Good question! USGS reports that “Before European settlement, forests covered nearly one billion acres of what is now the United States.” Data tables from the UN Forest Resource Asessment 2005 show that only 257, 439,329 acres (104,182,000 ha) of “primary” forests remain in the US (defined as where there are no clearly visible indications of human activities and the ecological processes are not significantly disturbed). This would indicate that roughly %25.7 of the US original forest cover remains as “primary” forest. Authoritative? Sure, but forestry numbers from the UN are notoriously bad.
Global Forest watch reports that
Approximately 20% of North American forests have been permanently cleared for agriculture and other uses, primarily within the last two centuries (Bryant, et al. 1997). Currently, forest cover is stable (Matthews, et al. 2000); however, in most of the lower 48 states and southern Canada, remaining forests have experienced significant human disturbance and do not possess the same degree of ecological integrity as the original forest. As human populations grow, forest fragmentation and degradation continues. One result has been the loss of extensive areas of old-growth forest. According to one estimate, stands of century-old forest now account for only 7% of forest cover in the United States (USDA-FS 2000).
Meanwhile, the University of Michigan says
Since 1600, 90% of the virgin forests that once covered much of the lower 48 states have been cleared away. Most of the remaining old-growth forests in the lower 48 states and Alaska are on public lands. In the Pacific Northwest about 80% of this forestland is slated for logging.
Thinking bigger? My summary of research on "how many trees are cud down every year" (globally) was an earlier topic of the Understory.