cutting cottonwood mother tree

[foothills]

We've cut several large cottonwoods last year. This Spring remaining cottonwoods 15-20 feet away are all dead. What might be the reason? I heard a funny theory that cutting the "mother tree" can do that. Is is true?

 

[ATH]

Did you herbicide the stumps of the other trees you cut?

 

[foothills]

Yes, the tree service poisoned the stumps, I think, not 100% sure, we talked about it.

 

[ATH]

Roots were likely grafted and herbicide very well could have spread through those root grafts.