January 14, 2015
January 18, 2015
These trees are expected to live up to 50 years, so I don't think it is old age that is killing them.
That looks like it is winter burn. Are all the trees browning in the same pattern? Has your area had harder than usual winters or has something around the trees changed that might change the flow of wind. This article will explain more:
It may also be a fungus. If the trees are browning in different patterns from each other, haphazardly, this would be an indicator that the problem is fungal. If this the case, a fungicide should help clear it up.