Transmissivity of solar radiation within a Picea sitchensis stand under various sky conditions

Abstract. We tested the hypothesis that diffuse radiation from cloudy and overcast skies penetrates the canopy more effectively than direct radiation from clear skies. We compared the flux density and spectral properties of direct and diffuse radiation (around solar noon (±1 h)) above, within and below a forest stand under sunny, cloudy and overcast conditions in a thinned Sitka spruce (Picea sitchensis (Bong.) Carr.) forest (28 years old, with a leaf area index of approximately 5.2 m² m⁻²). We recorded vertical profiles of radiation penetration (from 350 to 1050 nm), and we also explored the horizontal pattern of radiation along a 115 m transect.

We showed that in "clear sky" conditions, the photosynthetically active radiation in the lower parts of the canopy was substantially attenuated, more so than under cloudy and overcast skies. It was particularly depleted in the blue part of the spectrum, but only slightly blue-depleted when the sky was overcast or cloudy. Moreover, the red : far-red ratio under clear skies fell to values less than 0.3 but only to 0.6 under cloudy or overcast skies. Near the ground, the light climate was strongly influenced by the thinning pattern (carried out in accordance with standard forestry management practice).