Question
We recently lost a large limb on a 125+ yr old conifer tree in our yard. The limb was approx. 16" in diameter, and when it went down in a storm, the center core of the limb came out of the trunk, leaving a 12" long, 4" diameter hole in the trunk of the tree. the hole runs at a 45 degree angle towards the ground. There is also a gash in the bark of the trunk.

Is there anything we can use to fill the hole and dress the gash in the bark to prevent rot?  We hate to lose the tree!

Answer
No need to fill the wound just make sure the rough edges are cut smooth and the wound will heal on its own. More than likely the tree of this size has decay in the trunk. Decay fungi enter and "feed" on the woody dead cells but will not harm the living cells. You may want to watch and if more large limbs break off and leave hole in the trunk that are not solid then the tree has extensive decay in the trunk and could be a hazard if it fell. Usually the large limbs will fall first so I would not be concerned with the tree falling. I would fertilize the tree next Fall with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and water it in good. This will increase the overall health if the tree.