Characterisation, prediction and relationships between different wavebands of solar radiation transmitted in the understorey of even-aged oak (Quercus petraea, Q. robur) stands

Solar radiation transmission in forest stands affects many processes, including biomass and diversity of understorey vegetation and tree seedling regeneration (growth and morphogenesis). However, understorey light availability is not easy for forest managers or scientists to measure. Therefore, different models have been developed to predict light transmission in forest stands according to tree or stand structure. However, these models are generally too complex to be used operationally. This paper reports the assessment of light transmission according to stand parameters commonly measured by foresters in inventories. We measured transmittance in 29 even-aged oak stands in France for various wavebands, total solar radiation (TSR, 3003000 nm), PAR (400700 nm), red (R, 660 nm) and far-red (FR, 730 nm), and demonstrated that transmittance in a given waveband can be predicted from the measurement of another waveband. The R:FR ratio can be predicted according to TSR or PAR transmittance, but the opposite is also true; PAR or TSR transmittance can be predicted from the R:FR ratio. Transmittance variability was characterised, and the variation coefficient ranged from 5 to 45% with a trend to increase with tree density. By analogy to BeerLambert's law, we established that mean daily transmittance for the different wavebands can be assessed according to stand basal area and stand age with good accuracy (R2>0.74). Results are discussed in comparison with other models based on the principle of parsimony.