VARIABILITY IN THE FRACTION OF INTERCEPTED PHOTOSYNTHETICALLY ACTIVE RADIATION: EFFECTS OF IRRADIANCE CONDITIONS AND TEMPORAL SCALES

Light interception by plant canopies is one of the main drivers of biomass production. Quantifying spatiotemporal patterns of the fraction of intercepted photosynthetically active radiation (FIPAR) is essential in quantifying the CO2 assimilation at the landscape level, in ecosystem process modeling and for the interpretation of remote sensing data. To adequately characterize FIPAR, one important issue is temporal scale, as FIPAR is temporally dynamic and it is constantly changing under different irradiance conditions. Using ground measurements of FIPAR over various landscape ecosystems, this study addresses the impact of irradiance conditions and temporal scales on variability of FIPAR. This research examines 1) the variability of FIPAR based on hourly, daily, 10- and 30- daily integrated data, and 2) the impact of irradiance conditions on FIPAR as a function of solar zenith angle (SZA), weather conditions and seasonality. The goal is to examine the most optimal time and temporally integrated steps in generating FIPAR for better landscape characterization. Both the instantaneous and integrated FIPAR values for over 78 ecological sampling units (ESU) worldwide were generated and compared for different irradiance conditions and temporal periods. The findings suggest that diurnal time and solar zenith angle (SZA) of illumination are more important than weather condition and integration time when the instantaneous and integrated FIPAR values are compared. The instantaneous in-situ measurements serve as a good estimator of the integrated values when acquired in the morning or afternoon as a function of SZA. Both, non-linearity and high variability in the relationship between instantaneous and integrated FIPAR are observed at low SZA diminishing when instantaneous measurements are collected at around 9:30 or 14:30.