Question
A woodpecker has drilled a hole, about 1-1/2" in diameter about half way through one of my oaks, which is about 11" in diameter. How can I save this tree?

Answer
The woodpecker nests is a sign that the tree maybe hollow or at least soft in the middle due to decay fungi. The only real "health" issue maybe the lessening of the strength of the trunk from the hollow trunk. The holes will have little effect on the health. I would make sure the tree is fertilized each year to help with the health of the foliage and the living parts of the tree. The inner part (wood) is composed of dead cells and if a decay fungi gets in the fungi over time may hollow the tree. This does not effect the growing parts of the tree. fertilize with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Do this in the spring after the last frost date.