I don't think they find trees in those layers simple fact that no one is looking for them. Even if they find them all over, who is going to plublish on finding trees. People just don't get excited over a life of an ancient tree over say the t Rex. Or do you ever hear of kids when ask, what is your favorite fossil? They say, mine is a tree.
So just because you don't find something dose not mean it didn't exist. What I like to know from them is if they did not exist at that time then when we're they created?