Résumé
Temporal variations of environmental research satellite (ERS)-1/2 backscattering coefficients acquired over a mixed deciduous forest are analyzed with the aim of relating the observed radiometric variations to changes either in the vegetation seasonal cycle or in the structural parameters. Overall, the results are somewhat pessimistic. Temporal sigma-0 plots show chaotic variations, which are difficult to relate to the seasonal changes of forest parameters and particularly to the foliage dynamics. Furthermore, no distinction between stand types or between deciduous species is found to be possible, and nearly identical temporal plots are observed for all the stands, suggesting that the radar signatures are partly under the influence of nonforest parameters. Besides, the effect of meteorological events are difficult to evaluate. Discrimination between deciduous stands and conifers is nevertheless possible, since the radiometric difference between the two species is about 1 dB. With an overall sensitivity to standing biomass of about 0.1 dB/50 tons per hectare, ERS SAR's can be considered as almost insensitive to biomass variations. For the young stands, the C-band response is found to be dominated by stand structure, whereas the backscattering coefficient saturates for biomass values higher than 50 and 80 t DM/ha for deciduous and conifers, respectively. (Author)