Assessing stand structure of beech and spruce from measured spectral radiation properties and modeled leaf biomass parameters

Solar radiation is crucial for growth and competition within forest ecosystems. The spectral waveband between 400 and 700. nm is mainly responsible for photosynthesis and thus for plant growth. Within this spectral waveband, single spectral ratios (e.g. blue/red, green/red, red/far-red) influence and trigger different processes like leaf expansion, germination, stem growth and flowering. Spectral irradiance and biomass are heavily interrelated. Spectral radiation measurements covering the range 360-1020. nm were carried out with 130 sensors in six stand levels in a mixed plantation of Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica L.). However, direct measurements of vertical and horizontal distributions of foliage are very complex and time-consuming and for this reason foliage biomass parameters of leaf area index (LAI) and specific leaf area (SLA) modeled by the growth model BALANCE are useful parameters to give complete stand representations. The interaction between modeled biomass parameters of European beech and Norway spruce and measured radiation profiles through all stand levels in this unique spectral and spatiotemporal resolution was the aim of this study. Both species showed the typical response to variation in light availability for both modeled parameters. Results exhibit a significant negative relationship between LAI and the photosynthetical photon fluence rate (PPFR) for both species. The blue/red (B/R) ratio showed significant negative relationships to LAI of both species. The vertical distribution of green/red and red/far-red in respect to LAI varied depending on the species and their morphological crown habit. Analyses of SLA and radiation under spruce showed no significant relationship at all. In contrast, beech showed significant relationships of SLA and the spectral ratios of red/far-red and blue/red.