The huge amount of tree cover in New Hampshire historically speaking is also a pretty recent event. 100+ years ago NH was mostly farmland with little to no tree cover.
Yeah it's a little complicated historically. We used to have a nice "old growth" forest, with big oak trees etc. Nothing like the amount of cover we have now, but a lot. Then the British took basically all of the nice oaks for ships, and farming and so on cleared a lot of the rest. ~100 years ago people started feeling nostalgic about all the trees (plus farming in the area plummeted when soldiers from the Civil War returned with stories of real topsoil! and land you didn't have to remove rocks from every spring!) and a reforestation effort started. Unfortunately there is also an infestation of bugs that eat the tips off of trees so now we're stuck with tons of crappy pines with a few oaks and maples finally showing up.