I think the idea is that the piggies will root out the bracken which is choking out any re-forestation. Of course the deer and sheep will then probably come along and much the saplings before they can grow into trees. Scotland for the last 150 years or so has been a playground for wealthy individuals to come and shoot deer, grouse, pheasants, partridge, virtually anything that moves, and the whole management of land has been directed towards producing these species for sport. A tragic side issue of this has been the ruthless decimation by poisoning and other means such as trapping and, of course, shooting of all sorts of natural predators, buzzards, eagles, hawks, kites, foxes, stoats, polecats even badgers to maximise the yield with the result that there is no longer any ecological balance. The elimination of wild boar allowed the proliferation of bracken, the elimination of wolves allowed the proliferation of deer and so on and the proliferation of sheep ensured that any new saplings were quickly munched to prevent any re-forestation.