Quantifying the relationship between light quality and light availability at different phenological stages within a mature mixed forest

A quantitative comparison of light quality and light availability is performed over the entire vegetation period within a mature mixed Norway spruce (Picea abies [L.] Karst)-European beech (Fagus sylvatica L.) forest stand in Southern Germany. Data are obtained by a multichannel-system based on fiber optic technology and a diode-array spectrometer that measures incoming spectral photon fluence rates near-simultaneously at 130 positions within the canopy. Measurements are carried out by spatially integrating spherical detectors with high spectral resolution (0.8 nm) in the photosynthetically and photomorphogenetically relevant waveband from 360 nm to 1020 nm for both species under different sky conditions and at different phenological stages. Results show a non-linear relation between the light availability expressed by the relative photosynthetic photon fluence rate (PPFR_rel) and the light quality represented by the red/far-red ratio (R/FR). The relationship varies seasonally, type-specifically, and by meteorological conditions. A descriptive regression analysis reveals higher coefficients of determination for the rectangular hyperbolic function under clear skies than under overcast skies with only diffuse light penetrating the canopy. The variation is better explained for beech while foliated than for spruce. Throughout the vegetation period, beech exhibits a higher variability of the R/FR relationship than spruce. The presented results offer an easy technique to assess light quality by measurements of light availability and vice versa under several different conditions for a mature temperate mixed forest.