Estimating net primary production for Scandinavian forests using data from Terra/MODIS

A model for estimating net primary production (NPP) across Scandinavia has benn developed. The model is based on the light-use efficiency (LUE) concept, where NPP is calculated as a product of the amount of absorbed photosynthetically active radiation (APAR) and a LUE-factor (ε) controlling the efficiency by which vegetation transforms photosynthetically active radiation (PAR) into biomass.The fractional APAR (FAPAR) is obtained by a linear transformation of 250 m normalized difference vegetation index (NDVI) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). Prior to the transformation, the NDVI time series were seasonally adjusted by fitting local asymmetric Gaussian curves to the data thereby minimizing cloud contamination and other noise factors such as BRDF-related variation. The APAR is then calculated as a product of FAPAR and incident PAR, where the latter is obtained from the “Swedish Regional Climate Modelling Programme” (SWECLIM). The LUE-factor is modeled as a daily function of temperature, latitude, and day of the year (DOY).The FAPAR dataset was validated against measurements of fractional intercepted PAR (FIPAR) that were carried out at Norunda experimental site in central Sweden (60°5′N, 17°29′E) between August and October, 2001. The calculated FAPAR time series are in good agreement with the measurements.The modeled NPP is evaluated against flux measurements carried out in Norunda 1997–1999. The determination coefficients obtained when comparing modeled with measured data are R2 = 0.82 for 2000 (RMSE = 2.71 g C m−2 d−1), and R2 = 0.68 for 2001 (RMSE = 3.57 g C m−2 d−1) (time series averaged every 10th day).