Estimating light use efficiency by linking flux tower data and fraction of absorbed PAR at chlorophyll level (FAPARchl) derived from daily MODIS observations

We used daily MODIS imagery obtained over 2001-2005 to analyze the seasonal and interannual photosynthetic light use efficiency (LUE) of the Southern Old Aspen (SOA) flux tower site located near the southern limit of the boreal forest in Saskatchewan, Canada. This forest stand extends for at least 3 km in all directions from the flux tower. The MODIS daily reflectance products have resolution of 500 m at nadir and > 500 m at off-nadir. To obtain the spectral characteristics of a standardized land area to compare with tower measurements, we scaled up the nominal 500 m MODIS products to an area of 2.5 km × 2.5 km (5×5 MODIS 500 m grid cells). We then used the 5×5 scaled-up MODIS products in a coupled canopy-leaf radiative transfer model, PROSAIL-2, to estimate the fraction of photosynthetically active radiation (PAR) absorbed by the photosynthetically active part of the canopy dominated by chlorophyll (FAPARchl) versus that absorbed by the whole canopy (FAPARcanopy). From the tower measurements, we determined 90-minute averages for APAR and LUE for the physiologically active foliage (APARchl, LUEchl) and for the entire canopy (APARcanopy, LUEcanopy). The flux tower measurements of GEP were strongly related to the MODIS-derived estimates of APARchl (r2 = 0.78) but weakly related to APARcanopy (r2 = 0.33). Gross LUE (slope of GEP:APAR) between 2001 and 2005 for LUEchl was 0.0241 μ mol C μ mol -1 PPFD whereas LUEcanopy was 36% lower. Inter-annual variability in growing season (DOY 152-259) LUEchl (μ mol C μ mol -1 PPFD) ranged from 0.0225 in 2003 to 0.0310 in 2004. The five year time series of growing season LUEchl corresponded well with both the seasonal phase and amplitude of LUE from the tower measurements. We conclude that LUEchl derived from MODIS observations could provide a useful input to land surface models for improved estimates of ecosystem carbon dynamics.