High temporal resolution NDVI phenology from micrometeorological radiation sensors

Abstract

The boreal forest is a region characterized by wide swings in temperature and light levels over the course of a year. This seasonal variability strongly effects the vegetation of this biome. Normalized difference vegetation index (NDVI) values were observed at daily timescales for key land cover types of the boreal forest, developing a more detailed description of seasonal changes in NDVI than could be produced from satellite data. NDVI values were calculated from tower‐mounted photosynthetically active radiation (PAR) and global solar sensors measuring both incoming and reflected radiation above the canopies at four Boreal Ecosystem‐Atmosphere Study (BOREAS) sites. Comparisons were made between the tower‐based broadband hemispherical NDVI values and the narrowband nadir‐viewed NDVI values from helicopter modular multiband radiometer (MMR). The comparisons indicate that the tower NDVI values are close to the MMR NDVIs in value for the BOREAS sites, but the range in tower NDVIs is not so great as in the MMR NDVIs. In 1996, BOREAS towers operated from before thaw to freeze‐up, allowing a complete picture of growing season NDVI for fen, young jack pine, black spruce, and aspen sites. The tower‐based NDVI time series display different patterns for each vegetation type, showing the effects of snow cover and vegetation green‐up and senescence. Changes in solar zenith angles are shown to have little effect on the seasonal NDVI patterns.