Light spectral composition in a tropical forest: measurements and model.

I present a simple model that simulates vertical variations in the light spectrum within a forest canopy. The model considers only the vertical, downward transmission of light. The light in each canopy level was assumed to consist of non-intercepted radiation and radiation intercepted within the level and transmitted. The spectrum of non-intercepted light in each canopy level is the same as that of incident light above the canopy (input parameter), whereas the spectrum of transmitted light depends on leaf area index (LAI) and the mean transmission spectrum of leaves. The model was tested in a forest and provided adequate predictions of measured values. Stronger deviations were produced in the near infrared (NIR) waveband in lower canopy levels. Multiple regression between LAI, as the dependent variable, and spectral characteristics (Blue, Green, Red and NIR intensities) had an r(2) of 0.926. As a complement to other methods, I suggest light spectrum analysis as a non-destructive technique for estimating LAI in forest canopies.