Sorry Torq, I've tried for half an hour to try and re-create how I did it, but I can't find the brush I used . This is going to be fun as there is now no way I'll be able to edit the forests by painting more in.
The basic process involved using (the uknown) brush which painted a spattery type of green / brown pattern on it's own layer (using colour dynamics and scattering). Then I duplicated the layer and applied a grain filter (using photoshop CS2) to the underlying level which I then bevelled and embossed. I used 'multiply' layer style for the top layer.
I'll keep fiddling to see if I can find the brush I used.