One of the things about Radio waves is that they undergo different behaviors such as reflection, refraction and diffraction when in contact with obstacles such as walls, buildings etc. what i want to understand is that, which of the behaviors would occur the most in let's say an environment filled with trees and the slope is rough?
As pointed out by David, many metrics may be used and so there is not final and unique answer to your question. Anyway, by looking at the basics of the tree phenomena you can get a sense of what is happening.
1) Diffraction will happen when the wave encounters an object whose dimensions are comparable to the wave length. You need to be more specific here, because radio-wave length ranges from millimiters to kilometers, and so the answer may vary a lot.
2) Typically refraction and reflection happen when a wave incides through a wall. How much of the wave gets refracted, and how much gets reflected? The answer, once again, depends on the wave-length, but according to every day experience (the wifi signal can get through a wall, but you cannot hear FM radio while in a tunnel) full attenuation will happen after some meters. If the thickness of the object is shorter, you can say that refraction occurs more than reflection, otherwise vice versa.