Growing Season Start in Boreal Coniferous Forest from MODIS Time-Series and Comparison with Modelled Phenology
Böttcher, Kristin1; Markkanen, Tiina2; Aurela, Mika2; Mattila, Olli-Pekka1; Kervinen, Mikko1; Metsämäki, Sari1; Aalto, Tuula2; Arslan, Ali Nadir2; Pulliainen, Jouni2
1Finnish Environment Institute, FINLAND; 2Finnish Meteorological Institute, FINLAND

The beginning of the growing season in boreal coniferous forests can typically be recognised as sudden increase of gross primary production (GPP), which can be determined at local sites from CO2 fluxes, measured with the eddy covariance method. Here, a fixed fraction of peak growing season GPP was used as a threshold value for the growing season onset (hereafter referred to as Growing Season Start Date, GSSD) at three sites in Finland. Daily MODIS-derived time-series of Fractional Snow Cover (FSC) from homogenous areas in vicinity of CO2 flux measurement sites were compared with in situ GSSD for the period 2001-2010. The decrease of FSC during snow melt was used as proxy indicators for GSSD and good correlation between the satellite indicator and in situ GSSD was obtained (RMSE = 5 days). Satellite-derived maps of GSSD for Finland were used for the evaluation of modelled growing season onset. GSSD was determined from regional GPP predictions by the land surface model JSBACH (Jena Scheme for Biosphere-Atmosphere) of the Max Planck Institute (MPI), which is forced by meteorological data generated with the regional climate model REMO (MPI Meteorology, Hamburg). Modelled GSSDs showed a systematic delay (about 2 weeks) compared to satellite retrievals.