TRANSMISSION OF SOLAR-RADIATION IN BOREAL CONIFER FORESTS - MEASUREMENTS AND MODELS

A combined geometric-optical and radiative transfer (GORT) model allow s incorporation of multiple scales of clustering in conifer canopies o n the estimation of radiation transmission. Consideration of clusterin g of branches into whorls is the latest addition to this model. Modifi cation of the GORT model to include whorl orientation improves the abi lity to model the observed patterns of solar radiation transmission as a function of solar zenith angle and height in the canopy. Whorl orie ntation distributions are derived from multidirectional measurements u sing a geometric optical mutual shadowing model. For BOREAS test stand s, model estimates and vertical measurements of photosynthetically act ive radiation transmittance within the canopy show (1) general decreas es in transmission as solar zenith angles increase in the range of sol ar zenith angles dominated by beam irradiance, (2) increases in PAR tr ansmission at very high solar zenith angles where diffuse skylight is dominant, (3) maximum scattering and absorption occur in the middle of the canopy. Model estimates match measurements from the forest floor, indicating the value of the model for providing radiation inputs to s nowmelt models in forested landscapes.