Since the southern USA has lost its giant hemlock trees, our biggest old growth trees are now dominated by the Yellow or Tulip poplar. In fact, the Yellow poplar is not a true poplar, but is more closely related to the magnolia. It's a fast growing hardwood that gains a lot of height in a relatively short span of time. It tends to grow straight and true, sometimes with grand trunks and few limbs low on the tree.
Some of the best groves of poplars can be found in the Great Smoky Mountains National Park. Another spot for viewing huge old poplars is the Joyce Kilmer Memorial Forest in North Carolina.
Generally speaking, many communities in the southeast have some remnant poplar trees of great size. If one happens upon a truly huge tree in a southern mountain or Piedmont forest, it's likely a poplar.