Dielectric constant and dielectric loss of soils vary depending on texture structure of the soil, moisture content in the soil, soil salinity, temperature etc. Hope that our paper "Measurement of Complex Dielectric constant of Soils of Gujarat at X and C-band Microwave Frequencies" Indian Journal of Radio and Space Physics, Vol.37, June 2008, pp.221-229, may be useful to you.
If you are just looking to correlate soil moisture to different forest types, why not simply measure soil moisture in the different forests? Assuming they are on similar soils and similar topography, the forest cover and makeup will be the primary driver of soil moisture. I would think a depth profile looking at the soil moisture at different depth would then tell you what you want to know.