Monitoring temporal change in Alaskan forests using airsar data

MONITORING TEMPORAL CHANGE IN ALASKAN FORESTS USING AIRSAR DATA JoBea Way and Eric Rignot Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109 USA Kyle McDonald University of Michigan Gordon Bonan National Center for Atmospheric Research INTRODUCTION test site on March 13, March 17 and March 19, 1988. The data were calibrated radiometrically to +/-1.9 dB. The measured difference in L-band backscatter for the frozen and thawed days for the 12 stands and two clearcuts is large (2-6 dB) at all three polarizations with the greatest difference at vertical and cross polarizations. A large change in the bole dielectric constant between warm and frozen conditions can account for the large change in backscatter. The role of boreal forests in the carbon cycle is particularly important because experiments with atmospheric general circulation models indicate significant northern hemisphere high latitude climatic warming with doubled atmospheric CO2 concentrations (e.g., Schlesinger and Mitchell 1987). The ecological implication of such a climatic change is unknown, but the seasonal amplitude of atmospheric C O 2 concentrations in northern latitudes has increased with time, and this may reflect increased metabolic activity of ecosystems in northern latitudes due to warmer air temperatures and CO2 fertilization . PREDICTED SEASONAL MICROWAVE SIGNATURES A typical seasonal cycle in the soil-tree system for each of the primary floodplain species may be arbitrarily divided into several stages which should correspond to distinct signatures in the ERS-1 data; these are shown in Figure 1. A process-oriented ecophysiological model that summarizes our current understanding of productivity and decomposition in boreal forests has recently been developed (Bonan 1991a,b). This model requires stand level physiological measurements as well as characterization of the spatial (in terms of forest species) and temporal (in terms of phenologic and environmental state) patterns for validation. Recent results of radar signature modelling and measurement of multi-temporal changes in radar backscatter using the JPL aircraft synthetic aperture radar (AIRSAR) indicate it is possible to use spaceborne imaging radars such as the European Space Agency's (ESA's) Earth Remote Sensing (ERS-1) synthetic aperture radar (SAR) to monitor both phenologic and environmental change in forests, as well as map forest species by taking advantage of seasonal forest signatures. These results also demonstrate the importance of knowing the seasonal state of the forest when estimating canopy properties from SAR. Previous studies on the application of radar signatures to the water relations of trees have shown a marked change in the dielectric constant of the stem with (1) stem freezing and thawing (Way et al. 1990). and (2) diurnal water potential and/or water content variations (McDonald et al. 1990). Given the specified seasonal cycle described in Figure 1 and the stand characteristics, a microwave scattering model may be used to generate the expected backscatter over a seasonal cycle. The Michigan Microwave Canopy Scattering (MIMICS) model, a fully polarimetric first- order radiative transfer model developed at the University of Michigan specifically for modelling radar backscatter from tree canopies was used in this study (Ulaby et al. 1990). Backscatter as a function of seasonal state was simulated using the ERS-1 radar parameters (C-band, VV polarization, 23' incidence angle) for the 13 conditions shown in Figure 1. The results are shown in Figure 1. TEST SITE AND AIRSAR DATA SET In order to determine the feasibility of using imaging radars to monitor the seasonal cycles of boreal forests and map forest types, experimental work is ongoing at the Bonanza Creek Experimental Forest (BCEF) just southeast of Fairbanks, Alaska. The site is a floodplain succession forest consisting primarily of balsam poplar, white spruce and black spruce. Approximately 30 stands in BCEF have been identified for this study. To date, canopy characteristics have been collected for 12 stands. Airborne SAR data using the Jet For all three species, there are large total changes in the backscatter over the year at C-band V V polarization of 6.8 dB for balsam poplar, 4.3 dB for white spruce and 10.6 dB for black spruce. The last curve in Figure 1 shows the seasonal backscatter for balsam poplar, white spruce and black spruce plotted together. Clearly, at C-band VV, white spruce may be separated from black spruce/balsam poplar (BS/BP) best when the trees are thawed and the ground is frozen (conditions 3 and 4). Black spruce and balsam poplar are best separated when the soil and trees are frozen (condition 1). Propulsion Laboratory's (JPL's) P-, L- and C-band (.450, 1.26 and 5.3 1 GHz, respectively) quad polarized AIRSAR mounted in NASA's DC-8 aircraft were acquired over the BCEF CH2971-0/91/0000-1117$01.00 0 1991 IEEE