First of all, don't use the crappy trees from wc3 if you want a beautiful terrain, use custom models. Likewise with most other wc3 doodads. You can find better ones in the model section. Secondly, never spam a doodad type, like with the trees. In real life you'll find that there exist no natural environment with only one kind of plant. And thirdly, make it look natural, the more it resembles real life. The better.