One of the main things that prevents our boreal forests from not teeming with life is winter kill. Deep snow prevents movement, access to food, and in the spring wolves run the crust and often kill more than they need. One potential reason why there was 100 times more pounds of animals than there was just 13,000 years ago is because of winter trails. Here a herd of mastodons eat the younger spruce in a multi aged forests as well as the willows and shrubs of the understorey. They would also make excellent trails that other creatures could follow. A herd of 30 cows and calves would trample many hectares in a day and over a winter they would create an accessible landscape for many other creatures. Forget the spruce needles into elephant poop plus light for the understorey benefits, just the path making power that these creatures had.
Well maybe but plausible.
Roads out west have allowed wolves to avoid deep snow and depress game, especially caribou.