Here's why:

* Cold Temperatures: The tundra is characterized by extremely cold temperatures for most of the year, making it difficult for trees to grow.

* Permafrost: The ground is permanently frozen (permafrost), which prevents roots from penetrating deep enough for trees to thrive.

* Short Growing Season: The growing season is very short, limiting the amount of time plants have to grow.

While the tundra is known for its lack of trees, it does have other vegetation, including:

* Grasses: Short grasses are common in the tundra.

* Lichens: Lichens can survive the harsh conditions of the tundra.

* Mosses: Mosses thrive in the cool, moist environment.

* Dwarf shrubs: Some low-growing shrubs can withstand the cold.