Just an educated guess on my part but I suspect the problem is the decaying roots which are wood is sucking up and holding the nutrients. Otherwise removing shade should result in a flush of growth.

I am not real familiar with poplar but it might also be possible the decaying roots are releasing toxins.

I thought of that as well, but the dead area extends WAY beyond the root system of the tree.........

Can you post a picture, preferably taken on a cloudy day or in the early morning before the sun hits it?

Where do you live?

I live in Norfolk VA and I will try and get a couple of shots this afternoon....

Ok folks...

I have included some photos. The one with the tree stump was the poplar tree that was hit by lightening five years or so ago. The rest of the pics are just general shots showing the general condition of the yard...pretty sad, I know. Any suggestions (aside from starting over...which may be necessary) would be appreciated!

It only let me post one so here's another showing a close up of the prominent invader....

Based on your photo, it appears you live in an urban forest. Think I counted 11 trees. Don't know your overall amount of sun to your yard, but St. Augustine requires at least 6 hours of direct sun to thrive. The decaying stump is not an issue but I wonder why you have left the unsightly stump.

I have left the stump because it doesn't bother me and around here it would be very expensive to remove. the equiment would have to go over and old teracotta drain field that is still in use and would really reather not crush any of those tiles.

The yard was THRIVING before lightening took the tree i and it was a HUGE shade tree so the yar had even less light then it does now.

Eleven trees are just the tip of the burg sdo to speak. I would estimate that I have probably 75 or more trees on my 2.5 acre lot although I have never counted them. Many of them are medium to small pines that are in some cases densly packed. Thank you for taking the time and effort to respond.