The forest south of the tundra is called the boreal forest, also known as taiga.

Here's why:

* Tundra: A cold, treeless biome with permafrost (permanently frozen ground).

* Boreal Forest: A vast, coniferous forest characterized by long, cold winters and short, warm summers. It's found south of the tundra and stretches across large parts of Canada, Russia, Alaska, and Scandinavia.

So, the boreal forest is the natural progression south of the tundra as temperatures become slightly warmer and allow trees to grow.