Local-scale heterogeneity of photosynthetically active radiation (PAR), absorbed PAR and net radiation as a function of topography, sky conditions and leaf area index

The local-scale spatial distribution of photosynthetically active radiation (PAR), absorbed PAR (APAR) and net all-wave radiation (Q⁎) across the top of a forest canopy was investigated as a function of topography, sky conditions and forest heterogeneity for a forested hilly study site located in south-central Indiana, USA that is part of the FLUXNET and SpecNet networks. The method to estimate spatial variability of radiation components utilized theoretical radiation modeling applied to a topographic model combined with spatial distribution of leaf area index derived from IKONOS imagery and empirical models derived from data collected on a single flux tower. Modeled PAR and Q⁎ compared consistently well with observations from a single tower with differences typically less than 10%, although clear-sky conditions were simulated more accurately than cloudy conditions. Spatial variability of radiation was found to be very sensitive to topographic relief and could be scaled linearly by mean slope angle. Decreases in optical transmissivity and increases in cloudiness had a strong effect of reducing both the spatial average and standard deviation of radiation components. Spatial variability of APAR was 53% greater than PAR and the characteristic scale of variance was reduced due to finer scale and magnitude of variance of LAI. Clear seasonal patterns existed in both spatial average and standard deviation values with summer producing the largest mean values and weakest spatial variability due to smaller solar zenith angles and seasonality in both optical transmissivity (scaled linearly by specific humidity) and cloudiness. These findings of spatial variability illustrate the need to characterize the complex landscape patterns at flux tower sites, particularly where the goal is to relate flux tower data to satellite imagery.