The effective constitutive parameters of forests at high frequency range is analyzed. It is found that because of the occurence of resonance at high frequency range in tree trunks

magnetic energy can be stored and the effective permeability of the tree trunks needs not be determined conventionally to be unity. For vertically polarized waves

the effective permeability is found to be not significantly different from unity. For horizontally polarized waves

however

both the real and imaginary parts of the magnetic susceptibility

in a typical example

is about 10-4 and the imaginary part of electric susceptibility is about 10-3 and the real part of that is about 0.05.