[patall]

While the scale required is huge, mammoths are just one of the players here. I am not sure if you have heard of pleistocene park were they have started with horse, karibou, sheep, yak and a few others and already seen some effects. All those animals are seeking plants in some form or the other in the winter and by doing so move the snow cover that is insulating the permafrost, letting the cold seek deeper into it (or rather letting the warmth diffuse out). Mammoths are a key species as they as larger mammals are more prone to trampel down larger vegetation like trees to create the ecosystem all together.