Light-Transmission Profiles in an Old-Growth Forest Canopy: Simulations of Photosynthetically Active Radiation by Using Spatially Explicit Radiative Transfer Models

Light interception is a driving variable for many key ecosystem processes in forests. Canopy gaps, as natural irregularities, are common features of Pacific Northwest conifer forests and have profound importance on the within-canopy light environment. We used two spatially explicit radiative transfer models (OLTREE and SolTran) to understand better the vertical profile distribution of light penetration in an old-growth forest. Canopy access at the Wind River Canopy Crane Research Facility provided an opportunity to apply these models in a tall, old-growth, Douglas-fir-western hemlock forest. Both models required three-dimensional descriptions for every crown (location, orientation, and size) in a 4-ha area. Crowns were then simulated as foliage-filled ellipsoids through which light is attenuated following Beer's law. We simulated vertical profiles (2-m height intervals) of transmitted photosynthetically active radiation (PAR) in 16 gaps previously measured by Parker (1997). Point-by-point comparisons (n = 480) between measured and modeled results showed little agreement because small errors in crown location yielded large local differences in PAR transmittance. However, average gap profiles (n = 16) of PAR transmittance showed excellent agreement$(r^{2}=0.94)$between simulated and measured values. SolTran was used to simulate vertical profiles of daily PAR flux at different seasons for the whole 4-ha canopy, not just gaps. Overall, our results show that both models produced excellent simulations of spatially averaged vertical profiles of PAR transmission in the old-growth forest and are suitable for further investigations at other space and time scales.