Characterizing Carbon in a Northern Forest by Using SIR-C/X-SAR Imagery

A significant large-scale question in ecology and earth systems science pertains to the amount of carbon (C) stored in terrestrial vegetation. In this paper, a synthetic aperture radar (SAR)–based methodology is developed and evaluated for quantification of several key vegetative C components—both natural and human induced—of the northern forest. Specifically, the methodology provides estimates of C stored in living forest vegetation, above-ground C gain from annual growth (aboveground net primary productivity, or ANPP), and C removal due to managed forest disturbance in the form of clear-cutting. The inputs are shuttle imaging radar (SIR)-C/X-SAR- derived terrain classifications, SIR-C/X-SAR-derived biomass estimation, and allometric relations and equations developed for the northern hardwood and conifer forest in general and from local test stand data. Results are mapped quantitatively in the image domain for above-ground C storage, below-ground C storage, above- to below-ground ratios, total C in living woody vegetation, and forest absolute and relative ANPP rates. Numeric estimates also are extracted from each of these in tabular form; for example, results show that the forested parts of the sampled area contain 2.73×109 kg of aboveground C and 4.86×108 kg of below-ground C in 51,448 ha. When combined with a SAR-derived classification, similar quantifications can be extracted for each of the several forest communities present in the region. Estimates of forest ANPP show that it ranges from 0.5 kg/m2/yr of biomass to 2.7 kg/m2/yr in the test site, with an average of 1.09 kg/m2/yr. Estimation of C removal due to clear-cutting is done by using multidate classifications of SAR imagery and a procedure including image differencing and decision rules. Clear-cuts that were made between SIR-C/X-SAR Shuttle Radar Laboratory (SRL)-1 (April) and SRL-2 (October) are identified by differencing the classified imagery. When combined with the SAR-derived biomass image, results show that an estimated 300 ha of forest with 6.02×106 kg of C were removed in this 6-month period.