Here's a trio of trees and forest in square format, every very different from each other but at least I myself find all of these quite working solutions.
First is a composition of field filling in 2/3 of the photo and the objects are placed in the middle of the top part. The field part gives the photo depth and makes it more interesting as a picture. Bushes soften the area at 2/3 a little and yellow of the birches bring them out of the picture as clear point to look at. I find this works quite well together.
This second one is a black and white photo without any clear point to look at. So there's not much common with the previous photo. Trees on the left and right frame the view but as the left one is much stronger, I have left it as much smaller component than the tree on the right. Tree on the right is a much lighter element so it fills much more but as I see it, these two elements are quite balanced together like they are now. Rest is the view without any tricks. Soothing image you can look a little longer but it's not in any way "fast food" of photography. No "wow" effects, just plain relaxing photo.
As third of the bunch I've made a texture type of photo from treetops. Still also very different than the two above. It's a quite typical northern Finland forest and I guess almost any kind of composition of the forest would work from this upper point of view. To this I've managed to get some gradient from the sunset. It's brings in some depth. Anyhow, with texture type of photos, square format works pretty much every time.
Now go and compose in square!