dead shrubs

Help. All of a sudden my ceanothus of 8 years has suddenly died and also my Berberis Darwinnii they were both next to each other. My neighbour has been chopping down his trees but the rather large silver birch has been left but it looks dead. Could he have used something to kill this tree and the poison gone through the roots (as they are in my garden) and killed both of these shrubs? we dug down and had a look and all the tree roots are rotten. If this is so would it still be safe to replant anything in that spot . I am quite upset as these were both lovely shrubs.

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