Light transmittance in forest canopies determined using airborne laser altimetry and in-canopy quantum measurements

The vertical distribution of light transmittance was derived from field and laser altimeter observations taken in the same canopies of five forests of several ages (young to mature) and canopy types (eastern broadleaved and w estern tall conifer). Vertical transmittances were derived remotely from th e Scanning Lidar Imager of Canopies by Echo Recovery (SLICER) laser altimet er and in the field from measurements of Photosynthetically Active Radiatio n (PAR) made within the canopy using quantum sensors suspended from the gon dola of a tower crane or atop small balloons. Derived numerical characteris tics of mean transmittance profiles (the rate of attenuation, whole canopy transmittance, and the radiation-effective height) were similar for both me thods across the sites. Measures of the variance and skewness of transmitta nce also showed similar patterns far corresponding heights between methods. The two methods exhibited greater correspondence in the eastern stands tha n in the western ones; differences in the interaction between canopy organi zation and the sensor characteristics between the stand types might explain this. The narrower, more isolated crowns of the western stands permit a de eper penetration into the canopy of nadir-directed laser light than of dire ct solar radiation from typical elevation angles. Transects of light transm ittance in two stands demonstrate that the SLICER sensor can capture meanin gful functional variation. Additionally, for one stand with numerous overla pping transects we constructed a three-dimensional View of the transmittanc e field. Using geostatistics, we demonstrated that the spatial covariance m easured in the horizontal plane varied as a function of height. These resul ts suggest a means to remotely assess an important functional characteristi c of vegetation, providing a capacity for process-based ecological studies at large scales. (C) 2001 Elsevier Science Inc. All rights reserved.