deer prefer to eat saplings or fresh growth over established trees or old growth. In a healthy forest, they help prevent an excess of trees from overusing resources, while wolf populations keep them from eating all the new growth. Because wolves have been driven out of many places and have low numbers, they cannot keep down deer populations, and the perspective that deer are somehow sweet or innocent makes a lot of people hesitant to kill them when they would not balk at slaughtering a cougar or a wolf on their property. Because of this, attempts are reforestation in america are often stymied by the massive deer population feasting on the new growth and young trees, and the constant encroachment of suburbs keeps expanding both the exposed outer edge of the forest and places where deer are not predated. The best solution is to destroy suburbs and urban sprawl, reintroduce wolves and have massive breeding programs, and modify our wood consumption habits. The fastest solution would be to increase deer consumption and hunting or introduce more deer anti-breeding programs(making deer sterile or infertile, which has shown some success I think). america is taking neither course because of course it's not, so the problem just gets worse.