Years after logging at his Mariaville woodlot, Bob Seymour expected to see new hardwood trees growing in the understory, such as maple and birch. In fact, he took it as a given after decades in the Maine woods that natural regrowth would crop up so vigorously he’d have to thin it if any were to reach marketable size. Instead, almost all of the young trees in some sections are eastern white pines he had planted. He believes that’s largely because…