So your options are, depending on what you choose:
1) Make the trees larger, and manually place a few trees here and there to break up the seams
2) Paint a green area, and use the symbol with single instances to paint in the suggestion. In this case, less is more. That is how they did the old school maps back in the day.
3) Make a texture that mimics the forest and paint it around the area. You are doing a top down effect, so perhaps you can make another file of just trees with backgrounds, and save it and tile it in a graphical editor.
Hope that helps you out a little, and good luck!
Sandeman