tree looked fine

[fpyontek]

New England Jim, I believe it wasn't the co-dominant stems that brought this tree down, but the fact that the tree has a basal wood decay. If you could, please post another picture of base of the tree where it broke away from the stump. At the broken section of trunk you should find tan to rust colored wood that will easily break away in chunks. Other more normal colored wood you should be able to pull apart using your fingernail.
In the Northeast I have found that, in oak trees, many times the only outward sign of a problem is that the base of the trunk does not flow smoothly into the ground. The base of the trunk instead appears bulbous, for lack of a better description. I believe this abnormal growth is a result of an ongoing process of compartmentalization by the tree and reinfection by the fungi.
What happens is this particular fungi break down the cellulose fibers in the wood so that, while it looks solid, has little tensile strength.
New England Jim, you should look carefully at the base of the remaining tree for the same symptoms as described above. You may find that the remaining trunk also has trunk rot.

For more information see :
http://www.fs.fed.us/r6/rogue/swofidsc/stemdecay/brntrunkrot.html

Fred

 

[treeseer]

Good description af brown rot; wood is strong but stiff.

The link said "The stems of trees with heartwood decayed by this fungus are very prone to breakage."

How decayed? how prone? The prognosis and recommendation depend on these, etc. It ain't the meat, it's the motion.

 

[Grenfell]

On Mike's picture I see a phenomenon that I've noticed before. Take a look at a closeup of the tree, just below the crotch. There is a pattern of cracks in the bark which I suppose is normal for this species. But right about at the yellow line, the texture of the bark abruptly changes. There are a few cracks which go above that line, but overall, the texture changes almost as suddenly as if it were a Photoshopped picture that combined two different trees.

Why is that?

I'll add my unprofessional $0.02...

Like Mike said, the cracks are part of the bark's aging process.

In response to your particular question - i.e. the specific and isolated location of the cracks relative to the crotch/rest of trunk - perhaps that area of the trunk is slightly thicker, and has grown more rapidly, than the rest of the trunk due to compensating to movement from the leaning right branch above the crotch? Hence that specific area of bark is splitting/maturing slightly before the rest of the trunk.

I've often noticed that tissue near a narrow crotch junction tends to be bulbous or 'deformed' looking. I'm guessing that trees build up extra tissue in these areas in response to stress and movement of the individual limbs in the area - like a callous.

Again, just a wild guess on my part...