Going up and down is fine, but moving in any height, can get very confused as to where the Phantom is, and then you will crash, just look at the many examples on You Tube, most crashes from Newbies, going to high, moving then getting confused. It is not the trees that are always the problem, but not being able to see where your Phantom is, at distance is. Seems to many newbies are in to much of a hurry to go high, so have not learned to fly, so start moving, then crashing. Don't believe, lots of videos of Newbies getting confused in what direction the Phantom is going in, then either hit a tree, or crash into the ground.
Most tree crashes are due to being confused as to height your going, or confused in Direction and nearly all are newbies.
Take it in steps, don't rush it, you will get there. The Phantom costs to much to go out and take big risks before you know how to fly properly.
Also until you can fly Nose into you, don't even go high, else you will get into trouble. Nose in your controls are reversed, so can be confusing. this should be second nature before flying high, get plenty of stick practice in.
Practice going away from you, coming back, moving left right, slight turns, figure 8 etc, then when you can do this well, then try having the front towards you, and coming towards you.This can be scary as your control is reversed, but trust me, when flying nose in becomes easy, then flying at height becomes easy. Just going up and down is the easy part, moving at height is a different story. To many newbie take straight off and go right up before any flight experience and You Tube is full of these foolish people and crashing often.