A model to estimate light interception by tree crowns, applied to black spruce

CORONA is a spatially explicit model designed to efficiently estimate the quantity of shortwave radiation intercepted by individual tree crowns. Radiation interception is determined from intersections of light rays with tree crowns for rays originating throughout the sky hemisphere. CORONA comprises a flexible, asymmetric description of crowns, a computationally efficient ray-tracing procedure, numerical solution of ray × crown intersections, and several alternative within-crown light interception functions. The interception functions were calibrated and evaluated in two plots located in black spruce (Picea mariana (Mill.) BSP) stands, where tree positions and crowns were mapped. An interception function using only the number of intercepting crowns (i.e., a hits function) generally resulted in better agreement between modelled and observed light levels than an interception function using the total path length through crowns. There was some evidence that transmission was greater in parts of crowns within low-light-level environments. This effect can be incorporated into an interception function for more realistic modelling of interception. Errors in crown description are likely the main cause of discrepancies between modelled and observed light levels on a half-hourly basis. The agreement between modelled and observed light levels on a daily basis indicates that CORONA can provide useful estimates of light interception by tree crowns.