After foraging for 18 years I have come to the conclusion that primitive humans had to eat animals in order to survive in northern regions. This was not an easy realization to make for a hardcore vegan (at the time).
While wild greens and fruits are extremely nutritious, 95% of them are low in calories. And if they have calories (berries and nuts) they are in only season for a short amount of time and there is not enough of them to go around.
In the Pacific Northwest where I live, you'd have to eat insects at the very least if you wanted to survive the winter. The other alternative would be to gather your things and migrate south.
Just my 2 cents...