well, i suppose it's not impossible. i know i can't see through a tree, but i suppose it's conceivable that an alien race would be able to do so. as far as communicating, i don't know that any aliens are communicating with us, but it's certainly easy enough to shout through some wood.
No, because I don't believe trees come equipped with a visual system to watch us. As far as communicating with us, I'd soon as believe the trees themselves attempt to communicate rather than some aliens species vicariously attempting to communicate with us through the trees.
Perhaps it is possible but I don't believe it to be the case. Why not? Because aliens are not near us.
I think the aliens figured it's the best way to keep an eye on us, since there are so many trees around.
Yes, the trees then bark the information to the aliens. Personally I think they should leave them alone.
WARNING,WARNING: BAD PUN ALERT BAD PUN ALERT. It's not the trees that are watching, its the moss and lichens.
I love how some aboriginal people refer to trees as "Standing People." It is a sign of respect for the wonderful gifts trees provide. I believe trees are sentient on a certain level. I believe they can communicate with each other and perhaps some other living critters. But spying on us for the benefit of aliens? I really don't think so. If a superior race wanted to watch us, they wouldn't need to recruit trees to do the job for them. They'd just have to tune in to the Internet or watch our tv signals.