This is because during the 18th and 19th century, vast sections of forest was cleared in North America by settlers for farmlands. Now that farming has gotten much more efficient, we can grow much more food with much less acerage. The result is that most of these small farms have now been allowed to grow back to their original state. Just look out the window next time you fly from the East Coast to say, Chicago. It is virtually a never-ending blanket of green, broken up by a few towns and housing developments.
About 10 years ago Rush Limbaugh read a paper from some high falutin' dendronologist that unequivocally stated that we have substantially more forest today than we did 200 years ago. Nothing has changed.
The article is crap anyway, since outside of the urban cores, the climate is getting cooler fast.