Answer: If there are no signs of insects or diseases and the trees died from the base of the trunk and moved upward, it's probably a root problem. Roots can die from too much or too little water, and are especially prone to root rot if the soil doesn't drain quickly enough. I'd dig the trees up and inspect the root systems. You'll probably find the cause - mushy, wet roots with fungal growth or dessicated roots. If you can't determine the cause of the problem, take the smallest tree, roots and all to your local Cooperative Extension office. They can diagnose the problem for you or send it to the University of California plant pathology department for microscopic inspection. Best wishes with your landscape!
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