I borrowed a technique described at cartotalk, conveniently linked here somewhere in the tutorials forum. It was a bit hazy on the details, so I'll go ahead and reconstruct what I did here, but at a reduced resolution for speed and storage purposes. Reducing the res will make the scaling of the forest change, I think.
Starting with the shaded relief map, as in pic #1 below, use a hard-edged black brush to paint in where you want tree cover (pic 2). The borders will be a bit too regular, so select that layer and save the selection to a new channel: Select>save selection. In the channels window, go to the new selection and run the spatter filter on it: filter>brush strokes>spatter (pic 3).
Load the selection, invert it, and delete it from the trees layer, resulting in pic 4. Now, the next part of the process requires changing the color space to greyscale, and since you obviously do not want to damage the image, select the layer, copy it, and paste it into a new greyscale image of identical resolution (pic 5).