How does vegetation affect UHF and microwave signals?

Trees, bushes, and other vegetation contain a significant amount of water. As radio frequencies increase into the UHF and microwave regions, their wavelengths become short enough that they interact strongly with this water. The water in leaves and branches absorbs the RF energy, converting it into trace amounts of heat rather than letting it pass through. This absorption leads to a noticeable reduction in signal strength, especially for weak signals traveling through dense foliage in the spring and summer. Vegetation does not act like a solid sharp edge, so it doesn't cause knife-edge diffraction, nor does it possess any active electronic components to amplify signals.