Okay, I made this look like wood in Photoshop before layers existed. It wasn't easy, but now I can let you know how to do it too.
Make a square however large you want your tree type wood to be. A good color would be C40 M65 Y100 K0
Raster that shape.
Use your lasso/rope tool and go slightly around the inside of the square in an irregular manner. Once you've gone around the area select Shift/Apple/i and then delete. This will give you the shape you want.
Now duplicate this layer in Photoshop and make this invisible. Go back to your original layer.
Draw different layers of lines with the paintbrush going in a horizontal direction.
Now select the oval selection tool and make an oval in one area of the wood. In your menu go to Distort and twirl. Take the size of the twirl up to over 700. Repeat this step in a couple of different areas in different sizes.
Now in the menu Filter>Other> select High Pass.
Go to your 2nd layer now (Shape 1 copy) and set the pull down to Multiply. Duplicate this layer and set the new layer to Overlay.
I did a lot of fine tuning on my artwork to get it to where it is. So just play around with the look to get what you want.
PM me if you have any questions....but keep in mind, I did mine in PS 2 or 3. : )