Slitherine

It is hard to see at a glance whether a region is forested or not. For that matter, whether it is plains or hills is also sometimes unclear.

Could something be done graphically (add a few trees?)

the scale of the map makes this somewhat harder, as forests in a region doesn't mean the whole area is full of tree's etc, same as mountains etc etc, but yes see your point also

I would also be in favor of some trees popping up on the map as well and they already have some nice tree models and textures for the battle maps. It would be out of scale in the same manner as the settlements and the armies, but it should be fine.

Maybe not to everyone's tastes and could potentially hit performance on older machines, so ideally, it would be a toggle on the settings page.

It depends on what map overlay your using, but with no overlay you can see forests without too much trouble. The mouse over tool tips give you all the terrain info you need.

Here is a screen shot with no overlays. Easy to see forest. I mostly play with the ownership overlay on and I must admit it is hard to see terrain type with it on.
Region south west of the one with the mouse overlay is forest easy to identify, however the due south is hilly terrain, does look similar, but the tool tip says it all.

The terrain on the map could be a lot clearer to indicate what the primary terrain type is.

It's easy to see unless you use certain overlays. Perhaps there should be visual cues that aren't affected by overlays.