Hi there
The reason it feels stiff is due to your composition. When you center something like that you "lock" up the entire painting. Anything you put on either side of them will look out of place. To fix that just change it to a simple composition like a "rules of thirds". you will get a more dynamic feel to it and still maintain that towering forest.
I would recommend to spend some more time in the thumbnail stage. But the idea is cool just watch the values on the trees in the back, right now all trees have the same value so it gets a bit flat.
But I would love to see the finished piece. Keep it up