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.