Reforestation began in 19th-century New England, when farmers started abandoning marginal pastures and buying cheap feed grain from the rich, relatively flat lands on the other end of the newly opened Erie Canal. Later, petroleum-based fertilizers and gasoline-powered machinery made Midwestern farming more productive and draft animals obsolete, freeing up 70 million acres that were being used to feed them. Many farmers, meanwhile, opted for jobs in town. Trees took back much of their land and, after World War II, nonfarmers began moving onto it.
Today, the eastern third of the country has the largest forest in the contiguous U.S., as well as two-thirds of its people. Since the 19th century, forests have grown back to cover 60% of the land within this area. In New England, an astonishing 86.7% of the land that was forested in 1630 had been reforested by 2007, according to the U.S. Forest Service. Not since the collapse of Mayan civilization 1,200 years ago has reforestation on this scale happened in the Americas, says David Foster, director of the Harvard Forest, an ecology research unit of Harvard University. In 2007, forests covered 63.2% of Massachusetts and 58% of Connecticut, the third and fourth most densely populated states in the country, not counting forested suburban and exurban sprawl (though a lot of sprawl has enough trees to be called a real forest if people and their infrastructure weren’t there). . . .
Pre-Columbian period: 30 million
Today: 25 million to 40 million
Source: 'Nature Wars'
From the Wall Street Journal (reviewing "Nature Wars" (2012) by Jim Sterba) via Gene Expression.
The relocation of agriculture to other places in the United States more suited to it, and the emphasis on high urban densities in the Northeastern United States, where people actually do live, have been key elements of the reforesting on the region. More generally, environmentalism has gone mainstream in American politics and become a consensus value, so long as a non-absolutist approach is taken.
Long after the historical moment when the U.S. was considered to have closed its frontier according to the Census Bureau in 1890, much of its territory now fits that definition (although the larger than you would expect size of Alaska, which was not a U.S. State when the frontier was declared closed, has a lot to do with this stiuation):
How much of the U.S. is frontier?
Answer: Frontier is more of a concept than a specific definition, so the number of people living in the frontier and the amount of land that is frontier will vary depending on the definition you select. The North Carolina Rural Health Research and Policy Analysis Center's map, Frontier Counties, United States, 2004, identifies 440 counties that meet the frontier definition of fewer than seven people per square mile, with a total population for those counties of nearly 2.9 million people. Based on the USDA Economic Research Service's Measuring Rurality: Urban Influence Codes, over three million people live in rural counties that are not adjacent to a metropolitan or micropolitan county (having an Urban Influence Code of 11 or 12), and these counties cover an area of over 770,000 square miles. Using the counties and areas provided to the National Center for Frontier Communities by the State Offices of Rural Health, 56% of the land area of the United States is frontier and more than 9 million people, or less than 4% of the population of the country live in these isolated regions.