I've heard this before... do you have a source? I believe you, I've just never seen an actual source for that statement.
Well considering much of the Midwest was covered with savanna and prairie that only exist today in limited tracts, its not surprising at all. Fire suppression, among other things, has led to an intense reforestation of a large portion of the US (especially along the eastern prairie-forest continuum). Of 12 million ha of oak savanna in the Midwest, less than 1% still exist today (in terms of similar tree density). So definitely more trees today, well at least in the Midwest!
here are a few that touch on this, the focus is on the Midwest but they also talk about North America
Nuzzo, V.A. 1986. Extent and status of Midwest oak savanna: presettlement and 1985. Natural Areas Journal. 6(2):6-36.
Curtis, J.R. 1959. The vegetation of Wisconsin. University of Wisconsin Press. Madison, WI. 657 pp.