GRASS
Because our green "background" layer is a bit too flat, give it some life by creating a new layer above it called "grass". Fill that layer with an almost-white gray color, then run Filter->Artistic->Sponge, with settings around: brush size 3, definition 15, smoothness 4 (play with these to taste, of course).
Next set the "grass" layer's blending mode to "multiply" & down the opacity to 60-70% or so. (you may also wish to run a Gaussian Blur on it very lightly to downplay the edges.) [see image]
TREES
Of the many ways to add forests, I chose to use a seamless tile I previously created from a picture of some trees. And may I say that creating that one pattern was an amazingly useful project--I highly recommend you do the same! (You can easily be taught how to create both a seamless tile and how to define a pattern in about 10 minutes by doing an Internet search).
First click Layer->New Fill Layer->Pattern, and name it "trees"; in the next screen, choose your tree pattern at whatever scale you deem best. The entire image will instantly be covered with forests. Then use the paint bucket to fill the "trees" layer with solid black--all the forests will disappear.
Now with a white brush of your choice, draw wherever you want trees, and the trees will appear! I used a large brush to create forest areas and smaller brush to dab in individual trees; I also used some of the spotty brushes to break up the solid edges.
Now, your trees are placed, but they probably look very flat next to the other 3D-ish details of the map. So add a Drop shadow, Inner shadow, Bevel & Emboss (inner bevel, chisel hard) + texture set to the same tree pattern with which you filled the layer.
There you have it! [see image, below]
GROUND COVER
Finally, let's add some random shrubbery and ground cover. Create a new layer above the "trees" layer called "vegetation". Set layer style settings close to your "trees" layer [see my settings in the image, below].
Then choose a smaller stipple brush set to 40-50% opacity and fill. With various greens, browns, etc. either dab or draw in ground cover. Change brush tips, colors, and opacity/fill often for more detail and realism. (One note: the harder the brush & higher the fill/opacity, the taller the vegetation will appear, for the shadows and such will be more pronounced.)
You can even draw on top of the trees you just placed to lend them further dimension.
[see image below]