Actually, according to the following report, which in part is based on a 2000 artcile and a 1998 article in the Journal of Forestry, that is not the case.
Though the actual area of land populated by Forests had leveled off as of those dates and remained relatively static the 15 years before (which in the last 15 years has improved and is slowly actually growing - and this is talking about official forest areas and does not include the literally tens of million of acres of trees in urban areas that are not part of "forestry" per sey), timber inventory had risen 30% since 1952 alone, and it says directly that there are more trees now in America than there were in the late 1800s with regard to the timber industry.
The reason is because of three factors. 1) Our ability to get more trees out of every hectacre of land than before, 2) Our ability to increase the growth rate of trees (so we have more trees growing in each area that grow to maturity faster), and 3) The fact that we (meaning private, commercial and governmental) are managing the forests better...though the governmentaal part can be argured since Clinton because they have let the underbrush grow up without management (even the Indians managed this part) and have increased the negative impact of wild fires.
So, though our polulation and use of wood products are growing, the amount of land dedicated and in reserve is relatively static but improving, and the amount of wood and number of trees is increasing. Again, not including all of those trees in the urban areas. For example, Boise, Idaho is called the "City of Trees" because so many millions of trees are planted along the green belts, in people's yards, in small home orchards, in parks, etc. which are not a part of production or reserve...and in area that had no such tree growth in prior centuries.
Great topic for spin. We get to count trees planted in vacant city lots, trees planted in area’s that had been clear cut a century earlier. On and on.
Bottom line. Is the total number of trees higher now than EVER before in the US? Maybe. Is there more forested lands in the US now than EVER before, please.....