For me, these details are what brings alive a map, thou fantasy maping may not need this kind of details for most of them are/should be potrayed as literal illustrations, unlike this one that looks like an aerial "photography" of the area. In my case I went more for a "video game-like" map, where you can zoom in and actually use the place as the gaming area... Not necesarely bad but not a common approach.
First, I throw the general green shades over the areas to be "forested". This is a jungle city so I'm going for dark lush green. This layer is set with a little transparency so that it does not completely cover the background and get some of it color, thus integrating this layer of vegetation directly with its backgroud. You may notice that the base is also slightly texturized not to make it a plain color (also apparently I added a bit of lights to the rivers in this phase, messy me ).
This vegetation is not supposed to be the actual trees and bushes, more like the foliage and general color/feel of these areas.
Next, I threw in the actual "trees and bushes". Again, I cheated a bit and used a pregenerated brush which pretty much did 90% of the job for me (bad cartographer, BAD!). I think using this kind of brushes isn't entirely "bad" but try and create your own brushes so that your work is original. This brush is you typical random scattering one. This layer ain't transparent like the first one so that it actually covers the background and brings a better sense of depth and actual "3d" vegetation. You can compare the opacity of this layer with the transparency of the previous one and see the difference (and usefulness) between the two of them.
Next, I added the cursed bevel effect to the layer so that the trees pop up and get some fast n' easy volume. I left this simple step separated so that you could see the HUGE difference between flat trees and "beveled" ones. I think this is an excellent use of this effect and it doesn't actually feels so "plastic" if you throw a slight texture over it.
Last, I added a second color to the vegetation so that it doesn't look so flat. This color "flatness" ain't bad, but you can see how it improves if you throw a second color shade over it. It can work to "separate" areas to hint different kinds of vegetation. Try to go light on this kind of extra shades, for too much can ruin the whole thing. This is a simple transparent layer. Try playing with the transparency/overlay effects on the layers on this one, you can get different kinds of textures/contrast with a few clickings on these.
Again, this is a simple and fast step but I believe this one brings considerable life to the map.
Next: Shading and lighting. I'll wrap this step by step tutorial this monday, it is almost finished already anyways