Modelling Canopy Gap Fraction from Lidar Intensity

We reconstruct the vertical pulse power distribution returned from a commercial small footprint discrete pulse airborne laser terrain mapper within a mixed forest landscape. By modifying a Beer-Lambert approach, we relate the ratio of ground return power / total return power to the canopy gap fraction (P) as derived from digital hemispherical photography (DHP). The results are compared to the commonly cited and utilised ground-to-total returns ratio. Canopy gap fraction data were collected on five separate occasions from April to October of 2006, and analysed using standard DHP procedures. Five airborne lidar datasets were collected during dry conditions coincident with DHP, and all acquisitions were performed using the same sensor and survey configuration. It is found that for the mixed wood environment studied, a lidar intensity-based power distribution ratio provides a higher correlation with DHP gap fraction (r = 0.92) than does the often used ground-to-total return ratio approach (r = 0.86). Moreover, if the intensity power distribution ratio is modified to account for secondary return 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.