Monitoring coniferous forest characteristics using a multifrequency (5–90 GHz) microwave radiometer

The objective of this study is to analyze the potential interest of microwave radiometry to monitor coniferous forests. Microwave data were acquired by the airbone multifrequency radiometer PORTOS (5 GHz, 10.6 GHz, 23.8 GHz, 36.5 GHz, and 90 GHz) over a test site in Les Landes Forest (France). The test site consists of large homogeneous stands of maritime pines (Pinus pinaster). The stands are monospecies and even-aged and cover a large range of age and biomass conditions (the above-ground dry biomass ranges from 0 to about 180 tons/ha). The stands were flown over twice in May and August 1994. The microwave response to the coniferous stand characteristics is analyzed. A statistical study is carried out to investigate the potential interest of the PORTOS data to discriminate between the different stand categories. The sensitivity of the stand emissivity to view angle, frequency, measurement date, and stand characteristics is analyzed. It is found that the range of forest stand microwave emissivity is relatively narrow for a vegetation coverage exceeding 95%. Although different stand categories can be distinguished from the PORTOS data, the level of discrimination is not sufficient to allow the development of accurate rules of classification. No relationship could be found between low frequency measurements and the stand biovolume. On the other hand, a linear negative relationship is found between emissivity at 10.6 GHz and average tree variables of the stands (age, height, trunk diameter) for well-developed pine stands. Also, potential interest of PORTOS to estimate forest stand characteristics seems to be significant at 90 GHz. For this frequency channel, good correlation is found between emissivity and tree basal area B_A. The results presented in this work contribute to better assess the potential applications of the future ESA/MIMR instrument over coniferous areas.