Keenan,
I have made bows from standing dead yew. Here is only a guess as to why this works well....Trees/ plants obsorb water through their roots then this is transported through the cambium layer, when a tree is dying it (may?) still obsorb water at a slower rate, but over time photosynthesis is slowed to a creep, and water that is transported out of the plant through the leaves stops. Now, the natural process of a live tree is over, and what normally sets in is fungi's of some kind that leads to spalting of most white woods. In the case of yew, and or a lot of rot resistant woods it is much harder because of natural chemicals in the wood for this to take hold, and discoloration of normal sapwood to heartwood may take place as the beginning stages of fungi attack, BUT with a given amount of time, and the right conditions this does not take place. I can only assume the reason is a slow, but gradual loss of water through evaporation due to wind, and heat. The bark staying in place I think is key, and once the dead tree reaches a level equilibrium with the outside elements, bark being the key to NOT let any drastic change one way or the other. I think once the bark is damaged, or starts to fall off the game is drastically changed, and a more rapid process of decay can take place depending on species.
Brian