I was trained by the renowned Swedish landscape artist Larz Friberg when I was younger, and he always told me that the key to copy nature in a realistic way is to observe and practise. He gave me "challenges" now and then, and among them was to draw nothing but trees from all kinds of angles.
Because there is a big variety of trees, limit yourself to a few selected ones in the beginning until you know them in your sleep. Sketch them and refine your sketches when you are happy with what you've done. It takes time to learn what they really look like and to get the right proportions etc. As with so many other objects we see every day it's more difficult to pin down straight away.
Personally I was so sick and tired of pines and birch for a while I never thought I'd draw or paint them again

Christa