Relating a Spectral Index from MODIS and Tower-Based Measurements to Ecosystem Light Use Efficiency for a Fluxnet-Canada Coniferous Forest

Hyperspectral reflectance data collected diurnally from an instrumented tower were examined in conjunction with the eddy correlation fluxes and meteorological measurements made throughout a growing season at a mature Douglas fir forest in British Columbia, Canada (DF49). Here we present 2006 in situ results relating the Photochemical Reflectance Index (PRI551) to photosynthetic light use efficiency (LUE). Canopy structure information was used to partition the forest canopy into sunlit and shaded fractions. At each observation period, the PRI551 was examined for the sunlit, shaded, and mixed sunlit/shaded canopy segments as defined by their instantaneous position relative to the solar principal plane (SPP). We show that the PRI551 clearly captures the differences in leaf groups on sunny days. We also examined PRI551 from MODIS ocean band imagery acquired over DF49 during a five year period (2001-2006) from both Terra (late morning) and Aqua (early afternoon) platforms. The MODIS observations from Terra and Aqua were acquired in different viewing planes above the landscape over a range of view zenith angles, and sampled the backscatter (sunlit) and forward scatter (shaded) sectors of the forest's bidirectional reflectance distribution function. When tower-based bulk canopy LUE from 2006 was recalculated to estimate foliage-based values for the three foliage groups under their incident light environments, a strong linear relationship with PRI551 was demonstrated (r2~0.80). A similar relationship between the MODIS PRI551 and tower-based bulk LUE was obtained from satellite observations (r2~0.62), but only for the backscatter observations obtained at high light levels (APAR, >1500 mumol m-2 s-1). The MODIS observations, while not ideal in terms of spatial resolution (> 1 km2) or optimal viewing configuration, nevertheless provided a means to monitor forest under stress using narrow spectral band indices and off-nadir observations.