Submitted to: Geophysical Research and Atmosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 1999
Publication Date: December 31, 1999
Interpretive Summary: Remote sensing estimates of foliage density and percent coverage of boreal forest tree stands are needed for forest inventories, ecological studies and descriptions of the earth's surface for climatic general circulation models. Airborne data of spruce, pine, and aspen stands were analyzed to investigate relationships between reflectance and foliage density expressed as leaf area index (LAI). The model was used to correct the data for differences in solar and view angles, to a common viewing condition. Analysis showed a negative relationship between increasing conifer LAI and red reflectance. A similar, weaker relationship for conifers with near infrared (NIR) was found. There were no significant relationships with aspen stands. The conifer NIR relationship is different from the positive relationship between NIR and LAI for crops and grasslands. Further model analysis showed changes of the amount of shadow with changes in LAI strengthen the LAI-to-NIR relationship. This shows the importance of stand structure on scene reflectance as the under story optical properties are not significantly different from over story optical properties. These results indicate the use of traditional red-NIR relationships with boreal forest tree species needs revision and suggests exploitation of the NIR reflectance using a different approach.
Analysis of airborne radiometer measurements and imagery of boreal forest canopies was conducted to develop algorithms for retrieval of leaf area indexes (LAI) and percent crown closure. Data of black spruce, jack pine and aspen stands were acquired on different days and locations with various solar illumination angles and view geometries. A model was used to correct the data to a common solar zenith angle and view angle (nadir). Red reflectance decreased with increasing LAI. A similar, weaker trend was found with near infrared (NIR) for conifer stands. No significant relationship was found for aspen stands. Further model analysis showed the spectral relationships are controlled by the decreasing sunlit background fraction and increasing sunlit crown with increasing LAI. Since the under story and over story optical properties in the boreal forest are similar, changes in shadow fraction strengthen the responses of optical measurements to LAI. This information yields insights for developing parameter retrieval algorithms for use by foresters, land managers and those using remote sensing information as a lower boundary layer in meteorological general circulation models.