One of the fundamentals to successfully painting realistic trees is learning to recognize the characteristic shapes of different species. Look at the overall appearance of the tree and identify the overall shape of the tree.Is it shaped like a sphere, umbrella, cone, or tube, or is it simply irregular? Is it short or tall, fat or thin, straight or spread irregularly? Do the branches point upwards or downwards? Are the leaves dense or sparse? Has it spread naturally, has got broken branches, or has a gardener pruned it?
And remember to look at the tree's root system. Trees don't just stick up out of the ground.