Some observations on the spectral distribution characteristics of short‐wave radiation within Pinus radiata D. Don canopies

Abstract

Abstract. Measurements of the photosynthetic photon flux density (400–700 nm) and of the spectral distribution of photon flux density across the 370–800 nm waveband, were made under both clear and overcast sky conditions above and at various positions within two Pinus radiata canopies of different stocking but similar leaf area indices. The spectra obtained for the daylight conditions (i.e. above forest canopy) were generally similar to those published previously. The spectra for shadelight within the forest canopy showed no blue peak which was characteristic of previously reported measurements which were restricted to the diffuse radiation component. There was almost neutral absorption within the 400–700 nm waveband, and typical lower attenuation in the 700–800 nm waveband. The blue: red ratio was largely unchanged by either canopy type or sky conditions and varied between 0.57 and 0.81. The red: far‐red ratio in shadelight was between 0.22 and 0.41 under clear sky and between 0.68 and 0.95 under overcast sky conditions. Values for daylight were between 1.16–1.22. Calculated phytochrome photoequilibrium values in shadelight were approximately 0.35 under clear sky and 0.46 under overcast sky conditions. In each case there appeared to be no differences between the two canopies with respect to these minimum values.