Here's why:

* Tundra is characterized by extremely cold temperatures and permafrost (permanently frozen ground). These conditions are too harsh to support the growth of large trees, which are typical of both coniferous and deciduous forests.

* Coniferous forests (like boreal forests) are found in regions with colder climates than deciduous forests, but they still require warmer temperatures and deeper soil than the tundra can provide.

* Deciduous forests need even warmer temperatures and longer growing seasons, making them entirely unsuitable for the tundra environment.

Instead of forests, the tundra is dominated by low-growing vegetation such as:

* Lichens

* Mosses

* Dwarf shrubs

* Grasses

* Sedges