Historical forests in the Southeastern Mixed Forest province of the United States have been less researched than other regions using historical tree surveys. We used 81,000 tree records from surveys during the 1800s to quantify composition of this ecological province. Upland oaks and pines comprised about 75% of all trees, with relatively equal composition. Oak composition may have comprised ≥ 45% to the northern and eastern sides of the province. Hickories were about 10% of composition and a few species were present at 1% to 2% composition. Currently, pine has increased to 49% composition; loblolly pine was 46% of all trees. Upland oaks decreased to 8% composition. Paralleling other historically oak- or pinedominated regions, fire-intolerant species increased to 40% of composition, particularly early-successional sweetgum. Historical oak-pine forests mostly have converted to loblolly pine plantations and broadleaf forests in this region. A large extent of the eastern United States historically was dominated by oak or pine forests, which likely were open old growth forests due to a frequent, low-to-moderate severity fire regime that reduced tree densities and infrequently disturbed overstory trees. Open old growth forests should be recognized as distinct ecosystems with unique characteristics, ecological functioning, and associated management practices.