If you compress parallel to the grain wood is WAY stronger than if you compress perpendicular to the grain (i.e. growth ring orientation relative to compression).
Also, the distance of growth rings (i.e. winter and summer growth rates) affects the overall strength of the wood (any strength issues, not just compression).
I believe that if you fully compress a piece of wood (i.e. not a nail such that you exert a lot of pressure over a SMALL area), wood is FUNCTIONALLY equivalent to steel/metal. I just saw a house-moving show on History channel where the house was 190 tons. When they slid it over where the new foundation was going to be, they simply used 10x10 (or thereabouts) cribbings.
JL