Hi Somayeh.  I agree with Ram.  I think that is also one of the goals of the Sentinel, for biomass estimation. 

Kindly check the link below for more info on the application  of Sentinel.

https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-1-sar/applications/land-monitoring

Hi Somayeh. I think that Sentinel-1 can really be used for estimating AGB in woodlands.

But there are some constraints that you must be aware of:

I think the most relevant constraint is that Sentinel-1 works in C-band. It means that the signal it uses interacts mostly with canopy top and do not allow to get a direct analysis of the ground beneath the trees. Then, forest/woodland trees' volume cannot be assessed by a direct analysis, wich means that the analysis of AGB using Sentinel-1 data will certainly have an error that you must to assess if it is acceptable or not to your research objectives.

Natural Resources Canada has a tutorial on Radar Polarimetry that may be useful to you.

They say in this tutorial, for example, that in coniferous forests, HV polarization "can be more effectively used to characterize forest biomass", with R2 values of 0.749 for band C (which is the band used in Sentinel-1) and 0.957 for band L.

I think it would be a good idea for you to investigate how radar C-band characterizes biomass in the vegetation type of your study.

http://www.nrcan.gc.ca/earth-sciences/geomatics/satellite-imagery-air-photos/satellite-imagery-products/educational-resources/9579