SEMIVARIOGRAMS OF DIGITAL IMAGERY FOR ANALYSIS OF CONIFER CANOPY STRUCTURE

Semivariograms were used to exploit the spatial information inherent in digital imagery of a variety of Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] forest stands in the Pacific Northwest region of the United States. Calculations involved digital numbers from single transects of pixels representative of each stand and from a full spatial matrix of pixels from each stand. Digitized aerial video images having pixel sizes of 1 m, 10 m, and 30 m were used. At 1 m spatial resolution the ranges of the matrix semivariograms related to the mean tree canopy sizes of the stands. The sills responded to the presence of vertical layering in the canopies and to percent canopy cover. The transect semivariograms were less representative of overall stand structure, but exhibited periodicity that was suggestive of patterns in stand structure. Semivariograms based on 10 m and 30 m pixels contained significantly less useful information. The ranges of the 10 m matrix semivariograms revealed only whether the tree canopy sizes were less than 10 m or were between 10 m and 20 m. The sills were greatly reduced, but still related well to canopy layering and percent cover. Periodicity in the transect semivariograms was greatly reduced, and in some cases, eliminated. At 30 m resolution, only the sills of the matrix semivariograms contained useful information. However, actual differences between the sills were small.