> Trees and bushes are invisible to X-band (10GHz) SAR. It has to be an optical image underlaying the radar data

Trees and bushes are very much visible to X-band radar systems. See e.g. http://web.eecs.umich.edu/~saraband/KSIEEE/J54IEEETGRSJan00S...

Foliage Penetration (FOPEN) radars, in my experience, do not use X-band.