Here's why:

* Ice sheets melted: As the ice age ended, massive ice sheets that had covered much of the northern hemisphere began to melt.

* Land exposed: This melting exposed vast areas of land that had been beneath the ice for thousands of years.

* Harsh conditions: The newly exposed land was initially barren and infertile, with harsh conditions due to cold temperatures and lack of soil.

* Gradual change: Over time, the land gradually transitioned from tundra (treeless, cold, and often boggy) to boreal forests (coniferous forests adapted to cold climates) as temperatures warmed and vegetation started to grow.

It's important to note that the transition from ice sheets to tundra and boreal forests was a gradual process that took thousands of years. The specific vegetation that emerged depended on factors like latitude, elevation, and soil conditions.