Can a satellite-derived estimate of the fraction of PAR absorbed by chlorophyll (FAPAR chl) improve predictions of light-use efficiency and ecosystem photosynthesis for a boreal aspen forest?

Abstract

We used daily MODerate resolution Imaging Spectroradiometer (MODIS) imagery obtained over a five-year period to analyze the seasonal and inter-annual variability of the fraction of absorbed photosynthetically active radiation (FAPAR) and photosynthetic light use efficiency (LUE) for the Southern Old Aspen (SOA) flux tower site located near the southern limit of the boreal forest in Saskatchewan, Canada. 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 a 2.5 km × 2.5 km area (5 × 5 MODIS 500 m grid cells). We then used the scaled-up MODIS products in a coupled canopy-leaf radiative transfer model, PROSAIL-2, to estimate the fraction of absorbed photosynthetically active radiation (APAR) by the part of the canopy dominated by chlorophyll (FAPAR chl) versus that by the whole canopy (FAPAR canopy). Using the additional information provided by flux tower-based measurements of gross ecosystem production (GEP) and incident PAR, we determined 90-minute averages for APAR and LUE (slope of GEP:APAR) for both the physiologically active foliage (APAR chl, LUE chl) and for the entire canopy (APAR canopy, LUE canopy). The flux tower measurements of GEP were strongly related to the MODIS-derived estimates of APAR chl ( r 2 = 0.78) but only weakly related to APAR canopy ( r 2 = 0.33). Gross LUE between 2001 and 2005 for LUE chl was 0.0241 µmol C µmol − 1 PPFD whereas LUE canopy was 36% lower. Time series of the 5-year normalized difference vegetation index (NDVI) were used to estimate the average length of the core growing season as days of year 152–259. Inter-annual variability in the core growing season LUE chl (µmol C µmol − 1 PPFD) ranged from 0.0225 in 2003 to 0.0310 in 2004. The five-year time series of LUE chl corresponded well with both the seasonal phase and amplitude of LUE from the tower measurements but this was not the case for LUE canopy. We conclude that LUE chl derived from MODIS observations could provide a more physiologically realistic parameter than the more commonly used LUE canopy as an input to large-scale photosynthesis models.