Every bit of human habitation was surrounded by open areas denuded of trees. It actually looked unpleasant and sterile to see the lack of trees near everything man-made. Roads, fields, home...no trees anywhere in the vicinity.
I theorize that is due to the nearby trees all being used for firewood.
However, trees don't necessarily grow unaided in many of the spots on those pictures. Here's a village I know: http://www.winnem.com/Assets/images/frambergkirkesaether.gif shows lots of trees, a century ago there were none there. They were planted in a big campaign around 1950-1970, in places where no tree could grow alone but a dozen together might grow up and shelter each other if tended a bit. All the schoolchildren planted and tended trees.