You are suffering from a common thing when it comes to pattern/texture tiling. You can see obvious tiling/seam effects, and it looks gross.

You basically have a few options, and first you need to decide where you want to go with the map. Do you want it to look accurate? Do you want a "representation" of what a forest looks like? Or are you looking for something that outlines the forest?

See, the symbols are meant to show a representation of the forest, not actually be the texture of the forest. This is why it looks bad.

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