Question
Many of my Leyland Cypress trees that were planted last fall have turned brown at the base, thinning out, etc.  We had a severe drought this summer, and I'm afraid that, despite regular watering, they were stressed beyond saving.

Can a Leyland cypress "repair" itself, or is it a vicious cycle, and only a matter of time until the trees completely die?

Answer
As long as they have green foliage I would give them a chance to recover. Sounds like a combination of stress from the planting and the drought. I would mulch the area around the trees and do not over water. They need only about an inch of water per week during non rain periods. Place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop the watering.