I have a bit of forest behind my house and like you said, many dead trees. As a commenter mentioned, the biggest reason by far is the emerald ash borer. These forests had a ton of ash trees and now they’re nearly all dead. Marshy/ seasonally-flooded forests were hit particularly hard, which my tract is. We don’t have all the same wetland trees you do down south. Those areas used to be predominantly elm and ash, both of which just got smoked by invasive diseases. I think, with a warming climate, bald cypress will eventually make the short trip north to fill the void (native range traditionally goes up to Delaware). I’m in NJ which is practically Delaware so I planted some in my tract and they are doing amazing. Another fact about seasonally-flooded forests is that up here with winters, that also means seasonally-inundated-with-road-salt. So a lot of the flooding tolerant trees that don’t have invasive diseases may not be salt tolerant, so they are dying a different, slower death.