OOH sounds like a great idea. I had my room like a rainforest once. One of the fun things was hanging vines (stapled to wall). I also had a 'poet-tree' on my BB at one time.
How about these:
Show a path through the forest - 'pathway to learning' or 'follow the trail to ______'
We 'be-leave' in ourselves
One tree, many leaves - maybe using leaves for kids' names, or even subject titles
Cooperation - showing animals/plant life that co-exists in a forest
Growing taller every day
Reach for the sky
Reading provides nourishment so you can grow strong and tall
Many shades, one forest - show all the varieties of green leaves
Put down roots and grow strong
Hug a tree (are you old enough to remember that?)
Thank a tree - paper, wood, oxygen, shade, etc....
Of course, seasonal trees will be great. Use with life cycles, weather, earth day, conservation ...... What about a treehouse?
This sounds like fun.