I don't know if this technique has been posted here before, my search didn't bring any results. So I'll come up with a little tutorial for photoshop, how to create trees in a very easy and quick way. As my english is not the best, I hope you will understand, what I'm going to try to explain you.
If you would like to fill a map with a lot of trees in nearly no time, but not even with a pattern of any stylized trees, here is how it goes.
1: Create a new file, draw some trees or use some trees from your favorated brushset. Match the trees as shown in the screenshot (1a). It is important, that there are at least one or two pixels space between the trees. Then go to Filter -> Other -> Offset and accept with a value for horizontal and vertical, which is half the size of your document. Be sure, 'Wrap Around' is selected (1b). Fill the resulting gap with trees, so the pattern won't have any gap later (1c). Edit -> Define Pattern -> will let us finish this step.
For this example I'd choose very small trees, but size doesn't matter in this case. Important is only, that the trees don't border on each other and the contour is continuous.
2: Go into your map document, select those areas with the lasso tool (L), that have to become a forest, and fill them with your pattern. Don't deselect now! Invert the selection (Shift, Ctrl + I) and fill it with color (same color as the trees have). Deselect now (Ctrl + D), choose the magic wand tool (W) and click into the filled area (Tolerance: 30, Anti-Alias and Contigous selected). Press delete (Del).
3: The trees now stand alone without leaving any annoying half chopped trees. No more post editing required. Wasn't that quick? And the result is, as I think, quite handsome. If you want to fill the trees with a color (and not only the stroke), select the pixels by ctrl-clicking the thumbnail in the layer-window, go to Selection -> Modify -> Expand (1-2 px), do the same with Contract, both a few times, and you will have a full selection of the trees and can choose a backgroundcolor. For this example I'd use a matt green (mode: multiply, 50% opacity) for the background and the same color with the same adjustment by 100% opacity for the contour.