Question
Late last summer my mature pin oak dropped leaves early compared to other similar trees in area. The weather had been dry. This spring it appeared that 25% of the limbs were dead. Since then, many of the limbs have new growth and some of the smaller limbs appear to be dead but new leaves are forming on the primary limbs. Insects were not evident. Can you suggest possible cause. Have read of a  tree tonic, are you familiar with this.

Answer
Oaks during droughts will drop their leaves early. Pin oaks seem to do this faster than other hardwood species. I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. This will improve the health of the roots system and in turn produce more foliage.