(pronounced 'fre-nel' the "s" is silent) The area around the visual line-of-sight that radio waves spread out into after they leave the antenna. This area must be clear or else signal strength will weaken.
Fresnel Zone is an area of concern for 2.4 GHz wireless systems. Although 2.4 GHz signals pass rather well through walls, they have a tough time passing through trees.
The main difference is the water content in each. Walls are very dry: trees contain high levels of moisture. Radio waves in the 2.4 GHz band absorb into water quite well.
This is why microwaves -- which also use the 2.4GHz band -- cook food. Water absorbs the waves, and heat from the energy cooks the food.