Monitoring Spring Phenology of Boreal Coniferous Forest in Finland Using MODIS Time-Series

摘要

The possibility for the extraction of spring phenological events of boreal evergreen forest from Moderate Resolution Imaging Spectrometer (MODIS) time-series was evaluated in this study. The beginning of the photosynthetically active period, determined from CO_2 fluxes measured with the eddy covariance method, was used as primary reference for the onset of growing season (hereafter referred to as Flux Growing Season, FGS). Furthermore, phenological observations on pine trees, such as the beginning of height growth were included for comparison. Cloud-filtered daily time-series of Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI) and Snow Covered Area (SCA) from homogenous areas around observation stations (7 evergreen coniferous forest sites in Finland) were compared with in situ phenological events. The onset of FGS in coniferous forest showed correspondence with start of spring-rise in NDVI temporal profiles and coincided with the SCA change from full snow cover to patchy snow cover. The beginning of height growth of pine trees occurred later after complete snow melt, when NDWI profile showed a minimum. The start of FGS in different years was extracted from NDVI and SCA time-series with good accuracy (R~2>0.8, RMSE<7 days) for several years at three flux sites (two in northern Finland and one in Southern Finland). The method based on NDVI time-series was further applied to map the onset of FGS of coniferous forest in Finland for different years. Further work will concentrate on the enlargement of the mapped area and the combination with a mapping method dedicated to deciduous species in the boreal region.