Why At UHF and microwave frequencies the wavelength is only a few centimeters to a few tens of centimeters, comparable to the size of leaves and branches, and the water inside that foliage absorbs RF energy. The result is added path loss, so weak signals through trees or brush get buried in the noise. The higher the frequency, the worse the attenuation, which is why 2.4 GHz links suffer more foliage loss than 2 meters.
Watch out Knife-edge diffraction is a real propagation effect, but it comes from signals bending over a sharp obstruction such as a ridge or building edge, not from vegetation, and nothing passive can amplify a signal.
Wet leaves eat short waves: the higher the frequency, the more trees attenuate.