"This sounds like a job for the Masked Layer!!!" (cue cheezy superhero music and echoes)

Seriously, if you want to break up a drawing into different "sections", the best way is to use Layer masks. There is a great Tutorial by (correct me if I am wrong folks) jfrazierjr on the subject even if it isn't for gimp. RobA's tut also goes into the use of layer masks quite a bit, and I can not recomend that tut forcefully enough.

Basically a layer mask allows you to show only the desired portions of a drawings layer (like a forest noise layer) by hiding everything that is under the black portion of the mask. To create a mask (and here I might be mistaken and others should correct me if I am) create a layer (we will call it "mask") and fill it with black. Then using a white brush, color the areas that you wish to show through. Select the white area using either the majic wand or the color select, then go to Select menu and click on "Selection to Channel" (I think, cause I am at work and no access to Gimp). You now have a mask.

Using the mask is simpler. with a layer selected, either go to the Layer menu or right click on the layer, and select Add Layer Mask, check the Use Channel box and sellect the mask. Click OK. Bingo layer now has a mask and only the areas of the original layer under the masks white will show through.

Hope this helps.

You can create all kinds of masks for all kinds of layers for your drawing

:edit: and I am no expert