There are several thousand acres of wooded wilderness within the city limits of Dallas. I have not heard any discussion of the trees there dying off. Not only that, but Dallas county is also home to a second Audubon center in the SW corner of the county
That link is to a palmetto swamp in Dallas county. Other links on that page will show you several nature areas in the county of Dallas.
I am pointing these out, because Dallas is an urban area with a known ozone problem. It seems however that we have plenty of native trees that are doing well.
I grew up on the Cottonwood Creek Nature Preserve area on that list. Hunted and fished up and down that creek. We still have 13 acres for sale on the creek, 200 year old native pecan trees.
My own homestead is in the Ozark Mountains. I do have 65 acres of trees about 80 years old which is their climax age for the land. That is about as old as oaks get on this sort of ground. See, this land isn't naturally oak/hickory as it is presently. Natural vegetation here is Pine Prarie but about 80 years ago they started to cut the pine and ran cattle till scrub oaks moved in and created a "forest". It's not what naturally would be here at all, fire should rage through every 3-4 years and keep down all the red cedar oak and hickory, favoring native fire-resistant pine. Man kind has thus helped establish a false climax forest which isn't sustainable compared to what developed here since the last ice age. Much of our re-growth forest you see today just isn't what the original forest was. There are myriad reasons why some forest trees might not be healthy, and don't always assume it has one simple reason. Sometimes the reason for forest health was set in motion generations ago.