Quality and quantity of solar radiation within a mature mixed forest

The quantity and quality of solar radiation play a vital role for growth and competition within forest ecosystems. Radiation directly affects the increase of biomass by its intensity within the spectral band of photosynthesis. Moreover, it regulates growth by means of absorption in the blue and red fraction of the solar spectrum by pigments of phytoelements. The spatial, temporal, and spectral distribution of radiation within a heterogenous plant stand can vary to a large extent. For a realistic characterization of spatially highly variable growth dynamics and competition in mixed stands, detailed parametrization of radiation, especially of crucial spectral bands, is required. Thus, a novel multi-sensor monitoring system based on fiber optic technology and a diode array spectrometer was developed for measuring the radiation field within a 30 m tall mature mixed Norway spruce (Picea abies [L.] Karst) - European beech (Fagus sylvatica L.) forest stand in Southern Germany. At six levels and a total of 130 positions within an area of 25 m x 25 m the respective amount of radiation within the photosynthetically and photomorphogenetically relevant waveband from 360 nm to 1020 nm was determined by spatially-integrating spherical sensors near-simultaneously with a high spectral resolution of 0.8 nm. The measurements were performed over one year continuously, providing a huge dataset and exhibiting many novel results. A quantitative comparison of light quality and light availability for both species under different sky conditions and at different phenological stages shows a non-linear relationship that varies seasonally, type-specifically, and by meteorological conditions. A descriptive regression analysis reveals higher coefficients of determination for the obtained rectangular hyperbolic function under clear skies than under overcast skies. The variation is better explained for beech while foliated than for spruce. Throughout the vegetation period, beech exhibits a higher variability of the red / far red ratio (R/FR) than spruce. The results offer a technique to assess light quality by measurements of light availability and vice versa under several different conditions for a mature temperate mixed forest. Frequency distributions and statistical parameters of the quantitative occurrence of different spectral bands within different areas of the canopy and at the forest ground provide insights in the complex and highly variable radiation field within mature mixed stands. Observations show higher mean levels of radiation at the forest ground under beech at full foliation compared to spruce and a changed situation within the canopy. Temporary enhancements of the radiation quantity caused by sunflecks or penumbra are associated to modifications in the spectral composition. Phenological stages, such as leaf unfolding and leaf fall, can be described by measuring R/FR at an arbitrary position within the beech canopy with high statistical plausibility. The acquired extinction vertical profiles serve for determination of type-specific spectral extinction coefficients. A comparison of standard cosine-corrected flat plate detectors with the spherical sensors, which are superior to exhibit the actual quantity and quality of radiation impinging on a plant, shows major differences between the two measurement techniques. Particularly at low solar altitude, measured radiation values can differ by a factor of six, which can lead to significant errors in calculations of photosynthetical and photomorphogenetical processes.