You can't come in flat over a tree top. You have to come in a little high, then side-slip over the tree line and drop in shortly after it, burying the tail in right away to kill ground speed.
The tree will win every time if you try a direct approach.
Try it again with a side-slip... dip your left or right wing, depending on wind direction and add-in opposite rudder to the aileron position. You'll drop like a rock.