As promised, here is an easier-to-follow tutorial for the tree texture from Cartotalk. This is a refinement of the technique used in Mennin's Hallow, and the technique I am using for my current Challenge Map, Galerius Hallow.
First, set up a document at whatever resolution you intend to work at. For the purposes of this tutorial, I'll be working at 1280 X 1024 pixels. To start out, you'll need two layers. The top one will contain your texture and the bottom your base color for the forests. Mine are named Texture and Color.
Fill your Texture layer with solid black using the Paint Bucket tool.
Filter > Noise > Add Noise...
Set Amount to 1 - 3%. Distribution Uniform. Tick the Monochromatic box. Changing the Amount will alter the scale of your texture. A lower number will make bigger trees.
Set your foreground color to black. ("D" on the keyboard will set your colors to the default black foreground and white background.)
Choose Select > Color Range... Set the fuzziness to 5 and click "OK."
You should see a big mess of "marching ants" now. Hit the Delete key, and then Ctrl-D to deselect everything, leaving only a lot of specks everywhere.
Now it's time to start adding some color. Choose your Color layer and fill it with the base green you want your texture to have.