Spectral ratios of biologically active radiation in a mixed forest

Light quality and quantity affect different characteristics in growth and competition within forest stands. These parameters deliver crucial energetic information about the light environment to the plants. The spectral irradiance of the biologically active waveband is strongly influenced by the architecture of the canopy. Plant canopies operate as light filter concerning the spectral and spatial distribution of light at the ground level. Important ecological processes are controlled through changes in the light environment e.g. regeneration, establishment, and physiological processes of productivity. Dependent on the spectral composition, processes such as seed germination, stem growth, leaf expansion and orientation, flowering, and dormancy are triggered or inhibited. The ratio of red and far-red is determined as the status of the phytochrome photoequilibrium and important for the detection of competition, structure and architecture, and internode expansion. R light is strongly absorbed by photosynthetically active pigments, on the other hand FR is mostly transmitted or reflected. Blue light is primarily sensed by cryptochromes as photoreceptors and influences growth, development of higher plants, and promotes stomatal opening more than other spectral wavelengths. The blue range of the spectrum is characterized by low reflectance and high absorption in contrast to FR and near infrared. The aim of this work is to describe how light quality and quantity represented by PAR and the spectral ratios R/FR and B/R vary in deciduous and coniferous stands.

Measurements were carried out in the mature Norway spruce and European beech stand „Kranzberger Forst“ approximately 35 km northeast of Munich, Germany. A 130 sensor multichannel measurement system covering the range of 360-1020 nm was deployed for more than a year. The spectral measurements were performed for five different stand heights (3, 14, 17, 20, 23 m above ground) and above the canopy with a spectral resolution of 0.8nm.

When penetrating a canopy, light quantity as well as the spectral composition is changed significantly. Most of the absorption and spectral changes take place in the upper part of the sun crown. These processes are strongly dependent on solar elevation and sky conditions that determine the incoming radiation and the incident angle of direct irradiance. It is shown that under clear sky conditions more PAR can penetrate the canopy at higher solar elevation. This effect is more pronounced for spruce than for beech due to the conical shape that allows photons from higher angles to enter the gaps in between trees in contrast to the more homogeneous surface of the beech canopy. Solar elevation is not a relevant factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing both species or different sky conditions, while the frequency of sunflecks is not strongly dependent on either factor. Not surprisingly, the highest frequency of sunflecks can be observed in the upper parts of the canopies. Only very few sunflecks can be measured in the shade crown and on the forest floor of this mature stand during foliation of beech.

Sky conditions above the canopy have no great impact on the R/FR. Within the forest stand this situation changes in dependence of the species. The higher R/FR especially during OVC conditions is due to the fact that more diffuse unattenuated radiation can penetrate omni directional from the upper hemisphere into the forest stand and foliage gaps. The results show that deciduous canopies are denser than the clumped coniferous stands even under diffuse conditions where the impact of scattered light from all directions is quasi homogenous. R/FR determines the state of the phytochrome photoequilibrium and is to be seen as indicator for competition. The higher values in the upper layers of spruce during OVC and CS conditions are the effect of the special cone-shaped habit leading to a stepwise decrease of R/FR down to the shade crown. In comparison, beech shows a strong decrease within the layers of the sun and shade crown, respectively, linked to the dense, broadleaved architecture. The canopy density clearly affects the shading of the lower levels. Higher R/FR values at the forest basement are linked to scattered unattenuated radiation from the side, especially under low solar angles and OVC conditions. Low solar elevation angles are important for R/FR and the linked processes.

The analysis for B/R again shows the impact of biomass onto the ratio. All values under beech are lower than under spruce due to higher absorption of the broadleaved specie. For days with higher direct radiation an increase of the B/R ratio in the upper layers was evident for both species. Studies have shown that more blue light is absorbed by biomass than red light. In our case the opposite behavior can be observed in the upper layers which may be due to reflectance properties of sun leaves and bark. The B/R exhibits a strong dependence on the solar elevation angle. The variability of the values in the middle layers for spruce and the upper layers for beech show up to twofold increased values. The highest B/R ratios occur at low solar elevation independent of sky conditions.