Detecting phenophases of subarctic shrub canopies by using automated reflectance measurements

Boreal and subarctic plant phenophases are advantageous indicators of climatic change on a global scale. Remote sensing is a promising technique for assessing such changes over extended areas. An automated field measuring system collected seasonal reflectances of natural shrubs in visible, near-, and mid-infrared wavelength ranges. A boom-mounted four-band spectroradiometer was installed on a 4-m-high tower to measure seasonal radiances in green (520–600 nm), red (630–690 nm), near-infrared (765–900 nm), and mid-infrared (1570–1730 nm) spectral bands from undisturbed subarctic shrub vegetation during the 1994 and 1995 growing seasons (mid-June to mid-September) in northernmost Finland (69°45′N, 27°00′E, 105 m above sea level). The radiometer was vertically looking down on four fixed ground plots and a weatherproof reference panel continuously during all the daylight hours. The reflectance factor calculations, using the reference panel and solarimeter readings, included corrections for the reference panel degradation and non-Lambertian characteristics. Daily averages of visible and near-infrared band reflectance factors offered smooth seasonal trends in spite of the variation in solar irradiance at the times of data collection. The turning point dates in the trends of seasonal near-infrared (765–900 nm) and red (630–690 nm) reflectance factors might indicate the end of growth and the beginning of autumn changes, respectively. The normalized difference vegetation index and ratio of green (520–600 nm) to red (630–690 nm) band reflectance factors, however, seemed to be more accurate in monitoring them.