Measurement of forest canopy structure by a laser plane range-finding method: Improvement of radiative resolution and examples of its application

Abstract

In studies of flux in forests, the measurement and description of forest canopy characteristics are important but difficult tasks. In 1998, we assessed a laser plane range-finding method as a new, non-destructive way of measuring the three-dimensional (3-D) structure of forest canopies. Our ultimate goal is to develop the method and the instrument in such a way that we can accurately measure the 3-D architecture of forest canopies and automatically distinguish foliage and stems. In the current study we improved the system concerning the radiative resolution. This improvement was conducted by introducing a highly sensitive digital charge-coupled device (CCD) camera whose exposure time could be controlled by a personal computer and a proposed algorithm of plural exposure time. We expect the principle of using two wavelengths of laser light to be valid for distinguishing foliage from stems. Distinguishing each element of the canopy was conducted successfully by the regional segmentation method. With this system, we could acquire a hemispherical range-finding image in which each pixel had its own range-finding information. As an example of the application of this system, we proposed a new method to calibrate Leaf Area Index (LAI) value using the hemispherical range-finding image as follows: (1) normalizing the canopy into a horizontal unit thickness (1 m), (2) compensating radiative intensity for normalized canopy by using Beer’s Equations, (3) calculating LAI of the normalized canopy, and (4) reconstructing “Real canopy” and estimating “Real LAI” by multiplying by the canopy thickness. Practical examples executed in a real forest have proved that this new method is efficient.