Trees probably do not "communicate" in the same sense as sentient beings. What takes place there is pure chemistry and natural selection rather than conscious communication. There's no reason to suppose that there is any experience/awareness/consciousness going on. It's like two magnets repelling each other—they're not consciously deciding to push each other away but merely acting according to natural laws of physics.
A sign of consciousness/sentience is apparent volition or will. Gasoline combusts when a flame is held to it—it always does this because that's the natural laws of physics. The gasoline can't ever decide that it doesn't want to combust under identical circumstances to an instance in which it did combust in the past. The tree naturally reacts to certain inputs and, apparently, can't do otherwise. A dog, on the other hand, will react differently to the same stimulus/input, depending on its mood. The dog will come this time I call him but ignore me the next time. The dog may snap at me one day when I go to pet him but decide he likes me the next. This is a sign of volition, an indicator of consciousness—the ability to react differently based upon mood or whim rather than automatically responding in an identical fashion whenever a certain situation or stimulus arises.
And consciousness seems to be inseparable linked to brain function. If you introduce certain chemicals to the brain, there are certain predictable changes in one's conscious experience. That's how a lot of medicines work. If you perform a corpus callosatomy (separating the two halves of the brain), the patient will appear to have two separate centers of consciousness that are now unaware of one another. Splitting the brain in half literally results in splitting a person's consciousness in two, basically creating two independent conscious individuals within the same body. It is possible that consciousness can arise in other ways but human consciousness is very dependent on the brain and appears to be an emergent phenomena entirely dependent upon brain function. You can alter someone's conscious experience by messing with their brain but they can also get some idea of what a person is thinking about or seeing by looking at brain activity.
