This is pretty much all I can see from my yard. Everything is obscured with crunchy brown dead stuff. :o( There are some vines (the darker three-leaf thingies) nearby that don't seem to be a problem or related to the situation.

The suckers look like a Poplar of some kind.

I think it is pretty well known for it's ability to sprout back after appearing dead.

i had a 2 foot wide ten foot long poplar trunk .... laying on the ground.. that sprouted for upwards of 2 years ...

cut each sprout at ground level.. and use something like at the link to apply one drop of round up or stumpkiller.. to the cut ... the real key here.. is the one drop.. killing trees has nothing to do with spraying ...

keep at it.. though i dont know if you will ever apply enough.. to kill the behemoth behind ...

gotta love neighbors ...

ken

Here is a link that might be useful: link

After seeing your suggestions about Poplar, I did a bit of Googling and so far the signs point in that direction.

Thanks!