SEASONAL CHANGE IN UNDERSTORY REFLECTANCE OF BOREAL FORESTS AND INFLUENCE ON CANOPY VEGETATION INDEXES

One objective of the Boreal Ecosystem-Atmospheric Study (BOREAS) is to increase our understanding of the nature of canopy spectral bidirecti onal reflectance in the visible/near-infrared regimes for open canopie s typical of boreal forest stands. For such stands, the need to charac terize the reflectance of the sunlit and shaded vegetated understory i s critical. These variables are subject to temporal variability due to differences in species phenology and foliar display as well as diurna l and seasonal changes in solar illumination through a seasonally vary ing upper canopy foliar area. To provide for this need, a multiteam fi eld effort was mounted to measure the nadir midday understory reflecta nce for the flux tower sites during 1994 BOREAS field campaigns betwee n February and October, specifically during the winter focused field c ampaign (FFC-W), the spring thaw focused held campaign (FFC-T), and th e three intensive field campaigns (IFC-1, IFC-2, and IFC-3) between Ju ne and September, which sample vegetation phenological change. This wa s accomplished by measuring at near-solar noon the sunlit and shaded n adir reflectance of the understory along a surveyed leaf area index (L AI) transect line at each flux tower site. Site-to-site comparisons of understory reflectance spectra reveal stand differences that become m ore significant as the season progresses. Mean midday understory refle ctance spectra were observed to be remarkably consistent over the seas on for young jack pine stands, followed by somewhat increased variabil ity for mature jack pine, and significant seasonal variability for bla ck spruce stands. Derived vegetation indices for understories are gene rally consistent with extrapolations of previous relationships of cano py spectral vegetation indices (Vis) versus leaf area index to zero LA I. Inclusion of these ''zero-LAI'' understory-derived indices signific antly enhance the correlation in the linear VI-LAI relationships.