Differences in arctic tundra vegetation type and phenology as seen using bidirectional radiometry in the early growing season

Abstract

Ground-based nadir and off-nadir radiometric measurements were collected on the North Slope of Alaska in early June to determine their utility in 1) differentiating arctic tundra vegetation community types through remote sensing, and 2) examining differences in structure and phenology of arctic tundra vegetation communities. We employed a PS-II (personal spectrometer II) portable field spectroradiometer to measure narrow-band spectra at nadir and the PARABOLA (portable apparatus for rapid acquisition of bidirectional observations of the land and atmosphere) three-channel radiometer to gather bidirectional radiometric data over almost the complete ground hemisphere. Measurements were taken at a lowland wet sedge site and at two tussock tundra sites containing different amounts of woody deciduous shrub cover. PS-II nadir measurements alone allowed spectral differentiation between the highly senescent wet sedge site and the two relatively greener tussock tundra sites but did not allow clear discrimination between tussock tundra sites. Off-nadir measurements, however, permitted good discrimination between the two tussock tundra sites. For example, the reflectance-based normalized difference vegetation index (NDVIp) difference between the tussock tundra sites at a 75° forward-scatter view angle averaged 26% over a range of solar zenith angles (ϴsS). Similarly, NDVIp calculated from PARABOLA spectral hemispheric data averaged 6% higher in woody tussock tundra than in nonwoody tussock tundra. Differences between the tussock sites calculated by using NDVI based on hemispheric radiance (NDVIL) were greater than those calculated by using hemispheric NDVIp by approximately a factor of two regardless of ϴs Measurements taken when ϴs=75° generally exhibited the largest differences between NDVI at the two tussock tundra sites. Although nadir measurements exhibited minimal differences between tussock tundra types, bidirectional and hemispheric-based NDVI values revealed differences in tussock tundra woody vegetation cover because large view zenith angles were sensitive to the woody shrub foliage that extends above the hummocky tundra surface in early spring. These results may be useful for detecting changes in tundra woody cover through time.