THE APPLICATION OF A GEOMETRIC OPTICAL CANOPY REFLECTANCE MODEL TO SEMIARID SHRUB VEGETATION

The Li-Strahler [7] canopy model was tested, using SPOT HRV XS imagery, for semiarid shrub vegetation, based on 26 small (1-ha) sites in five classes of shrub vegetation, two dominated by tarbush (Flourensia cernua), one by creosote bush (Larrea tridentata), and two by mesquite (Prosopis glandulosa). The model was driven by reflectance values derived from June and September imagery. While predictions of crown size and density for individual sites had a large average error of 35%, the predictions of shrub size and density were reasonably accurate when grouped by shrub class. The aggregated predictions for a number of stands within a class were accurate to within one or two standard errors of the observed average value. Accuracy was highest but predictions were biased for some classes (size was underestimated) when the nonrandom shrub pattern was characterized for the class based on the average coefficient of determination of density. Results based on June data were not better than September because the hypothesized lower background "noise" (e.g., less green herbaceous cover that could be confused with shrub cover in the simple reflectance model) was not observed in the June data. This could have been due to the poor radiometric quality of the June image.