Answer: Based on your description, it is impossible to make a definite diagnosis. However, it is possible that cumulative stress is affecting them, particularly the past five years of drought capped by this terribly dry winter. Since you have an avenue of trees and they are large, you might want to consult with a professionally trained and certified arborist to see if that could be the reason. You might also want to make sure there is nothing else at work, meaning a pest or disease problem, that could affect the remaining trees and so you would want to be vigilant. This is the type of thing that can really only be determined based on an onsite inspection. I'm so sorry about your trees.
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