forest landscape. For the purpose of this analysis we make the assumption that overhead gap fraction (P) and overhead transmittance (T) are equivalent.