ORIGINAL: cfircav8r
If you want the skinny on wood strength check the homebuilt airplane sites. They have all the info you will ever need about strength and weight. Sitka spruce has long been the choice for F/S aircraft spars, however Douglas Fir is being substituted more frequently. It is heavier, but stronger, so you just make it a little smaller dimension and end up with about the same strength and weight.
Another source with good information on full scale aircraft wood working, and information about wood that will translate to our models is Advisory Circular AC-43.13 that is available through the FAA website. It provides good information on the strength comparison between different species of wood and the size differential for the same strength properties between species.
Hogflyer