Airborne SAR monitoring of tree growth in a coniferous plantation

Abstract

This paper examines three empirically based methods of monitoring forest growth between 1991 and 2000 from airborne synthetic aperture radar (SAR). In the first method, height change and volume change between 1991 and 2000 were estimated from the mean L‐band stand backscatter difference between the two dates. Height change and volume change over the 9‐year period were estimated with an accuracy of 0.23 m and 15 m³ ha⁻¹, respectively, when the stands were below saturation point for the first date. The accuracy of the results was lower for stands beyond saturation in both data sets. In the second method, the height change is calculated from the estimated stand height in 2000 minus the estimated stand height in 1991. The second method produced poorer results than the first method, but better results than predicted by the error propagation equation. The difference between the observed accuracy and the expected error (based on the error propagation equation) appears to be due to a systematic bias in both the 1991 and 2000 estimates, as the residuals are correlated for stands below 20 years old (r = 0.71 for stand volume residuals). The third experiment investigates the utility of data from two dates to classify the stands into three age classes. The results show that, with two images separated by 9 years, 85% of stands were correctly classified compared with 69% for a single date L‐HV image.

Acknowledgements

This research was funded under the BNSC‐LINK Carbon project no. R4/039 with support from Biffaward, EcoSecurities Ltd. and SGS Ltd. The AirSAR data are courtesy of NASA‐JPL. The E‐SAR data were acquired during the SHAC 2000 campaign by NERC/BNSC. We would like to extend our thanks to the Forestry Commission for making available the GIS database, and to all those involved in the Thetford fieldwork campaigns in 2000 and 1989.