If you have photoshop or similar, you might want to try this:
Go to Mayang's free texture site and have a look at the moss and lichen textures.
There is a really good one here (pic below) which looks like treetops. You might want to 'cartoonify' it a bit in photoshop so it doesn't jar against the general style.
At the end of the day, though if you're after that illustrative style, then you'll need to draw it (unless you're a photoshop genius). I'm not familiar with CC3, but it'll be pretty much drawing the boundary of the forest giving it a fill and then drawing your tufts on top of it - I would guess.
Looking at the forests in the really big map (the first one that jaerdaph) posted, this is what I would do to make the forests:
1. Draw the outline and fill it with the main green colour.
1a (Optional - if you want texture) Copy and paste the outline on top of itself to a new layer and use a cloud or noise texture filter (or you could try the mayang texture below). Set layer to overlay (or multiply) - best just running through them all and seeing which layer filter works best - and reduce the opacity of the layer to suit.
2. Copy and paste the outline on top of itself to a new layer and make the fill transparent. Use an 'inner glow' effect using olive drab and set the level to overlay. Adjust transparency to suit. Use the paintbrush tool to put in little additions to the inner glow so it doesn't exactly follow the outer border.
3. Copy and paste the outline again to a new layer and do an inner glow with a very dark green. Set the layer to dissolve and move down to about 30% transparency, that will give you the dots around the inner border. Put in the other little dots further into the forest by hand.
4. Merge the layers when you are done.
I'm afraid I don't know if CC3 has the abilities to do this, if it hasn't you can always use Gimp (which has) and import the image into your CC3 map. As you will see, a really good way to generate rich textures is to copy a shape, paste it on another layer and give it a different fill while setting the layer to overlay or multiply (mainly).
Looking at the second Link that jaerdaph posted, to me it looks like a straight drawing job (sorry). It would probably be a lot easier to do in pencil and to scan that into the puter.
The third link is a little small and hard to see, but it looks like what the cartographer has done is to draw the outline and apply a cloud filter fill using different tones of green and then applied a drop shadow to the shape.