Laboratory and Tower-Based Microwave Measurements of Spruce Defoliation

This paper presents the results achieved from microwave radar backscattering measurements of defoliating spruces. Individual spruces were measured in an unechoic chamber and in outdoor conditions and the natural defoliation was simulated by gradually removing the needles of the trees. The measurements were carried out using 5/10 GHz and 35 GHz radar systems. The influence of defoliation to the volume backscattering and extinction coefficient of spruce canopy has been determined from the measured data. The results show that defoliation has a considerable effect on radar backscattering for 10 GHz linear polarizations. At 35 GHz the achieved data indicate a rather complicated and controversial backscattering behaviour with decreasing degree of defoliation. At 5 GHz the effect of defoliation on the backscattering coefficient is negligible. Helicopter-borne measurements of natural defoliation using the 5/10 GHz radar are presented for comparison. Additionally, measurements using a 90 GHz radiometer were conducted. But, by the time of preparing this paper the analysis of these results was not finalised.