Canopy shadow in IKONOS satellite observations of tropical forests and savannas

The biological and structural complexity of tropical forests and savannas results in marked spatial variation in shadows inherent to remotely sensed measurements. While the biophysical and observational factors driving variations in apparent shadow are known, little quantitative information exists on the magnitude and variability of shadow in remotely sensed data acquired over tropical regions. Even less is known about shadow effects in multispectral observations from satellites (e.g., Landsat). The IKONOS satellite, with 1-m panchromatic and 4-m multispectral capabilities, provides an opportunity to observe tropical canopies and their shadows at spatial scales approaching the size of individual crowns and vegetation clusters. We used 44 IKONOS images from the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) data archive to quantify the spatial variation of canopy shadow fraction across a broad range of forests in the Brazilian Amazon and savannas in the Brazilian Cerrado. Forests had substantial apparent shadow fractions as viewed from the satellite vantage point. The global mean (+/-S.D.) shadow fraction was 0.25 +/- 0.12, and within-scene (e.g., forest stand) variability was similar to interscene (e.g., regional) variation. The distribution of shadow fractions for forest stands was skewed, with 30% of pixels having fractional shadow values above 0.30. Shadow fractions in savannas increased from 0.0 +/- 0.0 1 to 0.12 +/- 0.04 to 0.16 +/- 0.05 for areas with woody vegetation at low (<25% cover), medium (25-75%), and high (>75%) density, respectively. Landsat-like observations using both red (0.63-0.70 mum) and near-infrared (NIR) (0.76-0.85 mum) wavelength regions were highly sensitive to sub-pixel shadow fractions in tropical forests, accounting for similar to30-50% of the variance in red and NIR responses. A 10% increase in shadow fraction resulted in a 3% and 10% decrease in red and NIR channel response, respectively. The normalized difference vegetation index (NDVI) of tropical forests was weakly sensitive to changes in shadow fraction. For low-, medium-, and high-density savannas,a 10% increase in shadow fraction resulted in a 5-7% decrease in red-channel response. Shadows accounted for similar to15-50% of the overall variance in red-wavelength responses in the savanna image archive. Weak to no relationship occurred between shadow fraction and either NIR reflectance or the NDVI of savannas. Quantitative information on shadowing is needed to validate or constrain radiative transfer, spectral mixture, and land-surface models used to estimate material and energy exchanges between the tropical biosphere and atmosphere. (C) 2003 Elsevier Inc. All rights reserved.