I have a feeling she's not afraid of trees but a little freaked by something sooooo much bigger than her. Kids have all kinds of irrational fears, let me rephrase. We adults, think our children have all kinds of irrational fears but to them a giant that towers over them, blocks the sun and has many, limbs that sway and move to get em', not so irrational to our little ones. I find the best process (with my children) is the baby step process. Just a little at a time. If she is too scared to go up to and touch the tree, bring her some leaves, tear a chunk of bark off, show her an apple.... You get it. books are helpful but not really with my kids. On a page is one thing but in front of them where they can touch, smell, sometimes chew something gets them to a rational place much quicker. Also, make sure when your telling her there is nothing to be afraid of, you also make sure she know all the wonderful things trees do for us. Point out all the different fruit that comes from trees in the store, show her how nice it is to just sit in the shade, tire swings, blossoms, leaves changing color, how they are an invaluable part of our ecosystem and we need to fight to save the trees... Sorry just slipped into my hippy side. You get it. It'll be all good. Good luck!