Polarization characteristics of forest canopies with biological implications

Abstract

In this chapter we show that the pattern of the direction of polarization of sunlit grasslands and sunlit tree canopies is qualitatively the same as that of the clear sky. Since the mirror symmetry axis of this pattern is the solar—antisolar meridian, the azimuth direction of the sun, occluded by vegetation, can be assessed in forests from this polarization pattern. This robust polarization feature of the optical environment in forests can be important for forest-inhabiting animals that make use of linearly polarized light for orientation. Here we also present an atmospheric optical and receptor-physiological explanation of why longer wavelengths are advantageous for the perception of polarization of downwelling light under canopies illuminated by the setting sun. This explains why the upward-pointing ommatidia of the dusk-active cockchafers, Melolontha melolontha, detect the polarization of downwelling light in the green part of the spectrum. We show that the polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset. This is an optimal compromise between simultaneous maximization of the quantum catch and the quantum catch difference.