Why are our trees gradually dying off 1 or 2 limbs at a time?

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We have mainly birch trees that are dying, first the limbs go, usually top to bottom, and then the trunk. see photos
Jim
q why are our trees gradually dying off 1 or 2 limbs at a time
q why are our trees gradually dying off 1 or 2 limbs at a time
They are not very old either.
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