In order to complete this tutorial, you will need the following plugins:
Start out with a canvas (the default size is fine), and apply a radial gradient from corner to corner:

Next, duplicate this layer, and use the jitter plugin on the top layer (I used an amount of 20):

Merge the top layer down, and use the Polar to Rect plugin at its default settings:

The ring pattern is very symmetrical, so (this is an optional step) you can grab pieces of the image and stretch or compress them:


Eventually, you'll be done resizing things:

Crop the image to just the "wood" part.
Press [shift] and magic-wand in the transparent area to select all of the transparent pixels, then press [ctrl]+[i] to invert the selection.
Press [ctrl]+[shift]+[x] to crop the image to just the selection:

I think it looks better stretched taller, so I doubled the height of the image by using Image->Resize from the Image menu:

Now, we'll apply a color tint and get a basic oak board:

Playing around with hue and saturation, I got a respectable pine:

...and cedar:

A little more finesse with curves or other color or hue effects can get you a wide variety of interesting results.
Have fun!